Sunday 25 February 2024

Antilopine Antelopes: Tommy's Gazelle and Relatives

Thomson's gazelle
You probably don't need to live in Africa to be aware that there are a great many different kinds of antelope. (A couple of years ago I came across an online picture quiz of "can you name these African animals?" Over half of them were antelopes.) It's hard to say which of these are the most familiar to the general public, because quite a few of them probably are, at least in general terms. But one subtype of antelope that people will at least recognise are the gazelles.

Gazelles are smallish, fleet-footed animals; the word comes from the Arabic ḡhazāl, which literally means something like "slim/agile creature". Gazelles are widespread, perhaps surprisingly so, and there are many different species. Of these, the one that may be the most familiar to people outside of Africa is Thomson's gazelle (Eudorcas thomsonii) for the simple reason that it's the one that lives in the Serengeti and therefore gets into a lot of wildlife documentaries. Mostly getting eaten by big cats, to be sure, but it's a start.

In fact, Thomson's gazelle is found only in the Serengeti and neighbouring regions in and around the Great Rift Valley in southern Kenya and northern Tanzania. It was first named in 1884, after Joseph Thomson, who had explored the region for the Royal Geographical Society the previous year. It very much fits the standard image of a gazelle, a slender antelope with a cinnamon-coloured back and white underparts, separated by a wide black stripe beneath a pale buff band along the flanks and clear black-and-white facial markings. The horns are small on the females, but long and with prominent circular bands on the males.

Thomson's gazelles are grazing animals, with at least 75% of their diet consisting of grass. They will eat other plants, especially in the dry season, but the nature of their diet means that they prefer living in short grasslands and open terrain. The fact that they will avoid, so far as possible, areas with dense vegetation or even especially tall grass makes it easier for them to spot approaching predators but, at the same time, means that they are often far from water, inhabiting parts of the plains that can become parched in the dry season. That this doesn't seem to bother them indicates that they can survive on a relatively minimal water intake, even if they aren't quite desert animals.

Females live in herds, ranging from two dozen to two hundred individuals, but these don't seem to be very structured as they might be in other herd animals. Instead, adults tend not to interact with one another very much, perhaps staying together more out of convenience than any particular desire to be sociable. While young males tend to live in smaller herds, the older ones maintain individual territories, which vary considerably in size with the local population density, but typically cover 10 to 30 hectares (25 to 75 acres). They patrol the borders of these territories, trying to keep as many females within them as possible, and marking them with scent from the large glands in front of their eyes and with urine and droppings. 

The large groups that the females travel in make this challenging for them - there are only so many individuals that a given male can round up - so the female herds tend to be fairly free-ranging, occupying up to 3 km² (about 1 square mile) each and overlapping the territories of dozens of males. Even then, they are nomadic, moving between grassy plains in the wet season and savannah in the dry season. 

Cheetahs are the most common predators of Thomson's gazelles, although African wild dogs are also a threat and other animals, such as lions, will certainly eat them from time to time. The initial response of the gazelles to detecting a potential predator is, perhaps counterintuitively, to gather to look at it, moving about in slow circles and even approaching it to take a closer look. While this can be dangerous, it must be beneficial on balance, presumably because it lets the predator know that it has been spotted and that any attack might not be worth its while. 

When placed in danger, younger gazelles will attempt to hide, perhaps by dropping prone in long grass, but adults fast enough to have a chance at outrunning the predator (which may require a decent head start if it's a cheetah) will certainly try to do so. At this point, they will engage in a behaviour common amongst all types of gazelles: pronking or stotting. 

Whichever word you use for it, this involves the animal straightening all four of its legs simultaneously, propelling itself rapidly upwards in a vertical leap. Since this doesn't move them forwards, it actually slows their escape, not allowing them to use their full, and not inconsiderable, speed over flat ground. It also has to be exhausting to do repeatedly, when you'd think the gazelle would rather be using every bit of energy it had to make a run for it. Several reasons have been advanced as to why such a thing would be helpful, including that it might help warn other gazelles that there's a problem, or that it might be a signal of physical fitness - telling the predator that it should probably pick a weaker target. But, again, it may just be a very visible way of letting the predator know it has been spotted.

Like many herbivores, the need to stay vigilant means that they don't require much sleep; they take regular naps throughout the day but for no more than five minutes at a time. Nonetheless, they do spend around half their time resting, sunbathing in the morning and seeking shade in the heat of the afternoon. Although they are more active during the day than at night, they often wander around for an hour or two around midnight before having another lie-down. 

