Tribe Neotragini – Dwarf Antelopes

Home , Antilopini, Beira (antelope), Klipspringer, Neotragini, Neotragus, Oribi

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Module # 1 – Component # 3

TRIBAL TRAITS

The smallest antelopes, ranging from 20 cm tall, 1.5 kg royal antelope to the 67 cm oribi and 26 kg beira. Hare-like, with hindlegs much longer than forelegs (Neotragus, Madoqua), to gazelle-like conformation (oribi, steenbok). Narrow muzzle and incisor row, muffle bare and moist (most species) or hairy, with slit-like nostrils (dik-diks, beira); ears medium or large (steenbok, dik-diks, klipspringer, oribi); tail rudimentary to short (3.5 - 13 cm).

Horns: in males only (except one klipspringer race), short (2 -19 cm), sharp spikes, ringed at least basally (except Raphicerus), slanted backward (Neotragus) as in duikers, or more upstanding (steenbok, oribi).

Coloration: pale gray to dark brown, cryptic with generally inconspicuous markings apart from white rump patch or under-tail in oribi and Neotragus species.

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Mammae: 4 (2 in beira).

Scent Glands : the distribution of scent glands in the different genera, as presently known, is shown below. Out of 7 different sets of glands, the oribi possesses 6. However, some species and even subspecies may not have all the glands their congeners have: thus, preorbital glands may he undeveloped in N. batesi, and the southern race of Kirk's dik-dik lacks hoof glands.

Preorbital glands

Are exceptionally developed in the Neotragini, so big in the male oribi, suni, klipspringer, and dik-diks that the preorbital depressions in the skulls are about the size of the eye sockets. Dik-dik and klipspringer females are known to deposit preorbital secretions; the glands may be inactive in the female steenbok, oribi, and possibly other species, although the skulls of female suni, royal antelope, and steenbok I examined had distinct preorbital fossae (the depressions were absent or poorly developed in immature individuals of either sex – author's unpub. observ.). The glands are round or oval-shaped bodies that discharge a black, sticky secretion through a central duct, situated typically in a patch of bare dark skin. At rest the duct is concealed beneath a purse-like fold of skin which is opened wide preparatory to depositing secretion. Neotragus species have simpler glands that lack the skin fold; the preorbital glands are otherwise quite similar in both the Neotragini and Antilopini.

When a dik-dik preorbital gland is longitudinally sectioned, it is seen to consist of a completely black middle layer surrounded by a white layer (fig. 4.1).

The black layer consists of polygonal cells whose cytoplasm is packed with granules of melanin (black pigment). The glandular tissue, composed of strongly branched holocrine glands, produces 2 different kinds of secretion: larger drops containing melanin and whitish, transparent, fatty droplets rich in lipoids. The white layer contains strongly coiled, unbranched apocrine glands which produce a clear

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secretion that exits via wide, winding ducts into the middle, pigmented layer. All the secretions eventually empty into and exude from the central duct as a black, sticky compound.

DISTRIBUTION

In contrast to the duikers, which are much alike, this tribe includes such a variety of different types that taxonomists have divided the 13 or 14 different species into at least 6 different genera. This diversity reflects adaptations for very different niches:

?? lowland rain forest (royal and pygmy antelopes) ?? forest savanna mosaic and thornbush of eastern Africa from Somalia to Natal (suni); savannas (oribi, steenbok) ?? arid bushland of the Somali-Masai and South West Arid Zones (dik-diks) ?? cliffs (klipspringer); rocky hills and mountains of North East Africa (beira) ?? rolling, rocky terrain with dense cover (grysboks).

Apart from the 2 rain forest species, the others are all specialized for arid environments (more under Ecology).

RELATIVES

The neotragines are generally classified with the gazelles as a separate subfamily, the Antilopinae, on the basis of various anatomical and behavioural similarities among some of the species. In the following list of shared traits, those marked with an asterisk are largely limited to these two tribes and tend to bear out their relatedness.

Anatomical and physiological similarities: morphology of the preorbital glands; presence of carpal and inguinal glands in oribi (not other Neotragini), which also looks somewhat gazelle-like; structural plan of the feet and pedal glands; skull structure, so similar in dik-diks and small gazelles that young gazelle and adult dik- dik skulls may be confused; similar adaptations for arid environments.

Behavioural similarities

Linked urination/defecation, often preceded by pawing (territorial advertising); stotting gait; foreleg-lifting in courtship; and mounting without grasping or resting weight on female.

The unstarred shared traits are more widespread, such as inguinal glands, foreleg- lifting, and stotting. The Neotragini also have in common with other tribes and not with Antilopini certain morphological traits, such as the dik-dik's forehead crest, which otherwise occurs only in the duikers, and the preputial gland, which also occurs in the rhebok.

