Sunday 16 June 2024

Antelopine Antelopes: Blackbuck and Springbok

The gazelle-like body form has evolved at least three times within the "antilopine" subfamily, and arguably a few more times among antelopes more generally. A fast-running animal, able to outpace many of its predators, is clearly a useful thing to be if you're a herbivore. Only one of these three evolutionary events led to what zoologists would describe as the "true gazelles", although at least one of the others resulted in an animal so strikingly similar to gazelles that it's surprising that molecular evidence tells us it doesn't have an immediate common ancestor.

The blackbuck (Antilope cervicapra), however, does not look much like a gazelle. It is one of the five currently recognised species of antelope whose scientific name dates back to the origins of modern taxonomy in 1758. In 1766, Peter Simon Pallas first distinguished antelopes from goats, creating the genus Antilope to incorporate no fewer than seventeen species - including the Dorcas gazelle, which would later go on to become the defining species of the Gazella genus when that was created in 1816. While every other living species was eventually split off elsewhere, the blackbuck remained, and its genus gives its name to the entire subfamily. (The second part of the name, incidentally, translates as "deer-goat" and remains the name of the animal in French).

Despite being the immediate origin of the word "antilopine" and being the closest living relative of the true gazelles, and perhaps even nested within them, the blackbuck looks quite different. Living across most of India, and into the southern hills of Nepal, it is of similar size but more stocky than, a typical gazelle, with the largest males standing 85 cm (2' 9") tall at the shoulder and weighing no more than 35 kg (77 lbs). While there are several anatomical similarities, they also differ from gazelles in that the scent glands in front of the eyes are larger and the females lack horns altogether.

Only adult male blackbuck are, in fact, black, with females and younger males being tan in colour, although they are all white on the underbelly and the insides of the legs. The black colour is apparently linked to testosterone levels and males are noticeably darker during the breeding season. The horns also look quite unlike those of gazelles, pointing backwards and twisting along their length in an elongated spiral.

Unlike gazelles, they are not desert animals, and typically drink at least once each day. Instead, they live on grassy plains, venturing into scrubland and lightly wooded areas, but being outcompeted by the local chital deer where the forest becomes thicker. For much of the year, they feed on grass, although they apparently avoid eating the widely available halfa grass, probably because it's tall enough to hide them when they are resting. At times of the year when grass is either too dry or is dying off, they switch to leaves of thornbushes and legumes, and are often found on the margins of agricultural land where they can eat rice and other crops. 

Herding behaviour is highly variable in blackbuck, although the females are generally more sociable than the males. As a rule, the flatter and more open the terrain, the larger the herds, which can be all-female, include a single dominant male, or be more mixed-sex, perhaps depending on the availability of resources. Many males are solitary, defending territories, but younger males may gather into small bachelor herds. Where female herds are largest, typically in wide semi-arid grasslands, the males adopt a lekking behaviour, with each defending a small territory marked with dung piles, but keeping their territories close by one another so that they can directly compete for females. Where herds are smaller, the males disperse more, defending patches of high-quality food, hoping to attract females towards them.

While physical conflicts between larger males are rare, they will drive off younger rivals, and they occur often enough that around 4% have partially broken horns. They can mate throughout the year, although they are most likely to do so between August and October and between March and April. Although females only ever (so far as is known) give birth to a single young at a time, the young grow fast enough that a mother can mate again and give birth to a second fawn before the end of the year.

If blackbuck are the closest relatives of the true gazelles, the animal that physically resembles them the most is probably the springbok (Antidorcas marsupialis). Standing around 75 cm (2' 6") at the shoulder and  weighing around 30 kg (66 lbs) they are smaller than blackbuck, but not so different from some of the medium-sized gazelle species, and they have a very similar coat colour, including the dark stripe down the flanks. The horns are also of a similar shape and size, making it hard to tell that they are not, in fact, gazelles. Indeed, it was not until 1847, over sixty years after the initial scientific description, that they were placed in their own genus, having previously spent time in both Antilope and Gazella.

