A cornerstone of ethological theory is the belief that behavior comes in discrete packets. It is thought that behavior can be segmented in time into actions that have a characteristic "morphology" such that each can be recognized and therefore named. They arc the "natural" units of behavior. Lorenz (e.g.. 1950) spelled out the central role they must play in any coherent theory of animal behavior.
Most quantitative investigations rely on such segments of behavior. They are particularly relevant to the analysis of communication. However, despite their obvious importance they have been little studied. While investigators may recognize some variation, they generally accept it as simply part of the "slop" in the system.
In order to discuss the role of these seemingly unitary events in communication it is hrstnecessary to inquire into their fundamental nature. I will start with a consideration of the appearance of behavior that is recognizably patterned, followed by a discussion of stimulus control and the neurological basis of such behavior. Then I will treat some aspects of development as well as genetics before moving on to the role of patterned movements in communication.
Before doing so I have to take up the trouble some problem of terminology. The occurrence of relatively discrete motor patterns has long been recognized, for example, the bucking of a horse or the bowing of a pigeon. There is still no agreement, however, on a general term to categorize such behavior. Among the early terms that were used by Lorenz (reviewed by Baerends,1957; Schlcidt, 1974) arc "Instinct," "Instinctive Action." "Instinctive Movement," and "Hereditary Coordination." At one point a group of ethologists held a conference on terminology and settled on "Fixed Action Pattern" ( Thorpe.1951). Tinbergen (1952) accepted that terminology but later (1964) employed the more open expression "Typical Form." Lorenz (1965) subsequently resorted to further variants such as "Fixed Motor Pattern" and later (1973) "Hereditary Motor Coordination" and "Innate Motor Pattern." Yet other terms emerge in the literature, such as "Species-Specific," "Species-Characteristic," and "Species-Typical Behavior." Then there arc simply "Motor Pattern" or "Behavior Pattern." These are sometimes qualified, as in "Stereotyped Motor Pattern." It is small wonder, then, that individuals entering this area are sometimes uncertain as to which term to apply.
Labels such as "instinct," "innate," and "hereditary" should be avoided. As used here, they are not descriptive but rather are interpretive. "Fixed Action Pattern" (FAP)b is a particularly unfortunate construction, although currently it is probably the most widely used term to refer explicitly to the kind of behavior under consideration here. "Fixed" denotes a degree of stereotypy that is seldom seen. The concept has also been tied historically to a number of presumed properties (see below) that may or may not apply.
In 1968 I proposed an alternative term, "Modal Action Pattern" (MAP), which gets around both difficulties. It was put forth as a practical substitute for Lorenz's unitary concept of behavior patterns, but without implicit presumptions about causation and control. It emphasizes the statistical aspect of patterning as opposed to the fixed. I give it the following postulational definition:
The definition stresses that the behavior is a normal part of the biology of interbreeding individuals and is not meant to include prevalent human gestures that are obviously learned, such as those expressed upon meeting or departing. It allows for possible behavioral polymorphism and for the influence of the environment. The acronym MAP also appeals to me more than FAP—MAP relates nicely to the notion of a spatiotemporal map of behavior.
Because of the process of recognizing and naming MAPS, to facilitate quantitative analysis, the trend has been to focus increasingly on their stereotypy to the exclusion of their other, more interesting properties proposed by Lorenz (see reviews in Baerends, 1957; Barlow, 1968; Lehrman, 1953; Moltz, 1965; Schleidt, 1974). Most but not all of these are condensed here: (1) The behavior patterns are stereotyped in appearance. (2) The pattern is produced by a functionally organized network in the central nervous system (CNS). (3) Once triggered, the behavior runs to completion without further environmental control, i.e., without exteroceptive feedback. (4) The behavior is spontaneous; its occurrence lowers the probability that it will soon recur, and its nonoccurrence increases its probability so much that the behavior may happen without apparent stimulation. (5) The directional (taxic) component accounts for much of the variation and is not a part of the core MAPS coordination. (6) The behavior is heritable, its spatiotemporal patterning being little affected by experience.
It is a tribute to Lorenz that he formulated such clearly testable hypotheses about patterned behavioral output. Yet even though these hypotheses were put forth some time ago we still do not know, in a general sense, whether they are broadly applicable, whether they apply to some species but not to others, or, within a species, to some behavior and not to others. I would like to incite behavioral investigators to reexamine these hypotheses because they are important to fully understanding the mechanisms underlying communication. In the following therefore, I will summarize the usually inadequate information about these hypotheses. In doing so, I will play down findings on bird song; that information will appear elsewhere in this book, and it does not alter the conclusions reached here. First, however, a few words about the problem of delimiting which kinds of behavior are to be considered.
The problem of recognizing MAPS is that of deciding on the units. "The search for the units of behavior, their organization, and their empirical validation,. . . constitutes the central problem of behavioral analysis" (Condon and Ogston, 1967). Most observers rely on their own perceptual capability to select out those units (e.g., Altmann, 1965; Dawkins and Dawkins, 1974; Hailman, 1967; Lorenz, 1955, 1973; Tinbergen, 1960). It is a reasonable way to proceed and is at some point inevitable.
I shall not venture to prescribe just how one extracts units of behavior, for I think that there is no general answer and that specific answers are best dealt with by statistical procedures. I would like to mention, however briefly, that the difficulty lies in the complexity of motor output. Postures or movements may be graded, even in passerine birds (Blurton-Jones, 1968), although intermediates (Fig. 1) may be infrequent. The components of MAPS, often called "acts," may pop up in different MAPS (Stokes, 1960), requiring careful analysis and evaluation. Additionally, MAPS may overlap in various complex ways (Andrew, 1956; Barlow, 1962; Tinbergen, 1964).
An overriding consideration is that behavioral output is hierarchically arranged (DeLong, 1972; Hinde, 1953; Nelson, 1973; Tinbergen, 1950, 1951). An oversimplified view, but a helpful first approximation, is to think of neuromuscular effector groups organized into acts, which are further arranged into MAPS. The MAPS occur in stochastic sequences, which, if they become stereotyped, must be treated as units. At the other extreme, acts may appear alone and thus become simple MAPS, that is, units of behavior.
These difficulties, and more that I might mention, have been raised to bring out the imprudence of operationally defining MAPS. It may never be advisable to constrain the concept. The prudent approach is to seek the smallest distinguishable units of behavior, then to combine those that co-vary closely (Altmann, 1965; Cane, 1961; Dingle, 1972). In this regard, the statistical procedures developed by numerical taxonomists look promising (Sneath and Sokal, 1973). The techniques of ethologists for analyzing sequences may also be of use here (see review in Slater, 1973).
Analysis of MAPS
The realization that MAPS can be variable has been slowly growing; Cullen (1966) and Hinde (1966b) commented briefly on this as a general phenomenon. Examples in the earlier literature were provided in the study of wolves by Schenkel (1948), fruit flies by Bastock and Manning (1955) and Manning (1959), and primates by Hinde and Rowell (1962) and Rowell and Hinde (1962). A number of other papers have reported on the variability of displays and vocalizations in birds (Blurton-Jones, 1968; Hailman, 1967; Hinde, 1955-1956; Konishi, 1963; Stokes, 1960) and even displays in crustaceans (Crane, 1966).
Yet the general attitude remained that MAPS, especially if used in communication (displays), are rigid in their performance. Thus when Dane et al. (1959) published the first truly analytical investigation of displays, based on films of ducks, their data were received as confirmation of the fixity of displays. But the displays only seemed invariable. The duration of each was so brief that the variance was of necessity small. By using the coefficient of variation (CV), which is relative variability, the displays could be shown to be not only variable but variable in their variability. In fact, their variability is of about the same order as that for other biological systems (Barlow, 1968).
Since then a number of quantitative studies have appeared, and the use of CV is a growing convention. In the analysis of MAPS most investigators have done the obvious and easy thing first; they have simply measured duration. Consequently we have a limited view of what is meant by the stereotypy.
Our thinking about MAPS is further limited by the choice of MAPS for analysis. Observers tend to select the more stereotyped cases for study (e.g., Bucholtz, 1970; Wiley, 1973) (see also Table 2). Further, most of our examples have come from birds (Schleidt, 1974).
It is too early to prescribe a standard statistical methodology to characterize stereotypy. We may not have the statistical tools to deal with the more complex multiparametric analyses of MAPS(Schleidt, 1974). Nonetheless, some suggestions are in order as to how to assess stereotypy in simpler cases. Two basic procedures will be mentioned here. One involves central-tendency statistics, the other information theory.
When the behavior can be measured on a continuous-interval scale, as in the case of time and amplitude, a central-tendency statistic is appropriate, such as the mean and its variance. The coefficient of variation (CV) is derived by dividing the standard deviation (SD) by the mean. It is expressed as a percentage of the mean by multiplying by 100. As the stereotypy increases, CV decreases.
When analyzing MAPS it is convenient to speak of stereotypy rather than variability. CV can be converted to a statistic that I will call stereotypy, ST, which varies directly with the "true" stereotypy of the behavior:
The value 1 is added to the denominator to avoid the awkward situation of infinite ST when CV equals zero. The equation reduces to a more convenient form:
A few landmark values are given in Table 1 to show the relationship between CV and ST.
Equivalent reference values for the coefficient of variation (CV) and an index of stereotypy (ST).
