Equidae

Two distinct forms of social organization have evolved in the equids. Plains zebra (Equns quagga: Klingel, 1967), mountain zebra (E. zebra: Klingel, 1968, 1969a; Joubert, 1972), and horse (E. przewalskii: Bruemmer, 1967; Feist, 1971; Tyler, 1972) live in family groups consisting of one stallion, one or several mares, and their young, and in stallion groups. Both types of group are nonterritorial; they are characterized by their stability and coherence based on mutual bonds between the group members. This requires the capability of individual recognition of group members.

Grevy's zebra (E. grevyi: Klingel, 1969b, 1974), African wild ass (E. africanus: Klingel, in prep.), feral donkeys (Moehlman, 1974), and probably Asiatic wild ass (E. hemionus: Klingel, in prep.) live singly or in unstable anonymous groups of either one or both sexes. Some of the stallions are territorial and defend their huge territories only under certain conditions and only against their territorial neighbors, but are otherwise tolerant toward other male conspecifics. Only mares and foals know and recognize each other individually, and they are the only members of a population that establish mutual bonds that are maintained for some time.

SPECIES AND INDIVIDUAL RECOGNITION

In overlap areas, e.g., South West Africa (E.quagga and E. zebra) and in northern Kenya (E.quagga and E. grevyi) the members of the respective species tend to stay together in monospecific associations. Although they frequently use the same water holes, graze close together and even flee together, they do not contact individuals of the other species (Klingel, 1968, 1974). This indicates that species-specific differences are recognized. No information is available on how this is performed; it can, however, be assumed that knowledge of their own species' appearance is learned by the young through an imprinting process. On the other hand, in captivity it is possible to interbreed all the equine species. So obviously there are genus-specific characteristics which are recognized and which can act as adequate stimuli in abnormal situations. Individual recognition is established between a mare and her foal during the first few days after birth. According to Tyler (1972) the horse mare learns to know her foal, obviously by imprinting, when she licks it for twenty to thirty minutes after birth.

The foals have a following reaction as soon as they are able to walk. The imprinting of the foals on their mothers takes place after one to several days. It is enhanced by agonistic behavior of the mare toward all the other conspecifics during the first few days after birth (E. quagga: Walther, 1962; Klingel and Klingel, 1966a; Klingel, 1969c; E. grevyi: Klingel, 1972, 1974; E. przewal skiif. caballus: Tyler, 1972). In the nonterritorial equids the group members know each other individually by sight, voice, and smell (Klingel, 1967, 1968, 1969a, 1972; Tyler, 1972).

OPTICAL COMMUNICATION

The visual expressions of the equids are numerous. They consist of postures; movements of legs, tail, and head; and facial expressions effected by opening the mouth and by various positions and movements of the ears and lips.

Facial Expressions

The estrous mare, when approached and contacted by thestallion, shows the estrous face: she makes exaggerated chewing movements but without closing her mouth altogether; at the sametime her incisors are partly bared, the corners of her mouth arepulled back, and her ears are folded back (Antonius, 1940;Trumler, 1958, 1959a; Klingel, 1972). This expression is regularly observed in all the species with the exception of the horse, whereit occurs only rarely and only in young mares (Antonius, 1951)(Fig. 1).

A similar expression is the snapping (Zeeb, 1959) of young horses when they are threatened (Tyler, 1972). In snapping, the jaw movements are faster than in the estrous expression, and the teeth meet each time. Plains zebra, Grevy's zebra foals, and, rarely, adult plains zebra stallions make similar faces during greeting ceremonies with adult stallions; the position of the ears, however, is more variable. The significance of these three facial expressions is difficult to assess. They can all be considered submissive expressions, as they are always performed by the weaker partners. Tyler (1972), however, observed that young horses were frequently attacked and bitten or kicked in spite of having made the snapping response. Zeeb (1959) interprets the snapping as ritualized grooming movements, and Tyler (1972) observed that occasionally mutual grooming ensued from the snapping response (Fig. 2).

Fig. 1. Estrous face (donkey). Note position of ears.

Fig. 2. Snapping response: horse foal; plains zebra two-year-old stallion, with adult stallion.

