Antelopine Antelopes: The Giraffe-Gazelles

Gerenuk

Despite appearances, true gazelles are not the closest living relatives of the springbok. That honour is probably tied between two other species, although there have yet to be sufficient genetic studies to absolutely nail that down - one could be closer than the other. They likely diverged from the springbok over 10 million years ago in the late Miocene, earlier than the blackbuck is thought to have diverged from the true gazelles, so it's perhaps unsurprising that they look quite different.

They also look slightly odd, and very distinctive.

The better-known of the two is the gerenuk (Litocranius walleri). The name comes from the Somali word for the animal, but it is more commonly known as the "giraffe-gazelle" in many European languages, and it's easy to see why. It is, of course, much smaller than a giraffe, with males having a shoulder height of around 100 cm (39 inches). The colour is also different, a relatively uniform reddish-brown over the back, with a paler shade in the flanks, neck, and limbs, and stark white underparts. There are also white markings on the face, around the eyes. Only the male has horns, which rise almost vertically out of the skull before curving back in an S-shape.

It is, however, the bodily proportions that invite the comparison to a giraffe. Gerenuks have unusually long legs and a long neck that is even more striking. In fact, the neck is so long and muscular that it makes the head look small, enhancing the strange appearance. The tail is also noticeably longer than we would expect for a gazelle, and has a crest of dark fur running along it. 

Like the giraffe, the gerenuk's neck has exactly the same number of bones in it as the neck of almost every other mammal - seven. Instead, the bones themselves are longer, although their shape is modified to allow powerful tendons to support the head on such an attenuated structure. The muscles within the neck are also unusual; rather than reach all the way along, some are restricted to the lower part and some to the upper, with the fifth cervical vertebra marking the junction between the two. This likely allows the animal to separate the muscles that turn its head from those that provide stability, increasing the strength of the latter.

It was originally described by Victor Brooke in 1879 as a species of gazelle, but was given its own genus just eight years later, given its significant differences from any regular gazelle. Franz Kohl, who named the genus, never explained why he picked the name Litocranius, although it's thought that he probably meant "plain skull" because of its long and flattened shape. A supposed second species was named shortly after, and is still sometimes regarded as such, but it's more usually thought of as a subspecies today and there is little information on the distinction.

Gerenuks are found throughout the Horn of Africa, from northern Tanzania in the south up to Djibouti in the north, taking in large portions of Kenya and southern Ethiopia and essentially the whole of Somalia on the way. They seem to be reasonably common there, although it's obviously difficult to survey them in Somalia at the moment. They inhabit dry scrubby bushland, avoiding both dense forest and open grassland.

Gerenuks are essentially pure browsers, eating nothing but leaves, shoots, and flowers and not touching grass or low herbs. Where it lives, this means that acacia thornbushes form a large part of the diet, as they do for most true gazelles, but otherwise there is significant variation throughout the year. During the rainy season they eat climbing plants and the fresh buds of deciduous trees such as Hymenodictyum, while in the dry season, they switch to evergreen trees and bushes that still carry leaves. They apparently do not need to drink water.

The long neck helps the gerenuk reach higher than its relatives, but this is further aided by their ability to stand upright on their hind legs while feeding. This allows them to reach as high as two metres (6' 6") off the ground, beyond the reach of even most large antelopes. Although their hip joints are the shape we'd expect for a quadruped, their lower back does have an unusual curvature similar to that seen in humans, enhancing their ability to stand bipedally and even take a few steps while doing so. The narrow snout enables it to pick at leaves with precision, although the tongue is not unusually long for an antelope of its size, as one might expect it to be for a browser.

Gerenuks are diurnal, although they lie down when it rains and during the hottest parts of the day. Although small mixed-sex herds are occasionally seen, especially in dry country, the males in these groups are always subadults. The more usual arrangement is for a single adult male to defend a territory and be accompanied by one or two females and their young. The male marks his territory with random deposits of dung and urine, rather than reusing specific piles as many of their relatives do, and by leaving secretions from the scent glands in front of his eyes on twigs and similar locations that females and potential rivals are likely to encounter. 