The other approach is through the calculation of what has been called uncertainty, entropy, or simply, H. It is appropriate when the analysis produces discrete categories of countable elements. The use of H avoids the awkwardness of dealing with variance in a system that has few components. A short-cut tabular method of calculation has been given by Lloyd et al. (1968), together with suggestions for the particular formula to employ. For behavioral studies, Bril- louin's formula seems the more appropriate:
where the constant c is a scaling factor to convert from logarithm base 10 to a desired base such as 2 or e. N is the total number of occurrences, and n1, n2, etc., are the numbers in each category. (For applications to behavior studies, see Attneave, 1959; Dingle, 1972; Moles, 1963.) Limitations of the method have been spelled out by Quastler (1958).
H has at least two undesirable features. For one, it is a measure of uncertainty or disorder and is thus just another way to estimate variability. For another, the size of H depends on the number of categories. The latter shortcoming can be avoided by using another measure from information theory, redundancy (R). In a general sense redundancy can be equated to structure (Moles, 1963). It is calculated from
where Hmax is the maximum possible uncertainty. Redundancy has been employed as a measure of stereotypy by Altmann (1965), Dingle (1972), and Fentress (1972).
Another point needs making here about the analysis of stereotypy. The data should be sorted out according to individuals. The data from a number of individuals have sometimes been lumped because the mean value is of interest, as in comparative studies. Often the data are pooled because it is impossible to identify individuals (e.g., Marier, 1973). Yet Jenssen (1971) estimated that 98 percent of the variation in a population of iguanid lizards was between individuals, and therefore that only 2 percent was intraindividual. Such large differences between individuals are typical but that is not always the case. In the sage grouse (Centrocercus urophasianus) (Fig. 2), for instance, there is little difference between individuals in the temporal stereotypy of strut displays (Wiley, 1973).
There are other sources of error and of bias, but time does not permit their exploration. There are also other ways to explore the stereotypy of MAPS, as in the analysis of phase relationships between different components. Keeping this in mind, I will now review some of the literature that deals with a variety of ways in which patterned movement can be analyzed, such as through duration and amplitude, cyclicity, interrelationship of parts, completeness, and the separation of directional and core components.
DURATION AND AMPLITUDE
The literature reveals that MAPS may vary widely in their duration (Table 2). At one extreme are the exceedingly long static displays, as in the tail-spreading display of peacocks. Schleidt (1974) suggested that the bulk of MAPS, nonetheless, last somewhere between 0.1 and 10 sec, with most lasting about 1.0 sec, which tends to be borne out in Table 2. The table also indicates that stereotypy is usually not great, ranging from about 2 to 8 (CV roughly 35 to 15 percent). Only rarely does it exceed 40 (CV less than about 1.5 percent). A note of caution, however: in most cases the data were pooled from a number of individuals, a procedure that produces higher variances.
The stereotypy of MAPS thus presents a fairly wide range across species. Even within a species, different MAPS can express noteworthy differences in stereotypy (Table 2). But then temporal stereotypy is not a necessary condition for a MAP,for static displays can be enormously variable in duration.
When MAPS are examined internally, the little evidence available suggests that different components of the same MAP may show varying degrees of temporal stereotypy. When chickens drink or eat, for instance, the down stroke is less stereotyped than the up stroke (Dawkins and Dawkins, 1973; Hutchinson and Taylor, 1962). The feeding movements of the gastropod Urosalpinx cinerea follyensis are comparably simple, and the different components are also differently sterotyped (Carricker et al., 1974).
Turning to displays, the masthead display of the goldeneye duck (Bucephala clangula) may be broken down into three main components that are individually more stereotyped than the total duration of the display (Table 2). Variation apparently arises in the coupling of the components or in the interaction of their variations. These data must be interpreted with some caution, however, because they were pooled from several males (Dane et al., 1959), and there is some evidence (Dane and Van der Kloot, 1965) for behavioral polymorphism. In the barbary dove (Streptopelia roseogrisea), however, the six components of the bowing display are less stereotyped than is the total time for its performance (Table 3). But like the goldeneye, the main source of variation lies in internal pauses (Davies, 1970).
Statistical parameters of the duration of some MAPs, and in a few cases of the intervals between them.
Another aspect of internal variation was detected in the pushup display of the lizard Anolis anaeus (Fig. 3). The components of the display leading up to the terminal signature bob are relatively variable. But the signature bob is so stereotyped in a given individual that any variation it might have falls below the minimum resolvable difference, 0.02 sec, of film advancing at 24 frames per sec (Stamps and Barlow, 1973).
In comparison to the information on duration, that on amplitude or path movement is truly meager and has proved difficult and tedious to obtain. Rather than studying the path of movement, Hazlett (1972a) followed the change in angle relative to the body, made by the che- lipeds and the first walking legs in a spider crab (Microphrys bicornutus) (Fig. 4). The chelipeds and legs were held at higher angles when displaying than when feeding, and the final angle was also more stereotyped when used in communication than in the noncommunicatory movements involved in feeding.
The study of temporal and spatial stereotypy is in its infancy and promises fertile ground for further research. There are large and unexplained differences in stereotypy between species, within species, and within the MAPS of individuals themselves. We now have the technology to move on.
It is important at least initially to remove cyclicity, or spontaneity, from the realm of causation and to treat it at first as a problem of description. This means working out the likelihood of occurrence of MAPS.
Most investigators would think first here of clearly repetitive, rhythmic MAPS. Yet many degrees of intermediacy exist between these and what seem to be arhythmic events. Turkey calls are given singly and do not seem to be periodic. On a collapsed time scale, however, their calling is clearly rhythmic (Schleidt, 1964). Compressing time, as in fast-motion filming, reveals rhythmicity in many kinds of behavior that normally go undetected (see also Allison, 1971). In humans, this has been disclosed in religious ceremonies and in the pacing of a newspaper vendor (Eibl-Eibesfeldt, 1970).
There are two temporal aspects of repetitive behavior, inter-and intrabout intervals. The methodology for the analysis of one is applicable to the other since the one grades into the other. It is easy, then, to visualize how the compression of separate bits of behavior into bouts can lead to the evolution of new MAPS through rhythmic repetition of simpler elements, as in lizards (e.g., Ferguson, 1971; Gorman, 1968) and fiddler crabs (Crane, 1966, Salmon, 1967; Salmon and Atsaides, 1968).
The temporal stereotypy of components of the bow display compared to that of the display as a whole in the barbary dove (Streptopelia roseogrisea).
NOTE: The coefficient of variation (CV) and stereotypy (ST) are unweighted means derived from data for individual birds.
Source: After Davies, 1970: Table 6.
The intervals between MAPS, not surprisingly, are generally more variable than the duration of the MAP itself (Table 2). In the sage grouse, for example, the strut display has a high mean ST of 32 as opposed to 5.9 for interstrut intervals. Intervals between drinks or food pecks are less stereotyped in chickens than are the durations of drinking or pecking (Dawkins and Dawkins, 1973, 1974; Hutchinson and Taylor, 1962). The same holds for the intervals between pushup displays in lizards (Ferguson, 1971).
INTERRELATION OF PARTS
The prevailing view has been that the components of a MAP are linked, or coupled, in a strict sequential relationship (Hinde, 1966a). The ordering and pattern of overlap are held to be centrally programmed and essentially invariant. Qualitative observations suggest that this is a good approximation for most MAPS. However, it is not a rule.
Quite early Hinde (1955-1956) noted that the internal ordering may vary in the display of one finch species while not in another (the components themselves were said to be variable in some species but, again, not in others). Hinde then took a clearly statistical view of those displays that were variable: "display patterns cannot always be considered as distinct entities— rather the postures . . . refer to combinations of components which often occur."
Whether the sequential model fits depends in part on the fineness of the elements chosen for analysis. For example, in drinking by chicks the downward movement of the head is usually taken as one component, which is then followed by drinking, which is then followed by raising the head. Dawkins and Dawkins (1973, 1974), however, discovered that both the downward movement and the upward recovery movement can be broken into yet smaller elements depending on whether the chicks pause in their paths. An analysis employing these finer elements produced variable sequencing.
The succession of events in the pushup display of lizards seems stereotyped to the eye of an untrained observer. The ordering of individual components, however, can be shown to vary (Stamps and Barlow, 1973). The linkage between movements of the dewlap, tail, tongue, and bobbing of the head is also subject to variation (Ferguson, 1971; Gorman, 1968; Jenssen, 1971; Stamps and Barlow, 1973).
Unfortunately, there has been little attempt to analyze quantitatively the internal sequencing of MAPS. The use of H or R from information theory would be a start, detailing the stereotypy of sequencing.
Under different names completeness has long been recognized as a fundamental but accountable source of variation (Barlow, 1968). To illustrate completeness, consider a MAP consisting of three components occurring in the sequence ABC. In incomplete expressions the more terminal components drop out, as inAB, or simply A.
Schleidt (1974) has suggested a method for quantifying completeness. The number of acts in each performance is divided by the mean number of acts that occur in that MAP rather than by the maximum number ever observed. From repeated performances, the coefficient of variation is calculated, from which we may derive ST. The redundancy, R, of the different categories might prove to be more appropriate to this type of data, especially if the number of variants is small.
The concept of completeness need not be limited to MAPS with discrete components. It can also be applied to graded movements, as in Golani's (1973) analysis of the behavior of pairs of jackals (Canis aureus). The graded movements were divided into a number of arbitrary steps. In the same way the graded "smile" of primates and humans can be segmented for analysis (Van Hooff, 1972).
In the traditional view the fundamental form of a MAP is independent of environmental influence (reviewed by Eibl-Eibesfeldt, 1970; Hinde, 1966a). This view derived from the assumption that there is a core coordination for MAPS provided by the CNS. MAPS are directed or steered, whether toward one's self, another animal, food, nest, light, or whatever. The relation to external objects is bound to vary, so the directing of the MAP will be to that degree variable. This taxic component has been regarded as a secondary or overlying feature of MAPS and hence separable from them.