The greeting expression (Antonius, 1940; Trumler,1959a; Klingel, 1967, 1968, 1972; Dobroruka,1961; Tyler, 1972)can be observed, when two adult partners, e.gtwostallions orastallion and a mare, meet and establish nasonasal contacts. Therepresentatives of all species that have beinvestigatedextendtheir heads, usually have their ears directforward, and drawthe corners of their mouths up in a jerking movement, except forthe horse, where this last movement rarely occurs (Tyler, 1972). Plains zebra often open their mouths and make chewing movements with bared teeth; mountain zebra and donkey chew with their lips closed; Grevy's zebra and horse do not move their jaws (Klingel,1972). Thisexpression obviously denotes friendliness (Fig. 3)

Fig. 3. Greeting expression (plains zebra). Note position of ears.

The threat expression (Antonius, 1937; Trumler, 1959a; Klingel, 1967, 1968, 1972; Tyler, 1972; Moehlman, 1974) is similar to the estrus expression, but without the chewing movements. There are several degrees: when threatening lightly, the ears are only slightly laid back; at higher intensities the ears are straight back, the head is lowered and swayed from side to side, and the mouth is partly opened. This expression occurs regularly before and during fights and in stallions when driving mares or other conspecifics. Its communicative significance is much more obvious than that of the previous expressions: it demonstrates the readiness to fight, especially to bite, and conspecifics accordingly move out of the way, flee, or get ready to meet the attack (Fig. 4).

The flehmen expression consists of the animal's lifting its head up high, curling back the upper lip to expose the teeth, and lowering the lower lip; the jaws remain closed (Schneider, 1930, 1934; Klingel, 1972; Trumler, 1959a; Zeeb, 1959; Tyler, 1972). It is usually a reaction to some smell, usually urine, dung, or female genitalia, and facilitates the entrance of odorants into the vomeronasal (Jacobson's) organ (Este,

Fig. 4. Threat expression: Grevy's zebra cf; horse $ (note position of ears and tail).

1972), In a few instances it could be observed that flehmen can act as a signal and elicit responses by conspecifics (Dobroruka, 1961; Klingel, 1974; Moehlman, 1974) (Fig. 5).

Other facial expressions with doubtful or no communicative significance are lip clapping in plains zebra (Klingel, 1972), yawning in all the species, and extension of the upper lip in feral ass males when approaching females (Moehlman, 1974).

Leg Movements

Equids kick during fights with both front and hind legs. Ritualized kicking serves as a visual threat and is carried out in various degrees. It ranges from only slightly lifting one hind foot from the ground at low intensity to actually kicking with one or both hind feet in the direction of a pursuer at high intensity. Stallions display ritualized kicking with their forelegs during breaks between fighting bouts (Klingel, 1967).

Tail Postures and Movements

The communicative significance of tail postures and/or movements has hardly been investigated. During flight young foals hold their tails straight up, and this releases flight behavior in other foals (E. przewalskii f. caballus: Tyler, 1972). Grevy's zebra stallions, when driving mares or other stallions, raise their tails (Fig. 4a); during copulation they swing them from side to side as do horse stallions (Klingel, 1972; Antonius, 1937).

Body Postures and Movements

During estrus mares of all the species stand with their hind legs apart and their tails raised at an angle of about 45°, especially when being courted by a stallion. This posture is most conspicuously displayed by young plains zebra (Klingel, 1967) and by horse mares (Tyler, 1972) even when there is no stallion in attendance. Stallions are attracted by this posture even from a distance, and in plains zebra this results in the abduction of the young mare from her maternal group. It could be proved that the posture and not the smell is the decisive stimulus: drugged plains zebras, under the influence of Etorphin, Acetylpromazine, and Hyoscine displayed the same posture, and they were courted and even mounted by stallions (Klingel, 1967; Klingel and Klingel, 1968). Horse stallions are also attracted by urinating mares, as their posture is similar to the estrous posture (Tyler, 1972).

In the territorial equids the territorial stallions are distinguishable from other males by a number ofbehaviorpatterns, which can also be considered an advertisement of theirstatus to conspecifics: their posture is generally upright; they keep some distance from groups of conspecifics in the territory, which makes them very visible; they drive stallions and mares; they mount mares without erection, prior to copulation.

During the last two displays territorial stallions roar. This underlines the possible function of these two displays as signals, although they primarily serve to demonstrate the stallions' dominance to the driven animals or to test the receptiveness of the mares, respectively (Klingel, 1972, 1974, and in prep.).