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ANCESTRY

Only forms very closely related or identical to existing species, dating from the Pliocene, have been found as fossils. Nevertheless, according to one recent theory, the dwarf antelopes represent the most conservative and ancient line of bovids, from which all the other tribes except the subfamily Bovinae subsequently evolved. Sharp's grysbok and the beira, both seemingly relics of an earlier neotragine radiation, may be closest to the ancestral form, while the pygmy Neotragus species are thought to have invaded the forest after the duiker radiation, to fill a still vacant niche for an understory folivore.

ECOLOGY

Because of its relatively large surface area, a small antelope is more vulnerable to desiccation and temperature extremes than a big one. To live in hot, waterless lands, dwarf antelopes need to avoid overheating with the least possible loss of precious water – that is, they cannot afford to sweat. Evaporative cooling of the blood by nasal panting, an efficient mechanism for lowering body temperature employed by most plains antelopes, has been developed to an extraordinary degree by the dik-diks (see species account).

DIET

Dwarf antelopes are all concentrate selectors, except for the grazing oribi; they eat mainly foliage, shoots, herbs, seeds, pods, and some fruits. With their very narrow muzzles and incisor rows, coupled with small size, they can select the most nutritious available growth and glean a living within a minimal space on resources too sparse to support larger browsers.

SOCIAL ORGANIZATION

The dik-diks, klipspringer, and oribi are known to live, and the suni almost certainly lives, in monogamous couples, together with 1 or 2 immature offspring, within territories varying in size (both within and between species) from under 10 ha up to 1 km2. From 10 % to 20% of klipspringer and oribi families include 2 (very rarely 3 or more) females. The identity and relationship of the extras remain unstudied; they are most likely daughters that have failed to emigrate during adolescence. Although males are invariably more actively territorial, females participate in territorial advertising and behave aggressively toward female intruders. A dunging ceremony (see figs. 4.3, 4.5), performed by the couple and their offspring, usually on established dung middens, serves to maintain pair and family bonds (see species accounts).

Alarm-whistling in duet apparently serves the same function in the klipspringer and perhaps dik-diks. It is comparable to the duetting of many pair-bonded birds. Social

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grooming, on the other hand, so important in many other monogamous mammals and birds, is comparatively infrequent (except perhaps in the suni).

The steenbok and grysboks are more often seen alone than in pairs, and it is possible that the sexes remain separate most of the time. It seems more likely, however, that these species also live in monogamous but less-cohesive pairs than some other neotragines. It is already clear that such gradations exist, for dik-dik and klipspringer pairs keep much closer company than oribi pairs. The pygmy and presumably the royal antelope, for their part, are essentially solitary, like most folivores. The male N. batesi defends a territory of 2 - 4 ha which usually includes the ranges of at least 2 females. The females maintain spacing of 150 – 190 m; sometimes two, presumed to be mother and daughter, share the same range and may stay within 50 –100 m of one another.

ACTIVITY

Concentrate selectors generally feed and ruminate in alternating, fairly short bouts compared to roughage feeders, and forage actively day and night. Activity reaches a peak in early morning and late afternoon, as usual, continues at night up to c. 22h00, and is followed by another bout beginning at 04h00. The steenbok, grysboks, and suni appear to be more active by night, or it may be simply that they only venture into the open during darkness, as proved to be the case in the dwarf antelope.

POSTURES AND LOCOMOTION.

Neotragus species and dik-diks have remarkably long hindlegs, which are kept flexed with the femur and tibia folded up to the body, and the metatarsals, as long as the forelimbs and pencil-thin, extend to the ground, bearing the weight of the hindquarters upon the braced hocks. When the hindquarters are fully extended, as when stretching, the rump rises far above the shoulders (fig. 4.2). Presumably this arrangement is adapted to jackrabbit starts, long leaps, and quick turns. A pet royal antelope standing 26 cm at the shoulder could jump the 55 cm sides of its box and make 3 m horizontal leaps. The klipspringer has specialized hooves and peculiar conformation (see species account). The steenbok and oribi, which live in the most open habitats, have more evenly proportioned limbs, which may endow them with more speed and endurance than the rest.

SOCIAL BEHAVIOUR: COMMUNICATION

The variety and size of scent glands in this tribe (table 4.1), and the time put into distributing preorbital-gland secretions and droppings, suggest that olfactory signalling is of primary importance in neotragine communication. This is undoubtedly true for antelopes living in dense cover. However, for those that live in more open habitats, notably the dik-diks, klipspringer, and oribi, visual signals are important, especially the alert stance and alarm flight, also postures associated with territorial marking.

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White rumps accentuate alarm flight in the steenbok and oribi, and the oribi also stotts. Even the suni, which lives in dense cover, has a white under-tail, the wagging of which often draws attention to animals that would otherwise remain unseen. Nevertheless, acoustic signals may be more important in anti-predator behaviour than visual displays, certainly at night or in dense cover. A reedy whistle is characteristic of the oribi, klipspringer, and dik-diks; the dik-dik actually has a structure resembling a whistle in its nose (see species account).