The core coordination and the steering component can often be sorted out (see examples in Eibl-Eibesfeldt, 1970). The classic case is that of the greylag goose (Anser anser) retrieving an egg displaced from its nest. The goose places its bill on the far side of the egg and then pulls the egg back under its breast. The lateral swinging movement of the head is a taxic component compensating for the tendency of the eccentric egg to roll off the straight path (Lorenz and Tinbergen, 1939).
It is not always possible to make such a clear separation. Some MAPS can be recognized only by their orientation (Cullen, 1960). Many examples can be found in the ritualized paths of locomotion, as in the display flights of birds such as doves and hummingbirds, or in oceanic birds such as terns (Cullen, 1960; see also van Tets, 1965). In fishes, a good example is provided by display jumping (Fig. 5) in damselfishes (Holzberg, 1973). Stereotypy of interindividual orientation is illustrated in the aggressive interactions of the beautiful Asian fish Badis badis (Barlow, 1962) and the Arctic ground squirrel (Spermophilus undulatus: Watton and Keenleyside, 1974). Orientation is commonly ritualized in reproductive behavior, as in the spawning of the Florida flagfish (Jordanella floridae: Mertz and Barlow, 1966)
In many cases, then, regularity of orientation itself may be the key element that evolution has exploited. Yet in most instances the basic coordination and the taxis of the behavior can be sorted out. Closer examination will doubtless reveal a continuum of examples between these poles.
Originally MAPS were thought to be unique in relation to the stimuli eliciting them. Once trigger they were believed to be no longer subject to external (exteroceptive) influence. That was not taken to mean that proprioceptive feedback was ruled out, although its role was seldom considered.
Two different conditions were confounded. The first is that once a MAP has started further input is not needed. Here the MAPS is capable of producing patterned output, once stimulated, and without further guidance from the exterior. Note that external modulation is thus not ruled out if present and relevant.
The second condition is that once initiated further input is ignored. Even if the external situation changes significantly, the animal does not alter the performance of the MAP—it simply proceeds to completion.
Under either condition the effect of the stimulus is only to orient and release the response. Furthermore, the strength of stimulation is held to play little or no role in the form of the behavior itself. Thus any variation observed in the form of the MAP should be due to an internal change in motivational state.
Another aspect of stimulus control is that MAPS have been said to be set apart by virtue of their causation. Each MAP has common causal factors that differ from those of other MAPS.
IS FURTHER INPUT NEEDED?
Complex MAPS may be so independent of input that they can be performed in the absence of any apparent stimulation. This is the behavior Lorenz (1937) called "vacuum activity" to stress the competence of the nervous system to execute MAPS without external guidance. While there are many examples of such behavior, it is difficult to say that no relevant stimuli were present because suboptimal stimuli may suffice (Bastock, Morris, and Moynihan, 1954). For the larger question, however, this makes little difference. The point is that external stimuli are normally overt and conspicuous. Yet they may be so reduced that an observer does not recognize them. They may even be absent. The animal may, nonetheless, still perform particular MAPS.
It has been shown in a few cases that once a MAP, or a series of MAPS, has been triggered, the stimulus can then be removed and the behavior will still go on to completion. The classic example is again that of the greylag goose (Lorenz and Tinbergen, 1939). If the egg is removed after the retrieval movement has been started, the goose will continue the movement as though the egg were still present. Other examples are rare. Tin- bergen (unpublished) has found a comparable case in the courtship of a butterfly, the greyling (Eumensis semele; for details of behavior see Tin- bergen et al., 1942). A parallel case occurs in the sexual behavior of the smooth newt (Triturus vulgaris: Halliday, 1975).
These examples and those of vacuum behavior indicate that many species have the ability to perform complex MAPS, even sequences of them, without guidance from the object triggering their performance. Nonetheless, this is also an area that has been little investigated. We need to know if MAPS are generally so independent. And the question remains as to whether changes in external stimulation can modify the course of the performance in spite of the obvious motor capability.
ARE MAPS ALTERABLE?
The question here is whether the spatiotemporal pattern of a MAP can change adaptively during its performance. Change, therefore, is limited to a degree of variation ordinarily observable in the MAP, not a fundamental restructuring or a long-term change. The scanty evidence suggests a spectrum of alterability.
For many MAPS, such as those used in non- communicatory activités, it is clearly adaptive to respond to change. The form of feeding behavior employed by the fish Badis badis in extracting worms varies smoothly but radically depending on the resistance met (G. W. Barlow, 1961). But Badis badis feeds on a variety of prey and moves rather slowly. Thus there is both the need and the opportunity for exteroreceptive feedback.
In contrast, the feeding and drinking behavior of granivorous birds is simple and quick, so the upward and downward movements of the head have considerable momentum; the behavior is also more stereotyped (Hutchinson and Taylor, 1962). Dawkins and Dawkins (1974) tried to modify drinking in chicks by presenting a variety of aversive or disturbing stimuli. For the most part, the MAP was little affected if at all. But unusually shallow water caused the bill to be held in the water longer, a change the authors attributed to the lack of stimulation the fully immersed beak would ordinarily receive.
The dragonfly larva of Aeschna cyanea carries out high-speed strikes at its prey. Bucholtz (1970) reported that the strike could be stopped even after the fast phase had been entered, but she gave no substantiating data or details.
Alterability is a more contentious issue with MAPS that are clearly involved in communication. This is especially true for signals performed at high speed. It may be difficult for an animal to alter such MAPS because of inertia or momentum. Since the behavior is brief, it may be easiest for the animal to finish it, and at little peril, rather than to alter it. It may also be difficult to attend to the stimulus while performing, particularly if the head moves rapidly.
There is suggestive evidence, nonetheless, that even brief rapid communicatory MAPS may be disruptable. The signature bob of the lizard Anolis anaeus is the most stereotyped component of its pushup display. If a male is performing a signature bob when another approaches, it completes the MAP before responding further (Stamps and Barlow, 1973). Now Stamps (pers. comm.) has done pilot experiments on tame males. One male is perched on one hand. When it starts to signature bob a rival male on her other hand is suddenly brought close to the performer. The displaying male usually completes the signature bob, but often it does break off the display.
The extent to which MAPS are alterable, within their normal scope of variation, is an open question and one that has not been much asked. And it is not just a matter of external stimulation, for motivation also plays a role (e.g., grooming in mice: Fentress, 1972). We need to know more about the susceptibility of ongoing MAPS to external stimulation, and the relation to their brevity, stereotypy, function, motivational state, and so forth.
STRENGTH OF STIMULATION
While there is not much information, it is becoming increasingly clear that the differences in the trigger, the releaser, may influence the map. The problem has been neglected in part because releasers have usually been considered only in their essential features; that is historically understandable. The releaser concept brought out important aspects of the appearance of signals and the nature of perception. Nonetheless, there has been a glossing over of relevant variation in the stimulus, such as rate of approach, proximity, size, and so forth.
A simple type of stimulus-dependent difference is one in which the performance will vary according to whether the appropriate stimulus is present. This is not as self-evident as it might seem when one considers species that advertise. as do male fiddler crabs awaiting females (Fig. 6). The tempo of the claw-waving display is faster when a female is present (Salmon, 1967), although each wave takes about the same amount of time (Table 4).
The relative size of the mate in a cichlid fish called the orange chromide (Etroplus maculatus) influences the coupling between head quivering and flickering of the pelvic fin (Barlow, 1968). Size of mate also affects the frequency of performance, i.e., the cyclicity, of a spectrum of MAPSin this and in other cichlid fishes (Barlow, 1970; Barlow and Green, 1970a, 1970b).
Proximity may alter the very form of the MAP.Hazlett (1972b) found that the height of the cheliped display in a hermit crab (Pagurus longicarpus) varies with the distance to its mirror image (Fig. 7). Similarly, Stamps and Barlow (1973) presented a territorial male lizard (Anolis anaeus) with a rival male at varying distances. Both the components of the pushup display and their interrelations varied as a function of distance (e.g., Fig. 8).
The strength of stimulation can also alter the sequence of MAPS employed in grooming behavior.1Typically a mouse starts with its face and finishes by grooming its back. A mild irritant on its back sets off an episode of grooming, yet the mouse starts with its face. But when a strong irritant is applied to the back the mouse then grooms the back first (Fentress, 1972). Thus if the stimulation is strong enough it overrides the typical program.
The effect of the presence of a female on the speed of a claw-waving display of males in two species of fiddler crab.
NOTE: x = mean, SD = standard deviation, CV = coefficient of variation, and ST = stereotypy as defined in the text.
* The variation was probably below that accurately resolvable by the rate of film advance, which was not given; presumably it was between 16 and 24 fps.
Source: After Salmon. 1967.
COMMON CAUSAL FACTORS
This issue was explored in detail by Russell et al. (1954; see also Hinde, 1966a; Hinde and Stevenson, 1969). It was proposed that all the components of a Fixed Action Pattern depend on the same causal factors. The idea is sound. If the response is unitary, its causation should be unitary. This may hold for many MAPS. But there is always the problem of the null hypothesis. One has to test all possible relevant stimuli to assure that there are not differential effects on the components (Barlow, 1968).
As seen, however, MAPS often are not as unitary as was formerly believed. One should expect differential effects, at least for the less-unitary MAPS. In one instance this has been demonstrated. The height of the pushup display and the extent to which the dewlap is extended are separately influenced in the fan bob display of the lizard Anolis anaeus (Stamps and Barlow, 1973). To a large extent, however, this display shares causal factors.