Features that distinguish springboks from gazelles include the fact that the face is largely white, rather than tan between the facial stripes, the absence of hairy-tufted scent glands on the knees, and the fact that the first premolar tooth in the upper jaw is at best tiny, and usually missing altogether. The second half of the name comes from the presence of a pouch on the animal's back, which they flip inside out when they leap into the air, displaying the long white fur of the inner lining.

Beyond these characteristics, however, the best way to tell gazelles and springbok apart is simply that they don't live in the same place. The most southerly species of gazelle is Grant's, which doesn't reach beyond northern Tanzania, whereas the springbok lives much further south, across much of South Africa and Namibia and in southern Botswana. A small population also lives in the coastal regions of southern Angola, but they are far less numerous in the area than they once were - elsewhere in their range, springboks are about the most common antelopes you can find.

Like gazelles, springbok inhabit arid landscapes, mostly dry brush and savannah in their case, although they are also found in the Namib Desert. The latter is marginal habitat for them; they can survive, but avoid the sandier regions preferring areas with moderate vegetation, low but not non-existent rainfall, and rocky terrain for concealment. Experiments show that they can easily maintain a stable body temperature as the weather becomes warmer, and their unusually thin pelt probably helps them dissipate excess heat rapidly after energetic sprinting and leaping to escape predators.

Given their habitat, the diet of springboks is similar to that of the less desert-adapted gazelles, feeding on grass for much of the year, and switching to thornbushes and other shrubs in the dry season. The latter are apparently sufficient to allow them to go without water for extended periods, although they will certainly drink when they get the opportunity. 

Females live in small herds, with about six adults being typical. Adult males are solitary and territorial, with each patrolling an area of 25 to 70 hectares (60 to 175 acres), defecating on the boundary and thrashing bushes with their horns to intimidate rivals. A common tactic to display their bravery and fitness is apparently to simply lie down where they are clearly visible. Sometimes, a given male's territory will be occupied by only a single herd of females, effectively creating a harem system even if they don't normally travel together, but more often the females just wander over a much wider area than the males do. As with other antelopes, younger males associate in bachelor herds, which are smaller than those of the females.

In times of drought, springbok used to gather in large groups of hundreds of individuals, migrating across the veld, often following thunderstorms in search of the fresh grass that would grow in their wake. These "trekbokken" are no longer seen today, perhaps because many springboks now live in fenced game preserves, but perhaps also because of the decline in numbers they experienced in the late 19th century due to a combination of hunting and rinderpest outbreaks. Even so, in times of drought, male springbok often give up on maintaining their territories, finding it more important to travel in search of food.  

The springbok is so named because of its habit of suddenly leaping into the air while being pursued, straightening all four of its legs to do so. This behaviour is also seen in gazelles, presumably as a signal of some kind, but springbok can leap to greater heights, reaching around two metres (6 feet), and flip open their pouch when they do so, making them even more visible. Although there's not really much difference, some authors reserve the word "pronking" for the springbok and use "spotting" for gazelles; others use the two terms interchangeably.

Breeding takes place in the dry season, so that young are born in the spring when the rains return. Not normally vocal, males make deep bellowing grunts as they follow receptive females. They fight one another by locking their horns and twisting and occasionally by stabbing, but seem more concerned by intruders than by rivals in territories bordering their own. The young remain immobile for the first two days of life, relying on concealment rather than trying to flee if approached. Mothers and their fawns return to the herd three to four weeks after birth, and the young are weaned at five to six weeks.

At least two species of fossil springbok are known, the last of which, thought to have a more specialised grazer than the modern animal, died out as recently as 5,000 BC. But, apart from those, if gazelles are not the closest relatives of springboks, what is? Next time, I'll take a look at the two prime contenders...

[Photos by "Sagar735" and H. Zell, from Wikimedia Commons.]

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