TERRITORIAL BEHAVIOUR

Preorbital-gland marking, linked urination/defecation and dunging ceremony, prolonged, loud alarm calls (including klipspringer duets); static-optic display (e.g., klipspringer standing in silhouette).

In all species the male plays the main role in marking and defending the territory. Only dik-dik and klipspringer females are known to make preorbital deposits. However, linked urination/defecation (fig. 4.3), a territorial display limited to males (as is territoriality) in the gazelle tribe, is also performed by female and even by juvenile neotragines in a regular dunging ceremony, although differences in urination posture (males straddle, females squat) distinguish the sexes.

The male's performance seems clearly to function in territorial advertising, and does not depend on the female's presence. Whether the female's performance has any separate territorial significance is unclear, but it does trigger the pair or family dunging ceremony.

The male's tendency to approach and test the output of a urinating female is nearly universal in ungulates. But in the Neotragini the male adds his own excrement to that of his mate and offspring, often in conjunction with vigorous pawing and preorbital- gland marking. The whole ceremony follows a set pattern, which varies slightly between species, and typically takes place at territorial boundaries on established dung middens. Whatever reproductive and social-bonding functions it may serve, the neotragine dunging ceremony therefore clearly includes territorial advertising.

AGONISTIC BEHAVIOUR

Dominance/Threat Displays: erect posture; object-horning; medial-horn presentation; jabbing motions, grinding teeth (reported in suni).

Defensive/Submissive Displays: headlow/chin-out posture; udder-seeking (mainly by adolescent offspring, which sometimes approach and poke groin area in response to paternal aggression); lying-out (as when hiding).

Fighting: chasing, air-cushion fighting; horn-contact (on knees: suni); stabbing to body (mainly during chases).

Serious fighting is unusual in the whole tribe, perhaps because the danger of being wounded by their stiletto-like horns selects strongly for males that go no further than displaying. This is illustrated by the oribi, in which the hornless females butt heads but males, even when fighting mad, alternately attack and flee from one another and only rarely make contact. Dik-diks go through the motions of butting while maintaining an air cushion. Aggression is most common between father and adolescent son, and toward intruders into a territory.

REPRODUCTION

Extended or perennial breeding with or without definable peaks, gestation 5 - 7 months, postpartum oestrus, up to 2 offspring a year (dik-diks, steenbok, perhaps suni and others).

SEXUAL BEHAVIOUR

Approach in lowstretch, close following and persistent genital licking, foreleg-lifting, nose-to-nose greeting with or without grooming (nibbling, licking); mounting with forelegs folded, without resting on or clasping female, neck forward (dik-diks) or partially raised (oribi). Receptive females respond to genital licking by raising the tail and squatting slightly (like a calf licked anogenitally by its mother). (See species accounts for details and variations.)

PARENT/OFFSPRING BEHAVIOUR

Birth, care, and development of neotragine offspring are apparently typical of hider species. The gestation period of 5 - 7 months seems long for such small antelopes and could turn out to be shorter in at least some species. The few descriptions of birth and early development (based mainly on observations in captivity) suggest that calves are quite precocious (see oribi account), yet they do not regularly accompany the mother for 2 or 3 months. Fathers show parental care by exercising vigilance against predators, while their mates spend extra time feeding in order to sustain lactation (see klipspringer account). It is also possible, though unproven, that fathers actively intervene in response to a calf's distress bleating. Mothers give faint calls to summon calves from hiding and suckle them.

Weaning takes place by about 3 months, or around the time calves begin foraging with their mothers. Offspring become adolescent by 6 months in small species, by 10 months in the larger ones, and are adult-sized at 1 year. However, the horns may develop more slowly: klipspringer and dik-dik horns only reach full size at 1 and a half and 2 years, respectively. Harassment of male offspring may coincide with the appearance of horns – at 5 - 6 months in the klipspringer. The horns emerge at 2 months in dik-diks, but are initially concealed by the crest. Male offspring are forced to emigrate earlier than females, some of which may stay at home and even breed with their fathers (see klipspringer and oribi accounts).

ANTIPREDATOR BEHAVIOUR

Lying prone, skulking into or beneath cover, standing watch in alert posture, whistling snorts, stotting, rocking-horse run (klipspringer, oribi), sudden flight and dodging run, distress bleat. (Details in species accounts.)

SOURCES

Dragesco, Feer & Genermont 1979, Feer 1979, Feer 1982, Gentry 1978, Hofmann 1973, Kingdon 1982, O'Regan 1984, Owen 1973, Pocock 1910, Pocock 1918, Richter 1971, Tinley 1969, Walther 1984, Yalden 1978.