Generalizations are probably premature because there is so little to go on. The general impression that emerges, nonetheless, is that of a continuum. Some MAPS are cleanly triggered and relatively unitary, whereas others are to varying degrees under stimulus control. The original formulations were good first approximations. They brought out provocative and testable propositions by focusing on the more extreme cases. It is surprising that there has been so little attempt to test them critically.
Neurological Basis of MAPS
Out of necessity this section will be short and restricted primarily to a consideration of motor output. Obviously other aspects are important, such as sensory input and motivational states, plus learning, but they cannot be considered here. The proposition to be examined is that the spatiotemporal patterning of behavior, especially at the level of acts and MAPS, is a consequence of the functional organization of the nervous system (see Konishi, 1966).
Most of the relevant literature here deals with locomotion. But since many displays have been derived from locomotory behavior, and since some displays are little more than ritualized locomotion, the neurological basis of lomocotion is germane. The general conclusion is fairly clear. Locomotion is generated by central patterning, which is modified by both proprioceptive and exteroceptive feedback (Healey, 1957; Hinde, 1969; Hoyle, 1970; Wilson, 1966, 1967).
When one considers all motor output, a range of relationships can be seen (Hinde, 1969). At one extreme, swallowing (deglutition) and hind-leg scratching in mammals are machinelike in execution with almost no sensory modulation (references in Fentress, 1972). And Fentress (1972; see also Taub et al., 1973) concluded that "even complex movements in primates can be established and performed without any proprioceptive feedback"; he noted, however, that caution is called for in this interpretation (see Evarts, 1971).
There must be reasons why some MAPS are more dependent on peripheral modulation than others. Hinde (in Fentress, 1972) conjectured that those movements most involved in visual- motor coordination should be most severely handicapped by the loss of external information. Along similar lines, Konorski (1970) proposed that these more disruptable actions, requiring guidance, are more likely to encounter novel orientation. The obvious conclusion is that where it is adaptive to incorporate sensory information the system will have evolved to do so, and vice versa.
At one time it was thought that the organization of MAPS could be clarified through electrical stimulation of the brain. Unfortunately, the results are contradictory and difficult to interpret. Phillips and Youngren (1971), for example, concluded that "Complex 'natural' units of behavior are scare and fragments thereof are much more frequent." They were able, however, to produce sequences of aggressive behavior (see also Delius, 1973). The situation appears to be different in invertebrates such as insects (e.g., Loher and Huber, 1966) and gastropods (Willows, 1967). The MAPS is more simply organized, and some MAPS are found to be controlled by command fibers (Kennedy, 1971), ganglia, or central "tapes" (Hoyle, 1970).
It may be, however, that the failure to evoke coherent MAPS in the MAPS of vertebrates stems from a recent tendency to stimulate higher regions. Stephen Glickman (pers. comm.) suspects that the earlier workers had more success in this regard because they often stimulated lower regions of the CNS.
Even if diffusively structured, the organization of MAPS in the MAPS can range from complex to simple. Brown (1969) made the attractive and, one hopes, testable point that the evolution of stereotyped behavior can be achieved, or is permitted, by simple neural nets. In contrast, variably adaptive behavior such as learning requires a large brain with specialized areas and complex circuitry. If so, this could account for the apparent progression from predominantly stereotyped behavior among the invertebrates to the progressively more graded and complex behavior of vertebrates. (This "apparent" progression could stand closer, more critical examination.)
The general situation seems to be that the MAPS is functionally organized to produce patterned output. All motor output depends to some degree on central patterning. The organization is variously discrete to interwoven, and the output can be modified both by sensory information and by motivational state. Thus the mechanisms are present to produce a variety of motor outputs, ranging from stereotyped MAPSto smoothly graded and continuously modulated movements.
The Role of Experience
The neurological evidence indicates that MAPSreflect their functional organization in the CNS.That does not preclude a role for experience in that organization, however, particularly during ontogeny. Nor does it necessarily mean that once the organization is established it cannot be fundamentally changed through experience. By a fundamental reorganization I mean a statistically significant and persisting alteration of the spatiotemporal pattern of a MAP.
Now, no biological system develops independently of some type of experience, if experience be so broadly construed as to include one's own bodily tissues (Barlow, 1974b) and a variety of specific determinants (Bateson, 1974). It is therefore futile to propose a strict dichotomy between genetic and environmental factors (Lehrman, 1970). For analysis, however, it is convenient to disregard genetic variation and to ask rather if and how differences in experience might produce differences in MAPS.
The issue turns round the spatiotemporal pattern of MAPS themselves, not the development of associations between MAPS and stimuli. Historically there has been confusion on this point (e.g., Moltz, 1965), even though the distinction has been clearly spelled out (Eibl-Eibesfeldt and Wickler 1962; Marler and Hamilton, 1966; Tin- bergen, 1960). The very basis of conditioning depends on the unconditioned response, often a MAP, forming some new association with a stimulus other than its appropriate (unconditioned) one.
The most open question in ethology is whether experience contributes to the organization of MAPS, and if so, how? There is little to go on at present. There are abundant fragments in behavioral studies. Unfortunately, most are highly qualitative although interesting and provocative. Lacking a guiding hypothesis, they have seldom dealt with the internal structure of a given MAP.
A reading of the literature on the ontogeny of behavior disclosed that experience plays a limited role, confined largely to facilitating the development of MAPS and to regulating their frequency of occurrence, their association with stimuli, and their integration into coherent sequences. In a few cases, animals have demonstrated the ability to mimic what are, for them, novel MAPS. Imitation, however, seems to be limited to primates (e.g., Gardner and Gardner, 1969; Hinde, 1970; van Lawick-Goodall, 1970), toothed whales (Tayler and Saayman, 1973), and to vocalizations in parrots and various other birds (Thorpe, 1961; for a general review see Davis, 1973).
Caged animals develop highly stereotyped, often seemingly idiosyncratic, bits of behavior (Holzapfel, 1939; Morris, 1964, 1966). But these movements may not be so idiosyncratic. The peculiar stereotyped activities of chimpanzees raised without mothers, for instance, have a degree of congruence, and they resemble stereotyped movements seen in some institutionalized humans. Such movements may be "a common primate reaction to a variety of disturbances in the environment" (Berkson and Mason, 1964). This suggests that even seemingly individualistic but patterned movements are based on functionally integrated components inherent in the CNS.
That the basic coordination of a MAP can be altered through experience remains scarcely demonstrated. A possible instance is the loss of one component of pecking in chicks of the laughing gull (Larus atricilla: Hailman, 1967). In most cases the basic coordination seems resistant to fluctuations in the environment. Nonetheless, environmental influences may be subtle or more indirect, or may come early, as in embryogenesis, and therefore be undetected. The problem is that little attempt has been made to disprove the hypothesis that the spatiotemporal pattern of MAPS is not affected by experience. If the attempt were made perhaps a different view would emerge.
The growth of behavior genetics over the last ten to fifteen years has been extraordinary (Broadhurst et al., 1974, and Manning, 1975). It is now indisputable that genetic differences can produce phenotypic differences in behavior. Furthermore, in the broad usage of population genetics, behavior is heritable. As demonstrated through the technique of quantitative genetics, "heritability, which is a measure of the genetic variability accessible to selection, [is] relatively high for most [behavioral] characters and comparable to those calculated for morphological characters" (Ewing and Manning, 1967). As the field progresses, behavior is shown increasingly to be the product of the well-known and intricate complexities of genetic determination. These include pleiotropy, epistatic and additive effects, sex linkage, and the influence of the genetic environment on the expression of given alleles (Dobzhansky, 1972; Manning 1975).
Most of the analyses, however, have not dealt with the structure of MAPS. Investigators have centered on such things as rate of defecation or activity in an open field, frequency of courtship displays, susceptibility to audiogenic seizure, and many more (Manning, 1975). Yet MAPS seem especially well suited to the purposes of genetic analysis (Ewing and Manning, 1967).
MAPS appear to be polygenetically controlled (Manning, 1975). As a consequence, phenotypic variation between individuals is continuous. Differences of one or a few genes produce such small changes in the MAPS that they frequently go unnoticed (Franck, 1974). Thus qualitative differences within interbreeding populations are rare. (Of course, qualitative differences are merely large quantitative ones; it is a matter of degree.)
One of the more important small differences that can be produced by mutation is that of threshold of occurrence of the behavior. This is illustrated by Manning's (1959) observations on two closely related species of fruit fly (Drosophila melanogaster and D. simulans) that differ in only a few genes. They diverge in courtship behavior, but more in degree than in kind since they share the same displays. But the two species favor different displays. Manning contended that the divergence is due to differences in thresholds of the MAPS, perhaps as a consequence of different levels of arousal.
The threshold model is attractive. It could account for many differences that have arisen in the evolution of MAPS, as in the pushup display of an iguanid lizard (Uta: Ferguson, 1971, 1973). Franck (1974), however, warned that the threshold model should not be too generally applied "because it misleadingly implies a fundamentally uniform action of genes."
The most convincing demonstration of the genetic control of MAPS comes from the study of hybrids (see useful reviews by Franck, 1974; Marler and Hamilton, 1966). The most productive approach is to cross two closely related species that have distinguishable but homologous MAPS. Differences in the ¥\ phenotypes, compared to the parents, can be attributed to genetic differences because they can be replicated. They can also be verified through classical genetic methods such as producing F2 progeny and backcrosses. These studies confirm that MAPS are under polygenic control. In most of these studies, unfortunately, behavior is assessed by simply recording the presence or absence of the MAP, or the MAPS are described verbally with insufficient attention to essential detail.
