Sunday 19 May 2024

Antelopine Antelopes: The Largest Gazelles

Grant's gazelle
The word "gazelle", as used in everyday English is a little vague, referring to a general concept of slim, agile, antelopes but not to anything with a precise scientific definition. Strictly speaking, however, it refers to a specific group of closely related animals, and some species that are commonly called "gazelles" strictly speaking aren't. During the 20th century, all true gazelles were placed in a single genus, Gazella, and it's this that defines the more rigid definition of what does and doesn't count. 

In more recent decades, Gazella has been split in three, with the resurrection of two 19th-century names as "new" genera. The original Gazella is widespread, including animals from both Asia and North Africa, but the other two are exclusively African. Eudorcas (literally "true antelope" in Ancient Greek) includes Thomson's gazelle of the Serengeti and its various relatives, while the remaining genus is Nanger, whose name apparently comes from a local Senegalese word for one of the species.

By far the most populous of the three species in the genus is Grant's gazelle (Nanger granti). Since it, like Thomson's gazelle, lives in the Serengeti, it's also one that's likely to be familiar from wildlife documentaries even if their primary focus is on, say, big cats. 

Named for Scottish explorer James Grant, it is noticeably the larger of the two species. Thomson's gazelle stands 60 to 75 cm (2' to 2'6") tall at the withers, with the largest males reaching around the size of the smallest females of Grant's species. The males are larger, between 85 and 95 cm (2'10" and 3'2") tall, and weigh around 70 kg (155 lbs), compared with just 20 kg (44 lbs) for Thomson's gazelles. Otherwise, they do look similar, although the white patch on the rump extends up around the top of the tail, and the stripe down the side of the body is so much paler than, especially in the males, it's almost invisible.

While everyone agrees that Grant's gazelles lives alongside the "Tommies" in the Serengeti of Tanzania, how much further it extends is a matter of debate. Traditionally, it has been divided into multiple subspecies based on minor differences many of which, in retrospect, have proven difficult to justify. However, in 2007, it was suggested that the animals previously considered under the name in fact represented three different species. Many sources stick with the older, one-species model, albeit now with only three subspecies, but others maintain the 2007 split.

Under that scheme, the "real" Grant's gazelle lives in Tanzania and some parts of southern Kenya, while Bright's gazelle (N. notatus) lives through central and northern Kenya and a short distance across the relevant borders into the neighbouring countries of Somalia, Ethiopia, South Sudan, and Uganda. The third putative species, Peters' gazelle (N. petersii) lives in the coastal regions of Kenya, and also a short way into Somalia. The three, so far as we can tell never quite come into contact, and, while they can interbreed, haven't done so to any significant extent in the last 134,000 years - whether this is enough to make them distinct species is part of the debate.

Whether they are one species or three, Grant's gazelles as a whole inhabit dry plains, with some taking up relatively permanent residence in wooded areas with thornbushes, and others migrating through the year. The latter is most common in the Serengeti, where the gazelles' ability to survive with minimal water often leads to them migrating in the opposite direction to other local wildlife - away from the areas with the highest rainfall at a given time of year, rather than towards them. They are primarily browsers, with only about a third of their diet consisting of grass.

Herds are very variable in size, depending in part on how settled the group is. In wooded areas, herds consist of a single adult male and, on average, nine adult females, living in a territory delimited by visible landmarks and with the male defending against rivals. In these cases, the surplus males live in small bachelor herds of four or five individuals, each hoping for their chance at establishing a harem of their own. 

Territories are marked only with dung and urine; the scent glands on the faces of Grant's gazelles are much smaller than those on most other gazelle species and they don't seem to use them much. Males are more likely to deter rivals by standing in a prominent position in such as a way as to show off their muscular neck than by physically fighting. On the few occasions when they do have to enter personal combat, they lock their horns and twist violently with their necks - the winner is whoever can throw the other down onto their side.

On the more open plains, although such smaller groups may form in the rainy season, for the most part the gazelles live in large migratory mixed-sex herds averaging around 45 members each. They often travel together with Thomson's gazelles on the Serengeti, which may help defend against predators. Overall, however, Grant's gazelles seem to be attacked less frequently than their smaller kin; leopards may find smaller antelopes a more suitable target, while lions prefer larger prey such as wildebeest or zebras. For that matter, the fact that they can go so long without drinking that they rarely visit waterholes must also be an advantage.

