Antilopine Antelopes: Dwarf Antelopes of Eastern and Central Africa

Oribi

The term "dwarf antelope" can reasonably be applied to many different kinds of antelope that are smaller than, say a typical goat. In this common sense, it's not a precise term, and could refer to species that belong to quite different branches of the antelope family tree. When mammalian zoologists used the term in the 20th century, however, it was often more precise, referring to those thought to be most closely related to the gazelles as a branch within the "antilopine" subfamily of "typical" antelopes.

Technically, these dwarf antelopes were collectively referred to as "neotragines" and assumed to be a natural grouping within the antelopes more widely. Genetic analysis over the last couple of decades has muddied these waters considerably, not least because the genus for which the branch was named, Neotragus, turns out not to be closely related to the gazelles, and is something else entirely. Even if we look solely at those dwarf antelopes that we can still say belong to the antilopine subfamily, it turns out that they don't have a single common ancestor. Specifically, one of them is more closely related to gazelles than it is to any of the other members of its purported evolutionary branch. 

The upshot of which is that the term "dwarf antelope" now has little scientific meaning and, if "neotragine" means anything at all, it isn't what we previously thought. We're arguably just back to the common sense of the term: a dwarf antelope is any antelope that we think of as small.

The species that turned out to be in the gazelle half of the antilopine family tree is the oribi (Ourebia ourebi). This was first described by Eberhardt von Zimmerman way back in 1783, who stated that it lived in the vicinity of the Cape of Good Hope. They are no longer found so far south, and are now considered rare even in the more northerly parts of South Africa. However, they are widely distributed elsewhere across the continent, being found in much of the East African savannah and across the broad strips of land that separate the central jungles from the Sahara in the north and the Kalahari in the south. 

It's easy to see why they were thought to be closer to the other "dwarf antelopes", such as steenbok, than they were to gazelles, since they certainly look a lot like the former. Before the term lost most of its meaning, they were, however, considered the largest of the dwarf antelopes, albeit not by much. They stand 51 to 64 cm (1'8" to 2'1") high at the shoulder, and weigh 7.5 to 17.5 kg (16½ to 38½ lbs), with the males being larger than the females. Their colour is usually reddish or ochre, but some populations around Mali, Guinea, and Senegal are much greyer. This, and other variations in colour have previously been used (together with other features of size and form) to raise some of the subspecies to full species level, but this doesn't currently seem to be widely accepted.

The horns, present only in the male, are simple spikes about 12cm (5 inches) in length when fully grown, with rings around the base, but otherwise smooth. The scent glands in front of the eyes are also much larger in males, secreting a black liquid. An unusual feature is the presence of a patch of hairless skin below the ears in both sexes; although something similar is also seen in some non-antilopine antelopes, it is unique in this particular subfamily. Surprisingly, there is no evidence of scent glands within the patch, so its function is uncertain - conceivably, it might just help the animals identify other members of their own species.

Oribi live in areas of tall grassland with scattered trees, preferring floodplains and other relatively fertile patches, where the grass is likely to be lushest. At least in the Serengeti, they have been observed sticking to rocky areas in the dry season and moving to woodland patches of dense grass in the wet season. This is probably because the former provides better cover from predators in the dry season when the grass is not so thick and high. There is some variation in this, however, with those few remaining in South Africa preferring more open plains dominated by red grass, rather than the light woodland favoured elsewhere.

Their diet seems to be heavily dependent on what's available, which, given where they live, tends to be mostly grass. Red grass, gamba grass and dropseed seem to be favoured foods, but they will certainly eat whatever leaves and flowering plants may happen to be around, especially during times of the year when grass is less nutritious. 

Although they do sometimes gather in temporary groups, for the most part, oribi either live alone or in pairs. While this might imply monogamy, this is only the case when the population density is low, with oribi being flexible enough to be polygynous when the opportunity presents itself. In most places, where food is plentiful enough for several oribi to live close to one another, males defend territories that overlap with those of three or four females, and any pairings that do exist rarely last for long. Males mark their territories with dung and urine, and warn off potential rivals by scraping the ground or thrashing vegetation with their horns. If this fails to work, they will aggressively lower their neck, pointing their horns towards their perceived foe. This rarely escalates to an actual fight (although the hornless females have been known to headbutt each other) with one side running away as soon as the other darts forward.

Unusually, the males defending territories may share them with subordinates - usually, but not necessarily, their own sons. This apparently allows them to gang up on rivals, with the younger males hoping to get in some mating opportunities in exchange for their help in defence. Like some related species, male oribi mate standing upright on their hind legs. Pregnancy lasts six or seven months, and the young oribi spends the first month of its life hiding by lying still in long grass.

Beira

Despite belonging to the other half of the subfamily, the beira (Dorcatragus megalotis) is superficially similar to the oribi, albeit slightly smaller, at about the size of a steenbok. They too, are reddish brown in colour, with slender limbs and necks, large ears, and (for their size) long horns. There is even a hint of black flank-stripe seen in gazelles, although not quite as clear. They were only identified as a distinct species in 1894, and have been comparatively little studied ever since. In modern times, it doesn't help that virtually the entire population lives in Somalia, one of the world's most wartorn countries. 

In 1993, a tiny population was discovered just across the border in Djibouti, but the latest estimate places that at less than 100 individuals. Some may also exist in Ethiopia but, again, only very close to the Somali border and in areas increasingly being used for goat farming; no recent survey has been conducted there, so it's unclear whether any survive in that country.

Like most gazelles, but unlike the oribi, beira are true desert-dwelling animal, living in a part of the world that sees very little rainfall. While it may not live on sandy dunes, it does favour arid rocky slopes and plateaus where the primary vegetation consists of thornbushes... which, unsurprisingly, are their primary food source. Perhaps because of the nature of the terrain in which they live, their movements have been described as goat-like, especially in the way that they bound when fleeing from predators.

Beiras, like oribi, are either solitary or live in small groups of no more than five individuals. A key difference is that there is only ever one male in a group, who chases away any potential rivals. The females, so far as we can tell, are much more tolerant towards one another. Beiras lack the scent glands in front of the eyes that typify gazelles and their close relatives, so their only means of marking their territory is urination and defecation. Usually, a female marks the spot first and then the male deposits on top of it, presumably claiming her as his mate. 

Considering how barren the land is, the territory occupied by a given beira group or individual is relatively small, at just 0.7 km² (170 acres). It has been proposed that their simple social structure may resemble that of the very first antilopines, and possibly of the common ancestor of non-bovine antelopes, goats, and sheep. Although diurnal, they rest during the hottest parts of the day, standing and chewing the cud. Births are thought to be most common in the spring, but the details are unclear.

While they belong to the same branch of the antilopine family tree as the steenboks and grysboks, these are not beiras' closest relatives. These are even smaller animals, not quite the smallest of all antelopes, but not far off. Next time, I will wrap up the dwarf antelope section of the subfamily by looking at those...

[Photo by Rod Waddington, from Wikimedia Commons, drawing by Philip Sclater, in the public domain. Cladogram adapted from Bibi et al. 2013.]