Generally the behavior of the hybrid is intermediate, as is its morphology. However, the intermediacy is often less than perfect in that hybrids tend to favor one parent or the other (Davies, 1970; McGrath et al., 1972). Sometimes hybrids show intermediacy in one trait but apparent dominance in another, as in the display of a hybrid Anolis lizard (Gorman, 1969). If the species are sufficiently unrelated, as in a duck X goose cross (Poulson, 1950), the hybrid may fail to express many MAPS of its parent species. Of more general interest, if the parent species are quite but tolerably different, the behavior of the hybrids is often a mosaic of that of its parents rather than being intermediate (Buckley, 1969; Hinde, 1956; Leyhausen, 1950; Lind and Poulsen, 1963).
Uncoupling a MAP from its taxis has been seen in hybrid swordtail fish (Franck, 1970). Sequencing of MAPS, too, can become scrambled in closely related species with homologous displays (Lorenz, 1958; Ramsay, 1961; Sharpe andJohns- gard, 1966).
The evidence, then, reveals that behavioral traits are genetically determined in much the same way that morphological ones are. The genetics of MAPS can be studied with the same methodology and is subject to the same limitations of interpretation with regard to environmental interaction and analyses based on differences.
MAPS in Communication
So far I have seldom troubled to distinguish between communicatory and noncommunicatory MAPS. That has allowed us to consider patterned movement in general, to bring in all information available regardless of function. After all, one of the classic examples of stereotyped movement is that of the goose retrieving its egg, which is hardly a communicatory MAP. Additionally, it is often moot whether or when a MAPserves in communication.
Grooming and nest building, as two examples, might not seem communicatory. Yet preening behind the wing signals that copulation is imminent in pigeons (pers. obs.). Wing preening and other movements involved in bodily maintenance (Fig. 9) are an integral part of duck communication (Lorenz, 1941; McKinney, 1965). In penguins and other aquatic birds, a number of comfort movements may transmit information (Ainley, 1974). Allogrooming in primates has also evolved a communicatory function (Hinde, 1966b).
Communication is held to occur when a MAPperformed by the sender changes the probability of subsequent behavior in a perceiver. Within this broad definition there are some important distinctions. If there is a communicatory system, then, in the evolutionary sense, communication should improve the fitness of both sender and receiver (Klopfer and Hatch, 1968). The evolution of communication therefore requires a parallel progression of the signal and the perception of it, whether within or between species.
This restriction, however, excludes informative interactions between predators and prey, where evolution may be acting in a comparable fashion on the spatiotemporal patterns of MAP.Many sedentary species, for example, have evolved spectacular displays to put off their predators ("startle" displays of moths: Blest, 1957). There will be selection for those moths performing the most effective MAP, as well as for those predators that are not so easily startled. Likewise predators might evolve special movements or vocalizations that are more effective in driving or luring and capturing prey. The roaring of lions, if it confuses prey and facilitates their capture, could be considered a ritualized MAP. A number of fish predators (Shallenberger and Madden, 1973) and some turtles (Hediger, 1962) lure prey to them with stereotyped movements that connote food. Such cases present problems of signal detection, ambiguity, localization, and so on, that parallel those in mutualis- tic systems of communication.
The discussion to follow is built around the evolution of MAPS, particularly as tools of communication. Evolving into a signal often means becoming stereotyped, and that will be the organizing theme. In some respects this is an unfortunate approach, though a reasonable and convenient one. It emphasizes stereotypy at the cost of other important aspects of MAPS in communication, and that is not my intent. In addition, it detracts from recognizing the gradation extending from the more variable to the more stereotyped signals.
I also sidestep a precise meaning of stereotypy. It is not easy to spell out exactly what is meant when a MAP, in its totality, is said to be stereotyped. Many MAPS are stereotyped along one dimension but variable along another. Or, using the same parameter, say, duration, one component may prove stereotyped and another not. There is so far no satisfactory measure of all aspects of stereotypy of a given MAP. Nor is the problem of level in the hierarchy of behavior easily resolvable, especially for MAPS such as those of complex grooming in rodents.
In the following I review first the problem of producing patterned output by the neuromuscular apparatus. Then I touch briefly on these properties of perception that feed back on that output. Finally, I run through the various ways stereotypy or variation might be favored, depending on signal function and environment. I avoid trying to explain the origin and derivation of displays and the many types of changes that are possibly involved. That area, the topic of rit- ualization, has been covered well and in detail elsewhere, following the pioneering paper of Daanje (1950; e.g., Blest, 1961; Cullen, 1966; Hinde, 1959; Lorenz, 1958; Marler, 1959; Morris, 1957, 1966; Tinbergen, 1964).
Just about any regularly performed behavior becomes stereotyped, whether it is clearly learned or not. Thus religious ceremonies of humans and the play of animals become simplified and hence stereotyped (Thorpe, 1966), as do acts of "superstitious" or interim behavior of animals in Skinner boxes (Millenson, 1967; Stad- don, 1975). This progression toward economical output has been called the principle of least action (Wheeler, 1929), least effort (Tolman, 1932), or simply parsimony (Adams, 1931).
Spatiotemporal patterns of behavior characterizing populations or higher taxa doubtless become similarly parsimonious and therefore efficient. Since there is usually one most efficient method, the individuals of a population will converge on a particular method of performing the MAP. The capacity to develop such a stereotyped MAP will become genetically reinforced and common to the gene pool. The neural circuits underlying such stereotyped motor behavior are thought to be particularly susceptible to having their efficiency improved through natural selection (Brown, 1969).
If such behavior is especially frequent it tends to assume rhythmical repetition. Schleidt (1974) suggested that this is more likely if the behavior has been derived from originally rhythmic activity such as locomotion or respiration. It is probably also related to mechanical factors. Appendages of a certain mass and proportion, for example, will have an optimal period and amplitude for a given behavioral task. If the behavior needs to be repeated frequently it should assume an efficient rhythmic pattern. A good example is the fanning of eggs or larvae by a wide variety of teleost fishes. In Badis badis fanning is a stereotyped rhythmic behavior; its parameters such as tempo, bout length, and number of bouts are organized into rhythmic bursts in order to propel fresh water to the eggs (Barlow, 1964).
Mechanical factors are important in ways other than rhythmic repetition. Smooth, relatively quick motions are probably more efficient than slower, more erratic ones. Slowly performed movements, with many starts and stops, require more work to overcome inertia and momentum, respectively, and to hold the appendage or body in a particular position. Sheer size must also be considered. The smaller species or individuals are less restrained by mass and inertia. They can have quicker, more dynamic, and therefore more intricate movements (Cullen, 1966; Hutchinson and Taylor, 1962; Meyerriecks, 1960).
Irrespective of size, neuromuscular effectors tend to be conservative. Most communicatory MAPS have been derived from basic patterns of coordination, such as locomotion, respiration, feeding, elimination, and protective reflexes. The basic coordination, especially of locomotion, probably cannot endure the large changes that would be needed to produce radically different MAPS. The forms of MAPS are therefore limited to patterns similar to those of the more basic activities. Departures are obviously possible, but the nervous system has little scope for unique MAPS.
New MAPS are bound to share effector systems with more primary ones. They therefore have the potential to interfere with them. Sharing may occur quite far upstream. Consequently a basic coordination may service several behavioral tasks, so the same MAP may be employed to different ends. Context then becomes necessary in determining message and meaning (Smith, 1968, 1969).
Sharing of patterns of coordination in threat, attack, and prédation is relatively common (Albrecht, 1966; Barlow, 1968; Blurton-Jones, 1968), although the MAPS may be to varying degrees distinguishable. A basic motor pattern may be used even more widely. Neck biting in felids is employed in the unrestrained biting of prey, by males when fighting with other males, by a male toward the female during copulation, and by a female when carrying kittens (Leyhausen, 1973). Some differences, such as the force exerted, are obvious, but differences in coordination are not fundamental.
In summary, the nervous system of animals favors stereotypy when faced with a familiar task. Regularly performed behavior is parsimonious and therefore stereotyped to some degree. The nervous system gives priority to its more essential patterns of coordination, which limits the extent to which secondary MAPS may differ from them. It also reduces, thereby, the number of displays that may be produced. Consequently context often must supply the missing information about the meaning of a MAP.
A consideration of the influence of perception automatically limits the discussion to MAPSused in communication. Right off, we encounter a chicken-and-egg argument. To become a signal a MAP should evolve into a form that is best perceived by the intended recipient. Conversely, the recipient should be adapted to perceive key MAPS. At the more general level, properties of the receptors are probably the more fundamental. For finer tuning, both receptor and signal must adapt to each other. But if there were one best signal for a receptor, in general, a number of difficulties would arise. For one, a species might have only one signal in that channel. For another, closely related sympatric species would converge on the same signal or would be forced to change their receptor systems. If that means an alteration of spectral sensitivity, for instance, it could leave the receptor less well adapted for other essential behavioral tasks, such as detecting items of food. One solution is to alter temporal patterning and make the adjustment in the receptor system farther upstream. Yet there must be some limit here. One predicts a compromise in the more complex species, with the MAPSbeing constrained by the motor system, although with a substantial amount of adjustment to maximize detection by the receptor system.
There has seldom been any attempt to examine how the spatiotemporal pattern of MAPS is related to perception. One exception is the work of Magnus (1958) on the fritillary butterfly (Argynnis paphia). To reproduce, the male chases females, who are of a particular size and coloration and beat their wings at a characteristic rate. The faster the wings of a mechanical female beat the stronger the response of the male, until the rate exceeds the flicker-fusion frequency of the male's eyes. Then it stops responding.