When migrating, adult plains zebra often move their heads up and down conspicuously. This gives the impression of exaggerated walking and could serve as a following stimulus for group members or other conspecifics (Klingel, 1972).

Fig. 5. Flehmen (horse

ACOUSTICAL COMMUNICATION

Two vocalizations seem to be more or less identical and to have the same significance in all species. Both are snorts, but they differ in their duration. The short snort is obviously a warning sound, emitted when the animals are disturbed. The long snort is an expression of well-being (e.g., when the animals are eating). Its communicative significance is doubtful.

Contact calls serve for maintaining or reestablishing contact between two partners: mare and foal in all species, group members in the nonterritorial species. In the territorial species the same type of call is used by the stallions to mark their territories as well as in other situations (Klingel, 1972, 1974, in prep.).

In E. quagga, the contact call consists of one to three-syllable barking sounds, which are produced during exhalation and which can be transcribed by "ha, haha, hahaha." The frequency is ten to eighteen sounds in five seconds. During inhalation the animals often produce an "i" sound. These calls are individually different and are assumed to effect individual recognition (Klingel, 1967).

In E. zebra the contact call is probably the long squeal, which is repeated several times (Blaine, 1922; Antonius, 1930, 1951; Klingel, 1968).

The contact call of E. przewalskii f. caballus is the whinny or neigh, which is individually different (Tyler, 1972; Ödberg, 1969; Kiley, 1972; Tembrock, 1965; Waring, 1971) (Fig. 6a).

In E. grevyi the contact call is an intermittent roar, emitted when exhaling; it is succeeded by an "i" sound during inhalation (Antonius, 1951; Klingel, 1969b, 1972, 1974).

The contact call of E. africanus is the bray, a most varied and complex vocalization composed of thirty frequency bands. It consists of two syllables and is highly variable in temporal patterning, frequency bands, intensity, and other characteristics (E. africanus f. asinus: Moehlman, 1974) (Fig. 7).

The call of E. hemionus is described as a monosyllabic "kiang" emitted in fast succession; it is somewhat similar to the whinny of the horse and the bray of the ass (Antonius, 1937).

The contact calls of the various species, which areso different to the human ear, seem to have some typical, genus-specific properties that are recognized and responded to by congenerics, who, however, do not react to calls of none quine species (Antonius, 1951). Ödberg (1969) changed the whinny of horses by replaying tapes 50 to 75 percent faster (and therefore higher) and 50 percent slower (and lower). In all cases his horses still recognized the sound as a whinny and reacted accordingly. The calls of mules and hinnies are intermediate between those of the horse and the ass (Antonius, 1934, 1950).

Fig. 6. Vocalizations of the horse: a. whinny; mare when concerned for the foal's well-being. All b. nicker emitted prior to feeding; c. nicker emitted by stallion during sexual behavior; d. nicker emitted by mare when concerned for the foal's well-being. All these sounds are variable. (Photo courtesy G. H. Waring.)

The remaining vocalizations of the equids are listed by species. Their communicative significance is not obvious in all the cases:

E. quagga: (1) dissyllabic "i-ha" (warning); (2) high-pitched squeal (pain, distress); (3) long squeaking (pain, distress in foals). (Klingel,1967.)

E. zebra: (1) short bark, usually repeated once (warning); (2) two-syllable "i-ho" (warning?); (3) high-pitched squeal (pain, but also emitted during greeting displays and play fights). (Klingel,1968.) Joubert (1972) describes a total of four vocalizations, but does not compare them with previous descriptions.

E. przewalskii: (1) short squeal, emitted by a mare when approached by a stallion'and when fighting (distress, pain?); (2) nicker, a low-pitched guttural sound (contact call, greeting call, alarm call: Fig. 6b, c, d), (3) roar, recorded from stallions when meeting a nonreceptive estrous mare; (4) grunt (sign of content?). (Kiley, 1972; Tyler, 1972; Ödberg, 1969; Hafez et al., 1962; Waring,1971.) Ödberg (1969) distinguished three basic types of vocalization from his sonograms: whinny, grunt, and squeal with variations and intermediates.

