Sunday, 9 October 2016

Bovines: Bongos in the Jungle

Broadly speaking, the more cow-like bovine species tend to be found wild in forested habitats, often with quite dense foliage. In contrast, the antelope-like bovines tend to prefer more open wooded country or scrubland. But these are both very crude generalisations. Plains bison and yak, for example, both live in open terrain, and, on the other side of the coin, there are bovine antelopes that prefer forest to savannah.

Among these is the bongo (Tragelaphus eurycerus), which can be found in three, entirely separate, regions of equatorial Africa. The two main populations live along the southern coast of West Africa, from Guinea to Benin, and across the heart of the Congo Basin and surrounding jungles, from Cameroon and Gabon in the west almost to the Ugandan border in the east. The vast majority of bongos live in these two areas, which are nonetheless separated by over 1,000 km (620 miles) of intervening land (much of which is Nigeria). The third population is found only in a couple of tiny mountain refuges in Kenya, where they cling to the forested slopes, migrating up and down them each year as the weather changes.

It is generally agreed that the Kenyan bongos form a distinct subspecies - the "eastern" or "mountain" bongo (T. e. isaaci) - from their lowland relatives elsewhere. Given the distances involved, there are also some arguments that the western bongos may represent two or more subspecies, but we don't yet have the hard genetic data that would prove this idea one way or other, so, in the absence of any obvious physical differences, this is not widely accepted at present.

Bongos are quite large, standing about 120 cm (4 feet) high at the shoulder, and with adult males weighing anything up to 400 kg (900 lbs). They have a bright, reddish-brown coat marked with narrow white stripes; such stripes are common on spiral-horned antelopes, but are perhaps more prominently visible on bongos than on any of their relatives. The horns are large and widespread, especially in the males, and twist through a full circle, forming a sort of lyre-shape as they do so.

Bongos live in moderately dense rainforests, and, while they are larger than other forest antelopes, this can make them quite difficult to find and study. These are usually lowland forests, although the Kenyan populations, isolated as they are, live between 2,100 and 3,000 metres (7,000-10,000 feet) above sea level.

They are usually described as browsers, with perhaps 90% of their diet composed of shrub and tree leaves, rather than grass. They particularly seem to prefer leaves from the understory, or from forest edges, and apparently prefer areas with numerous forest clearings, where such food would be more common, rather than spending their entire time under the forest canopy. Having said this, they can twist small branches down from trees with their horns, breaking them off to eat the leaves, and the Kenyans ones at least seem to survive perfectly well on diets that are less nutritious than we might suppose (albeit presumably eating more to compensate).

One thing bongos do seem to require in the wild is a good supply of salt, and, like many herd animals, they tend to congregate around salt licks in groups of as many as forty, and their trails through the forest are often focussed around such areas. Away from the salt licks, however, their herds tend to be small, with no more than about ten individuals, mainly females and their young, accompanied by at most a single adult male. Unlike many antelopes, they are nocturnal, although perhaps slightly more active around dusk and dawn than through the middle of the night - naturally, this makes them even harder to study in the wild.

As a result, much of what we know about them comes from studies of captive animals in zoos, all of which belong to the Kenyan subspecies. From such observations we can confirm that their behaviour seems to be fairly typical for bovines, although it's obviously harder to know exactly what they'd do in their natural habitat. For instance, while we know that give birth to single calves after a nine month pregnancy, and that their mating habits are pretty much what we'd expect for related animals, we're not entirely sure when their wild breeding season is - it may well last longer under natural conditions in the tropics than it does in zoos in the temperate zone.

One thing we do know is that wild populations of bongo are declining. For the more widespread western bongos, while this is far from insignificant, it isn't drastic enough (yet) for the species to be considered truly threatened. For instance, a 1997 outbreak of infestation with stable flies in the Congo Republic resulted in a widespread die-off of bongos (and, to a lesser extent, other hoofed animals), but the effects seem to have been fairly localised.

The eastern subspecies has been far less lucky. Disease outbreaks in the later 19th and early 20th century dramatically reduced their numbers, while, in more recent times, loss of habitat to illegal logging enterprises, and increased hunting pressure, have been compounded by new infections, likely transmitted from nearby cattle herds. They now survive only in four forest patches, with a total population of no more than 140 adults, and maybe much less. Genetic tests on dung left behind by the animals suggests that their diversity is very low, presumably due to a high degree of inbreeding among the few survivors. Their low population, which is continuing to decline, suggests that, without drastic action, they may not last much longer in the wild, although there are a number currently living in zoos.