Hailman (1967) followed the same logic when testing the pecking response of chicks of the laughing gull. In contrast, the perceptual system here is adapted to the MAP. The chicks responded best to an intermediate speed of the model of the parent's head.
An indirect confirmation of signals' being adapted to receptor systems lies in the relationship between threat and appeasement displays. Darwin (1872) pointed out that they are antithetical (see Marler, 1959; Barlow, 1962). What needs emphasizing here is the contrast between them in relation to perception. Threat displays involve conspicuous movements, increase in size (hair, fins, or feathers, etc., extended), and approach. Some species also change color, contrasting with the background and sharpening their profile. The receptor system is played to.
In the commonest type of appeasement display the opposite occurs. The animal becomes relatively if not absolutely motionless, small, and seems to melt into the background. It fits itself into the "blind spots" of the receptor system, so to speak, to minimize detection and thus the evoking of attack.
As one might expect, there are appreciable differences in visual filtering between such phyletically and ecologically divergent animals as frogs, rabbits, and cats. Nonetheless, a modestly coherent theme emerges (Barlow et al., 1972). Gone are arguments about central versus peripheral filtering of stimuli. There is a continual process of filtering and coding, starting at the retina and continuing on into the brain. Different "trigger features" are encoded (compressed) at different anatomical levels ascending the optic pathway. Salient attributes include dark-light contrast, bars, slits, or edges, corners, ends, and orientation. These are all aspects of form. Attributes of motion, such as movement away from or toward the fovea, expansion, and so forth, play a role but are less well understood.
Probably the most important general feature of visual perception is the reduction of redundancy that is inherent in the input (H. B. Barlow, 1961). The nervous system tends to respond to change, as in "on," "off," and "on-ofF' fibers. This makes possible lateral inhibition and the consequent significance of contours. It also means that the nervous system, after a while, ignores sustained input—it is neophilic.
Thus it is paradoxical that communicatory behavior of animals, the output, is so often redundant. Why should this be, if repetitious stimulation is likely to be ignored?
Animals have a limited number of signals, which they must therefore repeat often if communication is at all frequent. Further, multichannel redundancy is almost the rule. For example, a displaying fish may posture and move, change colors, and vocalize while giving off chemicals. It is as though the animal has little to say but insists on being heard, so it delivers basically the same message repeatedly and through different channels.
As Bateson (1968) put it, redundancy exists if missing items can be guessed with greater than random success. Redundancy in this sense is patterning sufficient to permit part-for-whole communication. This turns the issue around. If the output is redundant it does not necessarily follow that the intended receiver is observing all the output. Indeed, animals that repetitiously broadcast—for instance, lek species—usually change their behavior when they get feedback.
The receiver is faced with a continuous stream of behavior by others. It is also barraged by sensory stimulation from the environment as a whole (H. B. Barlow, 1961). It has to pick out the relevant signals (segments). Part-for-whole communication becomes significant. To deal with the flow of behavior the receiver must properly segment it to decode it. Here it gets help from the sender.
The sender breaks up its behavior into relatively unitary pieces, whether for communication or for other reasons that I have already discussed. It produces variously discrete MAPS.
If a MAP is to be attended to it must contrast with other stimuli reaching the receiver. Now, contrast can be a slippery concept (Andrew, 1964). Here I mean only that it must differ sharply from the commonplace. If it differs, it is to some degree novel and therefore detectable. To assure that the pattern is recognized and decoded, once detected it must be stereotyped (redundant) to conform to the code.
Communicatory MAPS should have sufficient redundancy, therefore, to be extracted from the continual stream of heterogeneous input. But if the signal is highly redundant and inescapable, it may be responded to only initially, then ignored. A method of "having one's cake and eating it" is to regularize some parameters of the output, facilitating detection and recognition, while making other parameters variable to maintain attention. This can also be accomplished by changing the order in which MAPS are presented, that is, through variable sequencing.
MAPS that have evolved into signals are commonly given in strings or sequences. Then they often overlap broadly and to varying degrees. I have no desire to explore here the different models of sequence analysis (see Nelson, 1973; Slater, 1973), but such models are germane to the question of how complex MAPS are organized and thus how they evolved.
Do animals first recognize redundant sequences of MAPS and learn to respond to them as units? Through experience, humans can organize words or numbers into "chunks" of increasing length (Simon, 1974). Possibly animals do something similar, as when shifting from heterogeneous summation to Gestalt perception during ontogeny. This ability has been reported by Hail- man (1967) for laughing gull chicks (see also Bower, 1966, for human infants).
The maximum number of "pieces" per chunk evidently remains about the same, around five to seven in humans (Simon, 1974). This implies an upper limit to the number of units that can be organized into a new one. By coalescing smaller units into larger ones, however, greater complexity can be processed. Hence more information can be communicated in a given amount of time.
In animals, chunks would be equivalent to MAPS, or to groups of them, in a stream of behavior. The number of components making up a chunk in vertebrates must be relatively low. The few Markov chain analyses indicate, at best, second- or third-order dependence (Altmann, 1965; Fentress, 1972), and often less (Nelson, 1964). If animals evolved displays from sequences one would expect no more than two to four major components, and usually fewer, a seemingly reasonable first approximation. The displays of Anolis lizards are an excellent example (Gorman, 1968).
The anatid ducks, however, provide the best material here (Lorenz, 1941). Their displays are ritualized, rapidly performed sequences of up to three or four simpler acts. Differences between species consist in the main of re-ordered acts or the addition or loss of acts. "The elementary motor patterns are more widely spread among the species than their combinations" (Lorenz, 1955).
In fact, McKinney (1961) suggested that in some instances the sequences of MAPS, not just coupled acts, have become the displays. If so, the displays have moved up a level of organization in our conceptual hierarchy. McKinney speculated further that the highly stereotyped sequences of some species represent an evolutionary advance over the more "primitive" state, the relatively variable sequencing observed in other ducks.
This brief review of how the properties of the perceptual system might guide the evolution of MAPS as signals reveals how little is known in spite of progress in recent years. (A brief venture into the literature on pattern recognition by machines was of little help.) I feel the physiological information is there to bridge the gap, to say much more, but it has not been applied to this particular problem. It would thus be useful if physiologists were to pursue the question of how animals should structure signals to "exploit" the perceptual side of the system. What spatiotemporal patterns best lead to their detection, discrimination between them, and continued attention?
This, the last major section, is a bit eclectic. It is meant to identify those factors favoring stereotypy or variability in MAPS serving as signals. It is often put that if a MAP is used in communication it must be stereotyped (Barlow, 1968; Hazlett, 1972b). But this should not be construed to mean simply that the more obviously a MAP is employed in communication the more it will be stereotyped. The inference is only that if there is a communication system involving a sender and a receiver, they must have an agreed-upon code. Therefore the signal must have a recognizable pattern, and so it must be to some degree stereotyped. But different needs will set, through natural selection, different degrees of stereotypy.
Morris (1957) coined the term "typical intensity" to characterize situations in which the need dictates stereotypy. If it is all-important that only one message be transmitted, then the MAPshould remain about the same over a wide range of stimulation. In contrast is the system that utilizes an analog type of signal: The signal varies with the strength of stimulation. Here there is the potential to communicate more information. There is also more ambiguity because the signal is likely to be continuously graded or divisible into steps that differ little from the next one (see Marler, 1959). In either case the receiver must be able to distinguish small differences in the signal.
Working between these extremes, I will discuss the factors favoring stereotypy versus those favoring more graded, hence variable, MAP.This argument is essentially an evolutionary one. For coherence, I will bring in other factors bearing on the adaptiveness and evolution of MAPS.
Basic to all considerations is ecology—the physical environment, interaction with other animals whether benign or predatory, and feeding behavior. While recognizing the ultimate role of bioenergetics, I will not attempt to develop the nexus between the animal's ecology and the consequent form of its displays. Suffice it to say, however, that in spite of earlier views to the contrary displays are seldom truly arbitrary in form (Andrew, 1956 ; Brown, 1963; Crook, 1964; Cullen, 1957; Gorman, 1968; Hinde, 1956; Tinbergen, 1964).
The course of an animal's development should also prove important. Alexander (1968) noted that nonsocial insects, for example, seem to fit all of Lorenz's original criteria for instinctive behavior, including highly stereotyped MAP.Alexander interpreted this as an adaptation to the environment that prevails during development. First, no parents are present to exert their influence. Second, there is misleading environmental "noise" that must be overcome. Thus their MAPS should be resistant to irrelevant environmental stimulation and should not require parental molding.
Nonetheless, I suspect that the extreme stereotypy of arthropods is just as much attributable to the limitations of their simple central nervous systems. Note that many lower vertebrates, such as fishes, amphibians, and reptiles, experience a similar situation in ontogeny. While all these vertebrates show clearly stereotyped MAPS, they also perform graded, variable ones. Environment during ontogeny is probably a significant evolutionary factor in the development of stereotypy, but it cannot be considered of overriding importance.
A general factor influencing the evolution of stereotypy is noise. Interfering noise is usually proportional to the distance between sender and receiver. Different environments have different levels of noise so, for comparative purposes, both distance and environment need considering. The rule is that at greater distances and/or noise, signals are more redundant and individually stereotyped (Marler, 1965, 1968, 1973). This enables part-for-whole communication, so that the signal need not be received either completely or continuously.