Soemmerring's gazelle

Soemmerring's gazelle (Nanger soemmerringii) lives further north, in Ethiopia, Eritrea, and Djibouti, with a small population also likely surviving in Somalia. They also used to live in Sudan east of the White Nile, but died out there some time in the late 20th century. They are about the same size as Grant's gazelle, but generally paler, with a larger white patch on the rump, more distinct facial markings, and no stripe on the flanks. 

The species has not been studied in much detail, in part perhaps because of political instability in the region. From what we can tell, they are similar to Grant's species, inhabiting similar arid habitats although perhaps having a smaller proportion of grass in their diet. Today, they tend to be found in hilly areas but it may just be that these are more inaccessible to humans and so less likely to be used for agriculture. Although herds with hundreds of individuals have been seen even in recent times, most are far smaller, with about 17 animals per social group being typical according to one Ethiopian study.

At some unknown point in the past, a population of Soemmerring's gazelles became isolated on the island of Dahlak Kebir, part of Eritrea. When this happened is unclear; the gazelles were certainly there in the 18th century, so they aren't recent - for all we know, they could have lived on the island since it was connected to the mainland during the Last Ice Age. Mostly occupied by a national park, it has a human population of only 2,500, despite being almost twice the size of the Isle of Wight in the UK, or almost three times the size of Nantucket in the US. This is unsurprising, given that it's largely barren wasteland with a few patches of greenery running along dried-up stream beds.

This, however, is far more suitable for gazelles than it is for humans, and the animals seem to have done well there, expanding in numbers following a past population bottleneck. Indeed, apart from bats and introduced black rats, they are the only wild mammals on the island. Significantly, however, they have been there long enough to have become much smaller than their mainland kin, a case of insular dwarfism, where animals shrink over the generations to make the most of a limited food supply.

Dama gazelle

Although it is not formally an endangered species, Soemmerring's gazelle is considered "threatened" or "vulnerable", with a total population not thought to exceed 6,000 - compared with well over 100,000 for Grant's species. However, this is till far more common than the exceedingly rare dama gazelle (Nanger dama). 

First described scientifically way back in 1766, this was once widespread across the deserts and semi-deserts of North Africa, appearing, for example, in prehistoric rock art from across the region. Now only tiny remnant populations survive in isolated patches along the southern edge of the Sahara in Mali, Niger, and Chad. They are only slightly smaller than Grant's gazelle, but have a distinct dark reddish colour with white patches whose size tends to be highly variable between different populations - in the case of the subspecies in Chad (shown above), the "patches" are so large that the animal is more white than red. 

Feeding primarily on acacia thornbushes, grass, and wild millet, dama gazelles inhabit semi-desert regions and rocky hills or mountains and even travel into the margins of true desert, although avoiding the barren sand dunes. At least today, they live in very small groups, typically with a single male and only three or four females; the other males live alone, rather than forming bachelor herds. Males kept together in captivity become aggressive towards one another and quickly establish a distinct dominance hierarchy

It's unclear how much this happens in the wild, where small groups are wandering nomadically across the wastes in search of limited and often temporary food sources. They have been reported to travel at least 12 km (7½ miles) a day, sometimes travelling five times that far if spooked by predators or some other threat. Breeding is seasonal with most births in April.

The population of dama gazelles has crashed over the last sixty years or so, in part because they only inhabit poor countries that have little opportunity to provide effective protection against poaching. Sightings this century have been restricted to just five locations and it is thought that no more than 200 still survive in the wild, with a further 350 in zoos around the world. Although there is some doubt as to how distinct the three supposed subspecies really are, the assumed western one, which once lived from Morocco to Senegal, is entirely extinct in the wild. An attempt to reintroduce them in Morrocco in 2015 met with only very limited success, partly due to the animals' inability to recognise dogs as predators, but, more seriously, due to poaching.

As I noted at the beginning, there are other kinds of antelopes that are commonly called "gazelles". These are related to the true gazelles, in the sense of belonging to the same subfamily, but are not their closest relatives, belonging to a separate subgroup that happens to have similar habits and appearance. Instead, the closest relative of the true gazelles has quite a different look, while what's probably the most gazelle-like of all non-gazelles has a different name entirely. Next time, I will be looking at those two species.

[Photos by Doug Knuth, "Ji-Elle", and "Just Chaos" from Wikimedia Commons. Cladogram adapted from Bibi 2013 and Lerp et al. 2013.]

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