E. grevyi: (1) grunt (excitement). (Klingel, 1974.) E. africanus: (1) grunt; (2) growl, atonal vocalizations of different duration (agonistic context); (3) whuffle, atonal and of low intensity (approach, searching). (E. africanusf asinus: Moehl man, 1974.)

E. hemionus: (1) squeal (defense). (Antonius, 1937.)

Fig. 7. Donkey bray sequence, consisting of eleven exhalations. Note the large number of fre-

OLFACTORY COMMUNICATION

In equids communication by odors works only over short distances, when the animals engage in naso-nasal and naso-genital or other close contacts and when smelling urine and dung. Olfactory contact with conspecifics serves to effect individual recognition (in the territorial species probably only of mare and foal, in the nonterritorial species also of group members) and, with stallions, to test the reproductive state of the mares. The roles of circumanal, circumoral, and perineal glands are unknown (Schaffer, 1940). Equine dung and urine transmit information by scents to conspecifics. This is most pronounced in the reproductive context: the stallions smell the dung piles and urination spots, particularly those of estrous mares, and thereby receive the relevant messages. (They subsequently display the flehmen, which enhances olfactory perception, defecate or urinate on the dung, and urinate on the urination spots. The biological significance of this "marking" behavior remains obscure; Klingel, 1972.) Horse stallions and mares seem to recognize the dung of the group members by smelling, and use this capability when searching for their groups (Tyler, 1972).

In the territorial species, the territorial stallions mark their territories with dung piles, which are used for years and thereby reach considerable size. They do not serve to advertise the territories for other conspecifics, but obviously function as marks for the orientation of the territorial animal itself (E. grevyi: Klingel, 1969b, 1974; E. africanus f asinus: Moehlman, 1974; E. africanus somaliensis: Klingel, in prep.; E. hemionus: Klingel, in prep.).

Rhinocerotidae

The forest species Javan rhinoceros (Rhinoceros sondaicus) and Sumatran rhinoceros (Didermoceros sumatrensis) are solitary; the species of the more open habitats, Indian rhinoceros (Rhinoceros unicornis), black rhinoceros (Diceros bicornis), and to a higher degree, white rhinoceros (Ceratotherium simum) are often found in small, unstable groups (Schenkel and Lang, 1969). In C. simum the adult bulls are territorial, but tolerate one or more subordinate males in their territories (Owen-Smith, 1971). In D. bicornis the bulls live in well-defined, undefended, overlapping home ranges (Klingel and Klingel, 1966b; Schenkel, 1966; Goddard, 1966; Schenkel and SchenkelHulliger, 1969). A similar system seems to exist in R. unicornis (Schenkel and Lang, 1969), but Ullrich (1964) considers it to be territorial. The two remaining species are insufficiently known in this respect (Schenkel and Lang, 1969). In all the rhinoceros species the only permanent associations are those between a female and her young. In D. bicornis rather long-lasting associations between several adults and young have been recorded in the comparatively resident population of the Ngorongoro Crater (Klingel and Klingel, 1966b).

SPECIES AND INDIVIDUAL RECOGNITION

Rhinoceroses have poor eyesight. D. bicornis, for example, occasionally mistake a motorcar for a conspecific. The voices seem to play a certain role in intraspecific recognition, but as far as is known they are not sufficiently varied to be used for individual recognition (Schenkel and Lang, 1969). In an overlap area of C. simum and D. bicornis mixed grazing communities and interspecific play were observed (Steele, 1960), but such incidents occur only very rarely. They may be based on a "mistake," i.e., the members of the other species are taken for conspecifics. Dung piles are used by both species (Player and Feely, 1960).

OPTICAL COMMUNICATION

Facial expressions, consisting of different postures of the ears and opening of the mouth (including flehmen) seem to have no communicative significance, when one considers the limited visual abilities of rhinoceroses. The same is probably true for tail posture (for details see Schenkel and Lang, 1969; Schloeth, 1956; Goddard, 1966; Schenkel and Schenkel-Hulliger, 1969). The head-up posture and the fast charge toward an opponent obviously function as threat displays (Schenkel and Lang, 1969; Owen-Smith, 1971) (Fig. 8).