While many populations of sitatunga (Tragelaphus spekii) have, especially in the west, declined or even vanished, others have remained sufficiently large and stable that the species as a whole is likely doing better even than the western bongos. This likely has a lot to do with their preferred habitat, and their existence in large numbers in areas that are, from a human perspective, remote and inaccessible. This is because sitatunga, also known as "marshbuck", primarily live in swampland.

Indeed, sitatunga have such a strong preference for swamps, marshes, and other wetland that they are often described as "semi-aquatic" animals, spending much of their time in the water. They do, admittedly, venture out into neighbouring regions of relatively dry grassland, but only for a few hours at a time, always returning to the comfort of their swamps to rest. They are, as might be expected, excellent swimmers, and may travel for some distance while almost entirely submerged.

Sitatunga (female)
They are found throughout much of central Africa, from the coast of Benin, through the Congo Basin, as far east as western Kenya, and as far south as northern Botswana. That they once roamed even further is indicated by a few isolated populations elsewhere, including one found on the coast of Senegal and along the River Gambia. The males are not much smaller than bongos, standing about 100cm (3' 3") at the shoulder, but the females are dramatically so, being only around 80 cm (2'7") tall, and, at around 55 kg (120 lbs), weighing as little as 60% as much as their partners.

The males are a dull, greyish brown, and have long horns that are twisted like those of bongos, but lack the outward curve found in that species. The females, in contrast, have no horns at all, and have a much brighter, reddish, coat that makes the white stripes along their flanks more visible than they are in males.  Both sexes have physical adaptations to their watery habitat, of which the most obvious is a shaggy coat with an oily, waterproof, texture. Likely less apparent to the casual observer are the unusually long hooves that help them to walk on soft mud without sinking too far in.

Living where they do, they feed to a much greater extent on grasses and sedges than do bongos, along with waterside herbs and shrubs. In at least some areas, papyrus is a particular favourite, along with plants such as tropical knotweed. In places where agricultural land comes close enough to their native swamps, they will also eat maize and sweet potato, suggesting a fairly broad diet when alternatives to their usual food present themselves.

Sitatunga are definitely not herd animals; while groups of two or three adults may gather together, they spend as much as half their time alone, except for any young that they may have. They are most active around dawn and dusk, and least active during the heat of the day, when they use dense reed beds for cover from predators. (On the subject of which, while they probably have more to fear from crocodiles than most antelopes do, big cats are at least as big a threat). In another difference between the sexes, males are said to be noisier than females, often making loud barks or bellows when disturbed, while females are more likely to remain silent.

Although it's fairly difficult to catch them in the act out in the wild, mating seems to take place throughout the year. When she is ready to give birth, the female makes a bed of trampled reeds on a dry patch of ground concealed by dense swamp vegetation; the young remain in hiding there until they are able to venture out into the swamp proper.

One curious divergence from the usual pattern of the sitatunga lifestyle is found on Nkosi Island, at the extreme southern end of the Ssese Archipelago in Lake Victoria, Uganda. The island is tiny, and moreover, covered in dry, rocky, forest. Sitatunga were first discovered there in 1905, not long before the island, and its neighbours, were abandoned by humans in the wake of an epidemic of sleeping sickness. At the time, the animals were said to be so distinct from those on the mainland as to warrant description as a full subspecies. While that's very much a minority opinion today, the sitatunga of the island are clearly very isolated from their neighbours.

A visit to the island in 1925 found it almost entirely devoid of undergrowth, with the sitatunga apparently forced to rely on a few shrubs near the shoreline, and perhaps twigs and bark from the trees - all the remaining leaves being out of reach. Supposedly, there were over 100 antelopes on the island, but, given it's small size of 0.7 km2 (170 acres), it's hard to imagine that that's still true today, if, indeed, it ever was. The animals were, perhaps unsurprisingly, described as "mangy" and unfit, and to be remarkably silent. On the bright side, there was nothing on the island that might eat them, and this alone, perhaps, may have ensured their survival.

Bongo, and perhaps particularly sitatunga, are, due to their forested and swampy habitats, not often seen in the wild by European visitors. However, there is at least one other spiral-horned antelope that is highly ubiquitous, being found across wide swathes of sub-Saharan Africa, and a relatively common sight on safari trips. It is to that, and the remaining African species of bovine antelope, that I will turn next.

[Photos by "Chuckupd" and "Drow male" from Wikimedia Commons. Cladogram adapted from Bibi 2013 and Stankowich 2009.]

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