Field of Science

Gazelles and their Kin

Female steenbuck Raphicerus campestris, copyright Yathin S. Krishnappa.


Ever since biblical times, gazelles have been a byword for a kind of watchful elegance, always on guard against unwanted advances. It is not difficult to see how such an analogy arose: on their native savannah, gazelles are indeed always on the alert, wary of the threat of predators and quick to respond to alarm. It is a habit that has served them for millions of years.

The Antilopini are an assemblage of about thirty species of mostly smaller antelope found in Africa and Asia*. The smallest are the dikdiks of the genus Madoqua which may be only a foot or so in height and weight just a few kilos; the tallest, the dibatag Ammodorcas clarkei, stands about 90 cm at the shoulder and weighs about 30 kilograms. They are mostly associated with arid or semi-arid habitats: savannahs, deserts, steppes and the like. Some species form sizeable herds; others live solitary lives.

*Before I go too much further, I should note that J. K. Revell over at his site Synapsida has written a number of posts about bovids (antelopes, cattle, etc.) over the the past few years that I heartily recommend. To the best of my knowledge, he hasn't gotten to antilopins yet, so I should be safe on that front.

Female and male oribi Ourebia ourebi, copyright Bill Higham.


Modern researchers largely agree on dividing the Antilopini between four major lineages, recognised as subtribes. One contains a single species, the oribi Ourebia ourebi, a smaller species with short, straight horns found in eastern sub-Saharan Africa. The Raphicerina, including the dikdiks Madoqua, the steenbucks and grysbucks Raphicerus and the beira Dorcatragus megalotis, are similar small, short-horned species. The Raphicerina and oribi are solitary species with individuals maintaining exclusive territories (at least between members of the same sex). They advertise their territories through the use of defecation sites together with the marking of vegetation using scent glands in front of the eye. The Raphicerina are exclusively browsers, concentrating on higher-quality food sources; in contrast, the oribi is a grazer and consequently must occupy a larger territory than the other species. Females of Raphicerina and oribi are hornless; in most other Antilopini (with some exceptions noted below), horns are present in both sexes though the females' horns are shorter and more slender.

Przewalski's gazelles Procapra przewalskii, copyright Yilun Qiao.


The majority of the remaining Antilopini live in herds though males of most species will claim temporary territories during the breeding season as they attempt to gather harems of females. The central Asian gazelles of the genus Procapra are placed in their own subtribe; these are three pale, medium-sized species found on steppes and high-altitude grasslands between the Himalayan plateau and Mongolia. They have rear-swept horns that make them look a bit like a gazelle that is trying to pass itself as a goat. Procapra gazelles may not be immediately related within the Antilopini to the true gazelles in the largest of the four subtribes, the Antilopina. Until recently, most authors would have treated the great majority of the Antilopina species in the genus Gazella; however, questions about the monophyly of this genus in the broad sense have lead to the recognition of three separate genera of gazelles: Gazella sensu stricto, Nanger and Eudorcas. The Nanger species, which include the dama gazelle N. dama and Grant's gazelle N. granti, are relatively large gazelles with a conspicuous white rump that is absent in the other two genera. The genus Eudorcas includes perhaps the most familiar gazelle species, Thomson's gazelle E. thomsoni of Kenya and Tanzania, which forms much larger herds than other gazelle species.

Mhorr gazelles Nanger dama mhorr at Tierpark Hellabrunn in München, copyright Rufus46.


The remaining living Antilopina species are all placed in their own separate genera. The springbuck Antidorcas marsupialis of southern Africa also forms large herds that used to number in the tens of thousands before hunting and habitat loss reduced their population. Springbucks are best known, of course, for their habit of 'pronking', a mode of bounding with all four legs held stiff and landing simultaneously, most often seen when the animal is alarmed or at play. Pronking is not unique to springbucks (other gazelles do it too) but it is made particularly noticeable in this species by a crest of white hairs towards the rear of the back that is erected at the same time.

Springbuck Antidorcas marsupialis engaged in some pronking, copyright Hans Stieglitz.


In other species of Antilopina, only the males have horns. The gerenuk Litocranius walleri and dibatag Ammodorcas clarkei are two slender species found in eastern Africa that differ from other Antilopina in being browsers rather than grazers and maintaining permanent exclusive territories. Both these species habitually feed while standing erect on the hind legs, allowing them to browse at higher levels than they could otherwise; they are even able to walk about to a certain extent in this pose, albeit perhaps not in a manner that could be called graceful. Outside of Africa, the blackbuck Antilope cervicapra is found in grasslands and woodlands of the Indian subcontinent (there is also supposed to have been a small introduced population of them near Geraldton here in Western Australia, though it may have since been eradicated). Males of this species have long, spirally twisted horns; mature males are also the only 'blackbucks' that are actually black (at least dorsally) whereas females and young males are light brown.

Pair of juvenile dibatags Ammodorcas clarkei at a rescue centre, copyright F. Wilhelmi.


Perhaps the most distinctive member of the Antilopina, however, is the saiga Saiga tatarica. This is the only species that is known to never be territorial, forming large herds in its native habitat of the central Asian steppes (technically, the social habits of the little-studied dibatag are largely unknown but it would not be unreasonable to presume that they are similar to those of the gerenuk). It is more robust than other Antilopina species; indeed, there was long uncertainty about whether saiga are more closely related to gazelle or goats. The nostrils of saiga are inflated to a hanging proboscis that is usually presumed to function as protection for the respiratory tissues from the dust of their near-desert habitat. However, there may also be a display function involved; during the mating season, the proboscis of males becomes engorged while scent glands in front of the eyes produce pungent secretions (so maybe the function of the proboscis is actually to somehow protect the saiga from its own stench). Unfortunately, the saiga (among other Antilopini species) is currently regarded as critically endangered, with only a fragment of its historical population surviving. There was a time when the saiga was thought to be something of a conservation success story: after being almost wiped out in the early 1900s, populations built up to about two million by the 1950s. But in the last few decades, the combined effects of factors such as habitat loss, disease and the demand for their horns from everyone's favourite country to turn the extermination of endangered species into a pointless investment bubble have caused numbers to crash back down to an estimated 50,000 or so (as relayed by Wikipedia).

Pair of saiga Saiga tatarica, copyright N. Singh.


Fossil species have been assigned to the genus Gazella from as far back as the Miocene though there may be grounds for debating how many of them are true Gazella. For instance, Bärmann (2014) commented on preliminary results of a phylogenetic analysis including the Pakistani Miocene species G. lydekkeri (from the well-studied Siwalik deposits) that suggested that it might be placed outside the Antilopina crown group. Other fossils of Antilopini inform us that the modern blackbuck is the sole survivor of a lineage of spiral-horned antelopes that was previously more widespread in Eurasia. The saiga was more widespread in the past as well, with either the modern or a closely related species known during the Pleistocene from more northerly parts of Siberia (at which point, presumably, there may have been saiga in the taiga) and even in northernmost North America. If they do disappear completely, it will be a sad end to a long history.

REFERENCES

Bärmann, E. V. 2014. The evolution of body size, horn shape and social behaviour in crown Antilopini—an ancestral character state analysis. Zitteliana B 32: 185–196.

Macdonald, D. (ed.) 1984. All the World's Animals: Hoofed Mammals. Torstar Books: New York.

No comments:

Post a Comment

Markup Key:
- <b>bold</b> = bold
- <i>italic</i> = italic
- <a href="http://www.fieldofscience.com/">FoS</a> = FoS