ACOUSTICAL COMMUNICATION

Several vocalizations have been described for rhinoceroses but no detailed investigations have yet been published. It is therefore only possible to compile the various quotations:

C. simum: (1) snorting in intraspecific combats; (2) grunting of males when courting; (3) trumpeting of males when being rejected; (4) long, rumbling bellow as a threat; (5) squealing when fleeing and of young when in distress; (6) bass bellow; (7) shrieks when fighting. (Player and Feely, 1960; Schenkel and Lang, 1969; Owen-Smith, 1971.)

Fig. 8. Ritualized fight in the white rhinoceros, consisting of fast charges. Note position of ears.

D. bicornis: (1) puffing snort when alarmed, also when charging; (2) low-pitched squeal of females during copulation; (3) growling, and (4) grunting when fighting; (5) shrill scream (whistle); (6) crying during courting activities, possibly a contact call of the young (when this vocalization is imitated by the experimenter it can be used to attract female rhinoceroses). (Ritchie, 1963; Goddard, 1966; Schenkel and Schenkel-Hul-liger, 1969; Klingel, pers. obs.) R. unicornis: (1) bleating of the young; (2) bleating of the females to attract their young; (3) snorting as threat; (4) short grunting as a warning sound when fleeing; (5) high-pitched squealing (whistling: Schenkel and Lang, 1969) of females when being courted; (6) extended grunting of males when driving males or females; (7) extended squealing of driven males; (8) purring as a contact call. (Ullrich, 1964; Lang, 1961.) R. sondaicus: (1) harsh blowing sound (snorting?) when disturbed; (2) grunting when attacking, also when wounded; (3) squealing (whistling), produced when inhaling and exhaling as a contact call; (4) blowing of young. (Schenkel and Lang, 1969; Sody, 1959.)

D. sumatrensis: (1) snorting when alarmed; (2) quacking when fleeing; (3) squeaking when feeding undisturbed; (4) "low and rather plaintive" noise when wallowing; (5) squealing when wounded. (Hubback, 1939.)

OLFACTORY COMMUNICATION

Dung and urine are used for communication in all the species. Preputial scent glands have been described in C. simum (Cave, 1966), but their function is unknown. No particular scent glands and no behavior related to such glands have been described. Cows in estrus attract bulls by scent; bulls are capable of following a scent spoor made by a cow.

In C. simum the territorial bulls mark their territories by exhibiting specialized techniques of defecation and urination. Before and after defecation they perform backwardly directed kicking movements with the hind legs, which result in breaking up and scattering the dung. Dung is deposited at particular places only, and thus twenty to thirty dung heaps per territory are maintained. Urine is distributed in the form of a fine spray in three to five spasmodic bursts over vegetation or the ground, impregnating the territory with the characteristic odor. Cows, nonterritorial bulls, and calves use the permanent dung piles, but in a nonritualized manner (Owen-Smith, 1971) (Fig. 9).

Fig. 9. Ritualized defecation in the white rhinoceros: above, defecation on permanent dung pile; below, spreading the dung with hind legs.

In D. bicornis ritualized defecation and urination are carried out by both sexes, but ritualized urination is carried out by cows only during estrus. This display serves to inform all the passing members of the population of the presence and physiological status of the marking individual and will accordingly result in spacing or attraction (Schenkel and Schenkel-Hulliger, 1969).

In R. unicornis, dung is placed on permanent dung piles, but only rarely is it scattered by kicking movements of the defecating animal. Only dominant bulls urinate in a ritualized manner. During pre-mating activities both partners repeatedly display ritualized urination and thereby mark the mating area (Schenkel and Lang, 1969).

R. sondaicus defecate in an unritualized manner, i.e., without kicking movements, either on dung piles or in wallows. In the latter case the smell will be attached to the skin of the animal and from there to the tracks and vegetation. Urine is squirted onto the vegetation while the animals are walking. They also urinate when wallowing, thus impregnating themselves with the scent as well (Schenkel and Schenkel-Hulliger, 1969).

D. sumatrensis defecate into water (wallows) as well as on land, but normally seem to use permanent dung piles. However, in denser populations than now exist dung piles have been observed, and there may therefore be no qualitative difference between this species and the others. Bulls urinate in a ritualized manner, and the same has been observed in captive cows (Hubback, 1939; Strickland, 1967; Schenkel and Lang, 1969). Bulls rub their horns against trees, thereby wearing off the bark. This is frequently accompanied by pawing up the earth and sprinkling the surrounding vegetation with urine (Hubback, 1939).