Conversely those species that maintain close relationships tend to use signals that are more graded and flexible. This appears to hold in a variety of animals, such as primates (Marler, 1959, 1968, 1973), canids (Fox, 1970), and cichlid fishes (Barlow, 1968). There is a danger of oversimplification here, however.
Within species that associate closely, other factors may drive them toward stereotyped signals. Cullen (1966) noted that in colonial sea birds small territories necessarily heighten the stereotypy of signals involved in the reduction of attack. Another example is provided by hermit crabs. A relatively social species that lives in high population densities, and therefore has frequent contact with other members of its species, has highly ritualized aggressive displays. Species that live widely spaced are more aggressive and simply attack when they meet, rather than perform ritualized displays (Hazlett, 1972b).
The difference lies in the relationship between the animals in close contact. If they are members of a social group and are personally known to one another, the signals will tend to be graded. But if they do not live in social groups and are not likely to be known to one another, then stereotypy will be favored.
There are commonly differences within species in stereotypy of MAPS, even within the same signal category (Marler, 1968). These relationships are easier to visualize if categories of behavior are first compared. Crane (1966) stated that static threat displays of fiddler crabs are more stereotyped than those utilized to attract females. Hazlett (1972a) presented data to support just the opposite relationship in two other species of crustaceans: MAPS used in threat were found to vary more than those in mating behavior. This is more in keeping with Marler's (1959) observations on chaffinches and with my experience with cichlids and other teleost fishes. Such divergent conclusions suggest that the problem is one of secondary rather than primary correlations.
A universal factor is time. The briefer the encounter the more stereotyped the signal is likely to be, as in the mating of lek birds (Sibley, 1957). Wiley (1973) reemphasized the correlation after analyzing the strut display of the sage grouse, a lekking species.
The same conclusion derives from the stereotypy of sequences of displays: Camouflaged species of hermit crabs are secretive and expose themselves only briefly during interactions; their behavior is more stereotyped than that of related species that interact more frequently and openly (Hazlett and Bossert, 1965). Similarly, if there is great risk of being damaged, the encounter will tend to be brief; this may account for the stereotypy of behavioral sequences, and therefore reduction of ambiguity, in mantis shrimps (Dingle, 1972).
An important factor favoring brevity, hence simplicity and stereotypy, is exposure to prédation (Moynihan, 1970). It accounts for the quickness of displays in the camouflaged hermit crabs. This conclusion, however, is largely inferential although it gets some support from observations on comfort behavior by Adélie penguins (Pygoscelis adeliae). They are preyed upon by leopard seals (Leo pardus) mostly in the surf line. Young penguins bathe just beyond the surf and are easy prey. Experienced adults delay bathing until well beyond the surf. A most relevant factor here is that their bathing, unlike that of other waterfowl, is broken into short bouts, thus reducing the time they are continuously vulnerable (Ainley, 1974).
Frequency of display per se was mentioned earlier as a factor favoring stereotypy. But I have also maintained that MAPS less frequently performed must be more stereotyped. Both statements are true with more information, and for different reasons. Frequent behavior, whether or not employed in communication, will be stereotyped for reasons of neuromuscular economy. But if the MAPS are performed very frequently and in the service of close-up communication in a social group, there may even be a tendency for them to become less stereotyped (see below). Rarely occurring signals, on the other hand, must be stereotyped if the receiver is to detect and recognize them without fail. The evidence suggests that this is the case, as in seldomemitted warning calls of songbirds (Marier, 1957, 1961b). Since such signals must often of necessity be decoded without benefit of much or any previous experience, it is also essential that they be relatively simple. Otherwise, it is difficult to encode the signal in the genome (H. B. Barlow, 1961). Additionally, simple signals are learned more quickly than complex ones.
The example of alarm calls brings into consideration intra- as opposed to extraspecific communication. Many passerine birds have evolved two types of vocalizations that are similar in their essential features. One is an alarm call. Its important feature is that it is difficult to locate, minimizing detection of the sender by the predator. In contrast, the other vocalization, the mobbing call, facilitates localization and brings other birds to mob the "enemy." The resemblance of these calls across species is due largely to convergence on the physical parameters of sounds that make them difficult or easy to locate, respectively (Marier, 1957).
In addition, communication between species favors selection for (1) a simple signal and (2) extraspecific stereotypy. The elaborate threat displays of surgeonfishes, for example, vary between the species and are largely exchanged between conspecific fish. Extraspecific aggression, in contrast, is usually simple and similar across species (Barlow, 1974a). Thus extraspecific communication encourages stereotypy.
Most of the foregoing has focused on factors increasing the stereotypy of MAPS, although the argument could have been developed the other way around. For example, more time permits signals that are more graded. In some instances it is easier to think about the advantages of lessened stereotypy. This applies to the more social animals that live in relatively stable groups.
Two factors operate here, and both are functions of time. First, prolonged association permits the opportunity to learn finer distinctions between signals. The receiver learns what follows what, and errors are corrected through reinforcement. There is more opportunity for signals to be graded or similar to one another. In addition, context can play a greater role.
The second factor is the inherent tendency of the nervous system to filter out redundant input (H. B. Barlow, 1961). This is particularly critical in the pair situation. If the pair has a prolonged courtship phase the probability is high that the partners will habituate to one another. If each partner is to stimulate the other's reproductive physiology to achieve sexual union, they must be able to hold one another's attention to effect some degree of arousal.
Novelty, or variation, is important in attention and responsiveness. The most celebrated example is the Coolidge effect: When a male rat is apparently no longer able to copulate with one female it is presented with a new female, and copulation again occurs (Dewsbury, 1973; Fisher, 1962).
It is possible to achieve, in motor output, both pattern and variation. The animal can vary the sequence of stereotyped MAPS. It can also produce variation by overlapping MAPS. Even the MAP can be varied. For example, the orange chromide can maintain a recognizable pattern of quivering while varying the duration of bursts, the coupling of head movements with the flickering of the pelvic fins, orientation relative to the mate, and the superposition of other MAPS (Barlow, 1968). This is an antimonotony strategy. It was first suggested by Hartshorne (1958, 1973; see also Nottebohm, 1972) for bird song and was later applied to visually perceived MAPS (Barlow, 1968).
I should like to digress here to return to the theme of rhythmic behavior. It is relevant to the problem of monotony. MAPS are so often derived from the coordination of respiration or locomotion that one expects them to be rhythmically performed (Schleidt, 1974). The fanning movements of parental fish, the wing waving of colonial sea birds, and the pushup displays of lizards are obvious examples of rhythmic behavior derived from locomotion.
Given this, rhythmic displays are actually not as frequent as one might expect. They should be encountered most where redundancy is needed, as in communication over a distance. This seems to be the case. Examples can be obtained from the claw-waving behavior of fiddler crabs, the almost dancelike behavior of lek birds, and advertisement songs of many passerine birds.
Otherwise, selection should favor patterned irregularity. A good case is found in the comparative study of pushup displays by iguanid lizards of the genus Uta (Ferguson, 1971). In the course of evolution the intervals between each pushup have assumed different lengths, as have the distances of the up-down excursions. The result is a syncopated progression rather than a rhythmic repetition.
Yet there are times when rhythmic behavior is highly evocative and apparently essential to the proper response. This is evident in the arousal and synchronization of sexual behavior. In cichlid fishes the variable courtship becomes repetitive and highly redundant shortly before spawning. The rhythmic aspects of copulation in other animals hardly needs mention. This reversal, the requiring of redundancy to evoke a response, is probably a "fail-safe" mechanism, insuring that both sexes will synchronize their ultimate reproductive act.
Returning to the main theme, care is needed when applying to all species the idea that a prolonged pair relationship will result in variable behavior. Some species that live in pairs actually have infrequent and brief contact. Their interactions are comparable to those of species that come together only to achieve fertilization. In many birds the male and female share a territory but they are only weakly bonded; personal recognition is minimal. If one is lost a new, strange bird of the same sex may enter and become the mate (Welty, 1963). One would predict rather stereotyped behavior in such situations, as compared to paired species that move about together and show evidence of personal recognition.
It has been noted by a number of authors (e.g., Thorpe, 1961) that the variability of displays is related to individual recognition. Most of the research here has been on bird song (Nottebohm, 1972). Thorpe listed two main functions of song: (1) It should be sufficiently stereotyped to characterize the species. (2) It should be variable enough for individual recognition. Marler (1961a, 1961b) suggested that different aspects of the song serve these two functions; Falls (1969) found some evidence for this (see also Hutchinson et al., 1968; Ingold, 1973).
It should be clear what is meant here. The variation serving recognition is between individuals, not within them. The stereotypy of a MAP or of a component of it that is used in individual recognition should be higher than that shown for the group as a whole. Apparently it is. The variation of the means of temporal scores of displays of different individual Anolis is large, but it is small within individual lizards (Jenssen, 1971). In another lizard, the chuckwalla (Sauromalus obesus obesus), the pattern of head bobbing is erratic and varies greatly between individuals (Fig. 10). However, "the first three to four seconds of the display are unique for each chuckwalla" and are essentially the same each time the display is given (Berry, 1974). Thus a portion of the behavior communicates individuality, as Marler (1961a, 1961b) suggested for bird song.
Marler ( 1961 b, 1968) developed the view that when communicatory MAPS operate in the presence of a number of similar signals they tend to be more stereotyped, and there is evidence for this in bird and primate vocalization. This is a variation of the "noise" argument that was mentioned before. Marler had in mind closely related species occurring together. Ambiguity here must be avoided. "Key" signals should be executed with more precision, matching the "lock" in the perceptual apparatus of the receiver.