When males fight, it's in a similar manner to gazelles, wrestling with their horns and using their long, muscular necks as a brace; the loser submits by turning his head sideways. Friendlier relations, usually between adult females, are signalled by one gently rubbing her head against the jaws of her partner.

In most antelopes, the male indicates a desire for mating by approaching the female in a low crouch, but this is not the case in gerenuk. Instead, he just walks up behind her, raises his head, and then lifts one of his front legs, holding it straight out in front (most other antelopes that do anything like this would bend it at the knee). Assuming she's interested, he then rubs his head over her body, smearing her with the secretions from his scent glands, before standing up on his hind legs, taking a couple of steps forward and performing the act in that position, his forelegs hanging loosely.

Pregnancy lasts about seven months, resulting in the birth of a single calf. As with many other antelopes, the mother hides the calf in bushes, leaving it alone for much of the day, staying about 1.5 km (one mile) away so that predators don't realise she has something to protect. Young gerenuk have been seen trying to stand up on their hind legs within two weeks of birth, although they don't seem to succeed until they are at least a month old.

Dibatag

The dibatag (Ammodorcas clarkei), sometimes called "Clarke's gazelle", is closely related to the gerenuk and has a similar, if less extreme, appearance - in some Germanic languages "llama-gazelle" is a common alternative (or even preferred) name. It also inhabits much the same part of the world, being native to central Somalia and the Ogaden region of eastern Ethiopia. 

It's difficult to know how well the dibatag is doing at a population level. That's partly because there have been no relevant studies in Somalia since the 1980s, but also because they are quite good at hiding in thick bush where they are difficult to spot in aerial surveys. Nonetheless, the population does seem to be dropping, and they are no longer found in the more populous regions of northern Ogaden, where they were once common, so they are considered a vulnerable, but not yet endangered, species.

As a result, we also know relatively little about them beyond their physical appearance. They are slightly smaller than gerenuk, with a shoulder height of about 90 cm (35 inches) and the horns of the males have a C-shaped, rather than S-shaped, curve, with prominent widely spaced rings. As with gerenuk, the females are hornless. There are also other subtle differences in, for example, the shape of the hairless portion of the nose, and a longer white stripe down the sides of the face. The coat colour is usually described as tan or fawn, rather than reddish, although the underparts are similarly white. The tail is even longer, with a brush at the tip rather than a crest.

Dibatag are most often found in areas of moderate to dense thornbush, but they are said to be more likely to use tall grasslands than are gerenuk. They eat a wide variety of plants, primarily browsing on leaves, shoots, and flowers, but also eating some herbs. During the dry season, they have been reported to eat wild melons and other water-rich fruits, along with myrrh, which is similarly succulent. They can apparently feed while standing on their hind legs, although reports indicate that they prefer to brace their front legs against the trunk of a tree while doing so, rather than being truly bipedal.

They live in small groups of no more than five individuals, and usually less. One distinctive feature is that, when alarmed, they run away with their tail raised as a signal; this is thought to be the origin of the name "dibatag" which can be translated as "erect tail" in Somali. 

Exactly how broadly the term "gazelle" should be applied to fast-running antelopes is really a matter of taste, and, as noted above, the word is sometimes used for gerenuk and dibatag, especially in non-English languages. At its broadest, it includes not only they, the springbok, and the true gazelles, but a third group of antelopes native to the highlands of eastern Asia - a place that we perhaps less commonly associate with antelopes than Africa or the Middle East. Whether we can truly call them "gazelles" or not, it is to that group that I will turn next.

[Photo by Darren Obbard, from Wikimedia Commons. Illustration from the book Great and Small Game of Africa, in the public domain. Cladogram adapted from Bibi et al. 2013.]