Tapiridae

Very little is known of the life habits in the wild of this group; and only two species, the lowland tapir (Tapirus terrestris) and the Malayan tapir (T. indicus), have been studied in some detail in captivity (von Richter, 1966; Schneider, 1936). The two remaining species are the Central American tapir (T. bairdi) and the mountain tapir (T. pinchaque).

In the tapirs the only stable association is that of a female and her young. Even when several individuals are kept together in captivity they do not seem to pay any attention to each other, except when a female is in heat. The social organization and behavior of free-ranging tapirs are still unknown. They can be assumed to be either territorial or to space in large, but undefended, home ranges.

SPECIES AND INDIVIDUAL RECOGNITION

Little information is available on this subject. It can be assumed that a female and her young recognize each other mainly by smell. Hunsaker and Hahn (1965) consider the clicking noise as an aid to species identification (see below).

OPTICAL COMMUNICATION

In all the tapir species the tips or edges of the ears are conspicuously white and may enhance the perceptibility of ear positions and movements (von Richter, 1966). Only a few facial expressions can be distinguished, and they can only rarely be observed. The threat expression is similar to that of the Equidae: the ears are held back, and the lips are opened so that the teeth become visible (canine in the lower jaw, canini form incisor in the upper jaw, both sexes). Sometimes a fight ensues (von Richter, 1966). Flehmen and yawning do not seem to have a communicative significance.

ACOUSTICAL COMMUNICATION

In a detailed investigation of vocalization in T. terrestris four sounds were recorded (Hunsaker and Hahn, 1965). (The four sounds described by von Richter, 1966, in T. terrestris and T. indicus are obviously the same ones.) (1) Shrill, fluctuâting squeal as a response to fear and pain, but also when fleeing and in appeasement behavior; present in both sexes and all ages. It could serve as a warning call. (2) A sliding squeal is utilized in exploratory activities and is assumed to advertise the presence of the animals. This call is mainly uttered by the dominant individuals. It probably functions as a contact call and can be used by humans to attract tapirs in the wild. (3) A clicking noise observed in young and adult animals during exploratory behavior and when two conspecifics approach each other. In the majority of cases two clicks are produced at an interval of 0.125 sec. (4) A snort with a duration of 0.25 sec was recorded when the animals were threatening or charging.

OLFACTORY COMMUNICATION

T. indicus and T. terrestris in captivity were always found to defecate on the same spot. Defecation is often ritualized, as the animals scrape the ground with their hind legs, but usually without touching their dung. Whenever possible, the tapirs defecate when wallowing or bathing (von Richter, 1966; Krieg, 1948; Kuelhorn, 1955). Urination takes place in the water, and, in a ritualized manner, on the land. Males squirt urine onto certain marking spots; both males and females do so during pre-mating activities and in other situations of excitement. Urine and, to some extent, dung seem to be used by tapirs to mark their home ranges and, in females, to inform conspecifics of their physiological state.

Naso-genital contacts occur during precopulatory display; they most certainly have a communicative significance (von Richter, 1966).

References

Antonius, O., 1930. Beobachtungen an Einhufern in Schönbrunn, V. Bergzebras, Grevyzebras und Zebroide. Der Zoolog. Garten (n.F.) 2:261-74. Antonius, O., 1934. Beobachtungen an Einhufern in Schönbrunn, X. Zebroid und Maulesel, XI. Über den zweiten Schönbrunner Maulesel. Der Zoolog. Garten (n.F.) 7:165-79.

Antonius, O., 1937. Über Herdenbildung und Paarungseigentümlichkeiten der Einhufer. Z. Tierpsychol., 1:259-89.

Antonius, O., 1940. Über Symbolhandlungen und Verwandtes bei Säugetieren. Z. Tierpsychol1, 3:263-78.

Antonius, O., 1950. Fruchtbare Maultiernachzucht. Zoolog. Anzeiger, Erg. Bd., 145:28-33.

Antonius,O.,1951. Die Tigerpferde. Frankfurt/Main: Schöps.

Blaine, G., 1922. Notes on the zebras and some antelopes of Angola. Proc. Zool. Soc. London, 22:317-39.

Bruemmer, F., 1967. The wild horses of Sable Island. Animals, 10:14-17.