Mating takes place towards the end of the rainy season, so that, with a six-month gestation, birth can take place early in the next one. Like many other herd animals, the mother commonly isolates herself around the time of birth and hides her fawn in long grass, calling out to it when she returns from a feeding trip. This does not, however, appear to be universal, with some mothers staying with the herd and giving birth in short grass, apparently gaining from safety in numbers as much as they lose from increased visibility. Both methods seem reasonably effective, with the greatest risk to the fawn coming when it is old enough to leave cover and transition to regular herd living.

Red-fronted gazelle

For much of the twentieth century, Thomson's gazelle was considered to belong to the same genus as most other gazelles, but in 2000 it was split off into its own genus, Eudorcas. This was first named in 1869 for the red-fronted gazelle (Eudorcas rufifrons) but had been subsumed as, at best, a subgenus of other gazelles. It was restored due to some distinctive features that unite Thomson's and red-fronted gazelles but that distinguish them from other species. The most visible of these is the presence of prominent, widely-spaced rings on the horns, but others relate to modifications to the nasal bones that make them less suited to desert environments than most other gazelles.

Nevertheless, the red-fronted gazelle, like the "Tommies", inhabits parts of the world where rainfall is sparse and seasonal. In its case, this is the Sahel, the great band of semi-desert and scrubby savannah that stretches along the southern edge of the Sahara. Red-fronted gazelles are found from the Sahel's western bound on the Atlantic coast in Senegal right the way across Africa to the banks of the White Nile in Sudan, taking in many other countries along the way. Although they, too, do not need to drink water very often, obtaining enough from the plants they eat, they migrate south in the dry season, heading towards less inhospitable terrain before returning to the edge of the Sahara in the milder wet season.

They are about the same size as Thomson' gazelles, standing about 75 cm (2' 6") at the shoulder and have a similar build. They are, as their name suggests, a richer red in colour, and the black band along their flanks is not only narrower, but has a second red band just below it, separating it from the white underbelly. 

They are grazing animals, although perhaps less reliant on grass than their relatives. Females live in much smaller groups, perhaps due to the constraints of a relatively harsh habitat where there likely to be less food to go around; fifteen adults per herd is about the maximum, and any more than six is unusual. The males seem to have similar habits to Thomson's gazelle, staking out territories in much the same manner. The clearance of their already marginal native habitat for farmland and livestock grazing means that their population is almost certainly declining, although they remain well short of becoming an "endangered" species.

Heuglin's gazelle

There has been some debate over the last couple of decades as to how many species belong within the genus Eudorcas. When it was originally created, there were just two, but more recently there has been a move to split one or both of these further. The most accepted scheme these days leaves Thomson's gazelle as a single species, but splits off the two most easterly subspecies of the red-fronted gazelle into their own species.

Thus, east of the White Nile in South Sudan, we now find the Mongalla gazelle (Eudorcas albonotata). It has a narrower face than the red-fronted gazelle, with slightly longer horns in the male and much shorter horns in the female. The colour is less obviously reddish, closer to the cinnamon of Thomson's gazelle, and with a broader black stripe than the red-fronted species. Living in floodplains and seasonally watered savannah, it is also said to live in much larger herds.

Further to the north, also across the Nile, but in Sudan proper, and also across the border into parts of Ethiopia and Eritrea, we instead find Heuglin's gazelle (Eudorcas tilonura) named for the German naturalist who first described it in 1863. Here, the horns of the females are closer in length to those of the males than in the other species, and a narrow black band they all possess stretching vertically across the rump is missing. It's probably the least studied of the four currently accepted species, but is considered endangered, likely restricted to just a few national parks across the region, and with no more than 2,500 adults remaining, if that. It inhabits semi-arid highland plateaus.

A few sources list a fifth species, the "red gazelle" (E. rufina) which is said to have once lived on the north side of the Sahara in Algeria and to now be extinct. However, there is no evidence that this ever existed. It has never been seen alive, and the pelts it was named from had probably been shipped from elsewhere in Africa - the only one to be genetically tested proved to belong to a regular red-fronted gazelle.

There are, however, many other species of gazelle in Africa, less closely related to the animal most familiar from all those documentaries filmed in the Serengeti. Next time, I will be taking a look at some of those.

[Photos by "Bob" and Andrzej Barabasz, from Wikimedia Commons. Drawing by Philip Sclater, in the public domain.]

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