There is little evidence here for visual displays, and it does not clarify the issue. Isolated island populations of an iguanid lizard, Uta, are among the most- and the least-stereotyped populations (Table 2). In another case, the pushup displays of spiny lizards (Sceloporus) show character displacement where two species are sympatic (Ferguson, 1973). The expectation is that the displays should also be more stereotyped where the species overlap. That seems to be true for one species, but the opposite holds for the other. Thus there is no substantiation of greater precision, better fitting of the key to the lock, when visual signals are in competition. More studies are certainly needed.
This lock-and-key analogy is sometimes confused with a more general argument about signals without regard to competition between them. One should distinguish between two selective pressures. One is for detection of the signal from the background, which I have covered above. The other is for discrimination between comparably detectable signals. In some instances the two factors work in concert, but in many they do not. In any event, they are separable for purposes of analysis and exposition.
The discrimination of the correct signal from among others, as when closely related species coexist, leads to the evolution either of more complex or of more precise MAPS (Schleidt, 1974). Greater complexity permits less precision because it becomes increasingly improbable that competing signals will correspond as they become more complex. If the MAPS remain relatively simple there is a good chance of resemblance between them because of similar if not identical pathways of derivation. The small differences between them consequently should be more precisely maintained to be recognizably different.
Most species probably move toward increasing complexity when signal competition arises. There should be many instances, however, of isolated species with complex displays. They may have existed formerly in sympatry with a related species, or other considerations might be involved. One would be the need for subtler signaling in closed social groups.
Yet there must be an upper limit to complexity. At some point it becomes neurologically too expensive to produce. It taxes the perceptual capacity of the receiver as well (Moles, 1963). Excessively complex acts of communication would also become unduly long (Moynihan, 1970) and would therefore be costly in time and energy. They might also become so conspicuous and so preoccupying as to make the performer vulnerable to prédation. Hence displays should generally be limited in their complexity. There is also reason to believe that the number of displays themselves is limited (Marler, 1959).
When species split off from one another and remain, or come in contact, the signals used in reproductive behavior should differ. Thus for each such MAP formerly in common there should now be two. This phenomenon attains central importance when it is realized that animals have a limited number of "major" displays (Moynihan, 1970). When new ones evolve old ones must be given up, whether through alteration or replacement. A methodological difficulty exists here, however. While Moynihan (1970) gave some criteria for deciding which MAPS are "major" displays, it is difficult to apply the criteria uniformly.
Radically different estimates of the number of signals can be obtained for the same species. The longer an investigator works with a species the more signals he recognizes (Altmann, 1968; Dane and Van der Kloot, 1965). And when a species is observed in captivity, as opposed to in the field, the number of "major" displays is apt to be underestimated (Moynihan, 1970).
Different observers can arrive at different numbers, especially if they have different views about what constitutes a unit of behavior. Altmann (1968) compared the classifications of vocal signals for the howler monkey (Alouatta palliata) contained in three different reports. Twenty different vocalizations were listed. Thirteen were reported by him (1959), ten by Carpenter (1934), and five by Collias and Southwick (1952). Only two signals were common to all three classifications, four appeared in two of them, and fourteen of the twenty were unique to one classification or another. The number of MAPS and vocalizations2 reported for rhesus monkeys also varies drastically (Altmann, 1965, 1968; Hinde and Rowell, 1962; Rowell and Hinde, 1962).
Doubtless primates present the greatest difficulties here, and better agreement could be had in studies of lower vertebrates. While I have made no tallies, I generally find good agreement between the reports of different authors on the behavior of the same species of teleost fish, making allowance for differences in terminology.
Moynihan (1970) is doubtless correct in his thesis that animals have relatively few "major" displays, probably ranging from about 10 to 37, as he says. Nonetheless there is room for argument about the precision of the estimates, as noted. A quick histogram plot of his data discloses that most animals have about 13 to 21 "major" displays with a mode around 19 to 21. Using the same data, Wilson (1972) portrayed the number of "major" displays in animal groups, showing a progressive increase from fishes (mean = 17), to birds (21), to mammals (24).
Birdwhistell (1970) cataloged 50 to 60 "kines" for human beings. It is difficult to compare these directly to number of "major" displays in other animals because kines are recombined into a vast number of kinemorphs. If comparably fine distinctions were made, most animals would be found to have many more kines than displays. In fact, the communicatory acts of rhesus monkeys, as judged by Altmann (1965), are more comparable to kines or kinemorphs than to displays. This might account for the large discrepancy in number of signals reported by Hinde and Rowell (1962) and Rowell and Hinde (1962), whose approach was more clearly etiological in conception.
I have made a number of points large and small in this article but the main ones are three. First, the highly patterned movements that I call MAPS are of central importance in animal behavior. They form the basis of almost all quantitative analyses, with ramifications extending into communication, development, neurophysiology, genetics, and the evolution of behavior. MAPS are a direct readout of the central nervous system. They have been valuable in understanding how the MAPS works, and this should continue. But the physiological counterpart of overt behavior can never be observed directly in the nervous system. Only by careful and appropriate quantification of behavior can we expect to construct models that will illuminate the working of the MAPS. The same applies to the analysis of communication. The very notion of quantification means ultimately that one must deal with units. That is why it is so important to understand the structure of MAPS. HOW stereotyped should they be to be considered unitary? If significantly variable, how can the variation best be quantified and incorporated into a model?
This brings us to the second point. In spite of the importance of MAPS we are poorly informed about their basic anatomy. Most of what has been written about MAPS has been based on presumptions because the MAPS are themselves so easily recognized. There is need for careful description of the infrastructure of MAPS. Most of this work will have to be done by means of film analysis. Electromyograms seem promising, though they are limited by the restraints of the methodology (e.g., Martin and Gans, 1972).
The third point is that we are almost as ignorant about the way MAPS act as stimuli. By the same token, little is known about the ways stimuli affect the performance of MAPS. Both points are important to understanding the role of MAPS in communication. The scientific method, of stating hypotheses and attempting to reject them, needs more exercise here.
This is not the place to enter into a discussion of experimental procedures, but it should be mentioned that the favored methodology of ethologists here has been the use of dummies, fake animals. These static models, no matter how useful in other situations (Leong, 1969), have proved a disappointment in the study of MAPSand their effects (Cullen, 1972; Gorman, 1968).
Three other approaches suggest themselves. One is to display motion pictures to the subjects. I have discussed film playback with many ethologists who have had only failures. A few investigators nonetheless, have had success (e.g., Jenssen, 1970; Turnbough and Lloyd, 1973). The difficulty may be that the individual pictures are perceived by the subject as disconnected images because the rate of advance lies below its critical fusion frequency (Jenssen, 1970).
Another technique is the use of animated models or simulations of essential moving features, as in the work of Magnus (1958), who produced a pattern resembling the beating wings of a female butterfly. The two chief deterrents are that only simple animals will respond to the motions of crude models, and models that convincingly mime MAPS are likely to be expensive technological achievements. Even then, one has no assurance they will work.
The third technique is electrical stimulation of the brain. Ideally, the wired-up animal can be forced to produce a faithful example of the desired MAP whenever a button is pressed. The ideal is not readily achieved. There are several reasons for this (Delius, 1973; Phillips and Youngren, 1971). While this technique is not an impossible one, its formidable difficulties account for its rare application to problems of communication (see also Cullen, 1972).
Whatever technique is employed, it will not be enough to deal with idealized MAPS. We have to push on, to explore the communicatory significance of variation in performance, intermediate MAPS, superposition, and more. Our thinking must not be dominated by digital models. Perhaps some MAPS, the more graded ones, act as analog senders. And of course the context will have to be brought in, for signals do not work in vacuums.
In closing I must return to a philosophical point. Is there any reality to MAPS or FAPS? I find it impossible to give an operational definition that is general enough to meet all contingencies. That which I have given is postulational. It relies on the animals showing a recognizable pattern of behavior, without a precise definition of what is meant by recognizable. Moreover, there may even be instances in which a population is polymorphic—a large portion showing one MAP, the other portion a different MAP.
The difficulty is that virtually all movements performed by animals are to some degree patterned. None are uniquely different from the highly patterned movements seen in displays. The concept of MAPS directs attention to the more highly patterned movements, movements that are often derived from more basic ones. The concept is therefore most easily applied to displays, MAPS that are adapted to serve in communication.
Obviously, however, there is a continuum. At one extreme is patterned behavior, like locomotion, which varies from moment to moment in harmony with the environment. At the other extreme are precisely patterned movements that are relatively independent of external modulation and may even fit the Lorenzian criteria of the Fixed Action Pattern. There is no boundary.
Yet the concept of MAP remains useful. Note the parallel in arbitrarily dividing the age of an organism into categories. When does a youth become an adult? Or when is an adult old? When is a motor pattern a MAP? When a movement has become ritualized into a display it is clearly a MAP. Where is the cutoff? I think it is too early and perhaps inadvisable to try to delimit rigidly the lower limit of MAPS. I would rather live with shades of grey and await future developments.
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1. It is difficult to apply a simpleconcept ofsisrs to grooming because of its hierarchical organization. The distinction between acts, mats, and sequences is blurred Each component is to some degree independent within the hierarchy, S'et there is so much patterning and close temporal linkage of components that they can be arranged into higher levels of organization that are reasonably stereotyped albeit stochastic. furthermore, the choice of units of grooming influences one's conception of its higher-order organization (M. W. Woolridge. pcrs. comm.).
2. In a strict sense vocalizations may be maps. In the present sense they are. I treat them as separate here only for convenience of exposition.