Cave, A.J. E., 1966. The preputial glands ofCerato therium. Mammalia, 30:153-59.

Dobroruka, L. J., 1961. Eine Verhaltensstudie des Przewalski-Urwildpferdes (Equus przewalskii Poliakow 1881) in dem Zoologischen Garten Prag. Equus, 1:89-104.

Estes, R. D., 1972. The role of the vomeronasal organ in mammalian reproduction. Mammalia, 36:315-41.

Feist, J. D., 1971. Behaviour of feral horses in the Pryor Mountain Wild Horse Range. M.Sc. thesis, University of Michigan.

Goddard, J., 1966. Mating and courtship of the black rhinoceros ( Diceros bicornis L.). East African Wildlife J., 4:69-75.

Hafez, E. S. E.; Williams, M.; and Wierzbowski, S; 1962. The behaviour of horses. In: The Behaviour of Domestic Animals, E. S. E. Hafez, ed. London: Bailliere, Tindall 8c Cassell, pp.370-96.

Hubback, T., 1939. The Asiatic two-horned rhinoceros. J. of Mammology, 20 (l):l-20.

Hunsaker, D. II, and Hahn, T. C., 1965. Vocalization of the South American tapir, Tapirus terrestris. Animal Behaviour, 13:69-74.

Joubert, E., 1972. Tooth development and age determination in the Hartmann zebra, Equus zebra hartmannae. Madoqua, ser. 1, no. 6:5-16.

Kiley, M., 1972. The vocalizations of ungulates, their causation and function. Z. Tierpsychol., 31:171-222.

Klingel, H. 1967. Soziale Organisation und Verhalten freilebender Steppenzebras, Equus quagga. Z. Tierpsychol., 24:580-624.

Klingel, H., 1968. Soziale Organisation und Verhaltensweisen von Hartmann-und Bergzebras (Equus zebra hartmannae und E. z. zebra). Z. Tierpsychol., 25:76-88.

Klingel, H., 1969a. Dauerhafte Sozialverbände beim Bergzebra. Z. TierpsychoL, 26:965-66.

Klingel, H., 1969b. Zur Soziologie der Grevyzebras. Verh. d. dtsch. Zool. Ges., Würzburg; Zool. Anzeiger, Suppl. Bd., 33:311-16.

Klingel, H. 1969c. Reproduction in the plains zebra, Equus burchelli boehmi: behaviour and ecological factors. J. Reproduction and Fertility, Suppl. 6:339-45.

Klingel, H., 1972. Das Verhalten der Pferde (Equidae). Handbuch der Zoologie, 8, 10 (24): 1-68.

Klingel, H., 1974. Soziale Organisation und Verhalten des Grevy-Zebras (Equus grevyi). Z. Tierpsychol., 36:37-70.

Klingel, H., Social organisation, behavior, and ecology of African and Asiatic Wild Asses (in prep.).

Klingel, H., and Klingel, U., 1966a. Die Geburt eines Zebras (Equus quagga boehmi Matschie). Z. Tierpsychol., 23:72-76.

Klingel, H., and Klingel, U., 1966b. The rhinoceroses of Ngorongoro Crater. Oryx, 8, 5:302-306.

Klingel, H., and Klingel, U., 1968. Equus quagga: Paarungsverhalten. Encyclopaedia cinematographica, E 1044 Publ. Wiss. Film, Göttingen, A 11:461-66.

Krieg, H., 1948. Zwischen Anden und Atlantik. Munich: Hanser. 492pp.

Kuelhorn, F., 1955. Säugetierkundliche Mitteilungen aus Süd-Mattograsso. 3. Teil. Säugetierkundl. Mitteilungen, 3:77-82.

Lang, E. M., 1961. Beobachtungen am indischen Panzernashorn (Rhinoceros unicornis). Zoolog. Garten (n.F.), 25:369-409.

Moehlman, P., 1974. Behaviour and ecology of feral asses (Equus asinus). Ph.D. diss., University of Wisconsin.

Ödberg, F. O., 1969. Bijdrage tot de Studie der Ge-dragingen van het Paard (Equus caballus L.). Lie. thesis, Ghent.

Owen-Smith, N., 1971. Territoriality in the white rhinoceros (Ceratotherium simum) Burchell. Nature, 231:294-96.

Player, I. C., and Feely, J. M., 1960. A preliminary report on the square-lipped rhinoceros (Ceratotherium simum simum). The Lammergeyer, 1:3-24.

Ritchie, A. T. A., 1963. The black rhinoceros (Diceros bicornis). East Afr. Wildl. ]., 1:54-62.

Richter, W. von, 1966. Untersuchungen über angeborene Verhaltensweisen des Schabrackentapirs (Tapirus indicus) und des Flachlandtapirs (Tapirus terrestris). Zoologische Beiträge, 12:67-159.

Schaffer, J., 1940. Die Hautdrüsenorgane der Säugetiere. Berlin-Wien: Urban und Schwarzenberg.

Schenkel, R., 1966. Zum Problem der Territorialität und des Markierens bei Säugern am Beispiel des Schwarzen Nashorns und des Löwen. Z. Tierpsychol., 23:593-626.

Schenkel, R., and Schenkel-Hulliger, L., 1969. Ecology and behaviour of the black rhinoceros. In: Mammalia depicta, WolfHerre, ed. Hamburg, Berlin: Parey.

Schenkel, R., and Lang, E. M., 1969. Das Verhalten der Nashörner. Handbuch der Zoologie, 10 (25): 1-56.

Schloeth, R., 1956. Zur Psychologie der Begegnung zwischen Tieren. Behaviour, 10:1-80.

Schneider, K. M., 1930. Das Flehmen, I. Der Zoolog. Garten (n.F.), 3:183-98.

Schneider, K. M., 1934. Das Flehmen, V. Der Zoolog. Garten (n.F.), 7:182-201.

Schneider, K. M., 1936. Zur Fortpflanzung, Aufzucht und Jugendentwicklung des Schabrackentapirs. Der Zoolog. Garten (n.F.), 8:83-99.

Sody, H. J. V., 1959. Das Javanische Nashorn (Rhinoceros sondaicus) historisch und biologisch. Z. Säugetierkde., 24:109-240.

Steele, N. A., 1960. Meeting of rhinos of two species. The Lammergeyer, 1:40-41.

Strickland, D. L., 1967. Ecology of the rhinoceros in Malaya. Malay Nat. J., 20:1-17.

Tembrock, G., 1965. Untersuchungen zur intraspezifischen Variabilität von Lautäusserungen bei Säugetieren. Z. Säugetier künde, 30:257-73.

Tembrock, G., 1968. Perissodactyla. In: Animal Communication, T. A. Sebeok, ed. Bloomington: Indiana University Press, pp.377-82.

Trumler, E., 1958. Beobachtungen an den Böhmzebras des "George-v.-Opel-Freigeheges für Tierforschung e.V." Kronberg im Taunus, 1. Das Paarungsverhalten. Säugetierkundliche Mitteilungen, 6 (Sonderheft): 1-48.

Trumler, E., 1959a. Das "Rossigkeitsgesicht" und ähnliches Ausdrucksverhalten bei Einhufern. Z. Tierpsychol. 16:478-88.

Trumler, E., 1959b. Beobachtungen an den Böhmzebras des "George-v.-Opel-Freigeheges für Tierforschung e.V." Kronberg im Taunus, 2. Die Hautpflege. Säugetierkundliche Mitteilungen, 7 (Sonderheft): 104-25.

Tyler, S. J., 1972. The behaviour and social organisation of the new forest ponies. Animal Behaviour Monographs, 5, 2:87-196.

Ullrich, W., 1964. Zur Biologie der Panzernashörner (Rhinoceros unicornis) in Assam. Zoolog. Garten (n.F.), 28:225-50.

Waither, F., 1962. Beobachtungen über die Mutter-Kind-Beziehungen und die Rolle des Hengstes bei der Aufzucht eines Fohlens von Equus quagga boehmi. JB. George-v.-Opel-Freigehege Tierforsch3:44-51.

Waring, G. H., 1971. Sounds of the horse(E. caballus) . A.I.B.S. Sept. 1971. Paper read at meeting of Ecol. Soc. of America. Colorado State University, Fort Collins, Colorado.

Zeeb, K., 1959. Die "Unterlegenheitsgebärde" des noch nicht ausgewachsenen Pferdes (Equus caballus). Z. Tierpsychol., 16:489-96.