All the World's Antelopes: Duikers of Bush and Forest

Common duiker

While the term "antelope" probably conjures up the image of something like a gazelle or a blackbuck, a great many species are much smaller than these animals. Often collectively referred to as "dwarf antelopes", they are scattered across the wider antelope family tree, belonging to several different groups. Being small and relatively inconspicuous helps when you're a herbivore, even if it isn't the only route to survival.

In terms of number of species, the duikers, technically referred to as the subfamily Cephalophinae, are one of the more successful groups of such small antelopes. The exact number of duiker species has fluctuated widely over the years, with some estimates reaching as high as 25. The latest version of the Mammal Diversity Database, however, lists 21, so that is what I am going with.

The best-known species is the bush duiker (Sylvicapra grimmia), also known as the "common duiker". It is the most widespread of all antelopes, being found through essentially the whole of sub-Saharan Africa outside of the rainforests. It was also the first species to be described, back in 1758, when (like all antelopes at the time) it was listed as a type of goat.

Perhaps ironically given its "common" moniker, it is also arguably the least typical of all the duiker species. Like other duikers, it is a small antelope, standing between 39 and 68 cm (15 to 27 inches) tall at the shoulder. This is taller than many other duiker species, due to it having proportionately longer legs, although the body is otherwise similar in form. The colour varies from grey to yellowish-fawn across different parts of its wide range, and males have straight, vertical horns, typically around 10 cm (4 inches) in length.

The reason for these longer legs is that, as its name suggests, the bush duiker lives in moderately open country. Living across so much of Africa, the details of this vary considerably, including bush, veldt, and savannah, but they avoid dense forests just as much as they do inhospitable deserts. They have even been found close to the snowline at 4,300 metres (14,000 feet) elevation on Mount Kenya. Having said that, they do require longer vegetation to escape into, and so will avoid barren areas or those covered only with short grass. 

Tall grass and scattered ferns are ideal, but, while they are unlikely to forage in wooded areas, any shrubby vegetation will suffice. Although they can jump up to 1.8 metres (6 feet) if they have to, when threatened, they are more likely to dart for cover, ducking below any suitable obstacles that might impede a predator. This is where the name comes from, since it is the Dutch/Afrikaans word for "diver". (In English, if you're wondering, it rhymes with 'biker', although it doesn't in the original language.)

They are browsing animals, feeding on a wide variety of low vegetation, mostly leaves and stems, although they will pull up some plants to get at the roots. In the drier parts of their range, on the edge of the Kalahari, they eat melons for their water; otherwise, while they can get by for several days without drinking, they will take water whenever they get the opportunity. Unusually for an antelope, they have also been recorded intentionally eating animal matter (as opposed to the odd insect that happens to be sitting on a leaf), and even hunting some small ground-dwelling birds.

Like most small antelopes, they are solitary, meeting up for just a few days each year when the females come into heat. Both sexes have scent glands in front of their eyes with prominent slit-shaped openings, and the exact scent produced varies between males and females. Males leave their scent on branches, rocks, and other landmarks, marking out territories of around 0.25 km² (60 acres) each, which they defend through combat with their rivals. Having said which, if they aren't competing for mating opportunities, bush duikers are quite friendly with one another, grooming each other if they happen to meet up and hanging around together for several hours before parting again.

There is no particular breeding season, although births are most common around the start of the rainy season - the timing of which varies depending on where in Africa they happen to be. Otherwise, their reproduction is similar to that of other antelopes. Courtship consists of the male chasing the female until she relents and slows down, advertising her receptivity with urine scent marks. They give birth to a single young about 200 days later, hiding it in cover and browsing nearby, only returning to allow it to suckle. 

Blue duiker

The blue duiker (Philantomba monticola) is the smallest of all duiker species, standing just 30 to 38 cm (12 to 15 inches) tall at the shoulder. The colour is somewhat variable, although describing it as "blue" is perhaps something of a stretch. It is most typically a brownish grey, but this varies between subspecies, with some having reddish flanks, which may be vaguely or clearly demarcated from the more typically coloured rump. Both sexes have horns, but they are only 3 to 5 cm (1 to 2 inches) long in the males, and even shorter in the females.

It is found across central Africa, from Nigeria to Angola on the Atlantic coast and from Kenya to Mozambique on the eastern side of the continent. Two subspecies live in an entirely separate area along the east coast of South Africa, and have sometimes been considered to represent a distinct species, although this is not followed by most modern sources. 

As with all duikers other than the bush species, they are forest dwellers. They inhabit a wide range of different forest types, from the dense jungles of the Congo to the more scattered low-reaching woodlands of black witch-hazel in South Africa. They tend to prefer forests with a good mix of tree types, and are most commonly found foraging in clearings where there is comparatively little undergrowth, but nearby cover is still easy to reach.

Blue duikers prefer to feed on freshly fallen fruit and leaves, although, especially in South Africa, where the forests are less dense, they will also feed directly from trees and bushes. The specific fruit that they eat varies with locality, but studies have identified those from umbrella trees, manguella, and African nutmeg as favourites in some parts of their range.

Like other duikers, they are diurnal, but are most active shortly after dawn and just before sunset. They are usually described as solitary, although reports of apparently mated pairs are more common than that might suggest - the fact that they are small animals prone to hiding in undergrowth may also mean that such pairs are underreported. Indeed, pairs are most often recorded from nighttime surveys, where they can be spotted sleeping next to one another, rather than when they are foraging during the daytime.

Population estimates are similarly unreliable, varying depending on what method is used, but they seem to be quite numerous in most places they are found. They typically range over areas as small as three hectares (7½ acres), although males may make short-range forays beyond. Both sexes are territorial, although males scent-mark more often than females, perhaps indicating a more defensive attitude. Even young leaving home to set up their own territories only travel about 1.5 km (just under a mile) before settling down but even so, humans disrupting their forests can have a negative effect.

When alarmed, they are most likely to freeze in the first instance. They let out a shrill alarm call if they feel the need to flee, sometimes adding to this by thumping the ground with their hooves as they set off. When this does happen, however, they don't run for long, taking refuge in nearby undergrowth at the earliest opportunity.

Maxwell's duiker

Maxwell's duiker (Philantomba maxwellii) is very similar to its close relative. It is slightly larger, but is perhaps most readily distinguished by the presence of two reddish stripes on the head, although these become fainter with age. It lives in the forests of West Africa, from Senegal to Ghana, living in much the same habitat as the blue duiker. Its diet is similar too, with estimates suggesting that at least three-quarters of it consists of fruits and legumes, most often freshly fallen from trees rather than plucked. Often, they are observed hanging around underneath groups of monkeys foraging in the trees, picking up whatever gets dropped. The remainder of the diet consists of a mix of leaves, fungi, and flowers.

The evidence for living in mated pairs is stronger with Maxwell's species as well. While each member of a pair has its own separate territory, these overlap by about 80% with each other and by less than 1% with those of their other neighbours. These are around the same size as those of blue duikers, with the animals showing a similar aversion to travel. Males mark the boundary of their territories with dung and other scent marks, and are highly aggressive in defending them. 

With most antelopes, the defender will engage in some form of ritualised posturing to deter intruders before any energy is wasted on fighting. But Maxwell's duiker doesn't seem to have time for that; they charge straight in to headbutt each other with such force that one combatant is often flipped into the air. The following fight is intense but brief, with at most two minutes before the loser makes a run for it, angrily pursued by the victor until he is well out of range.

In 2010, a review of the animals at the eastern end of Maxwell's duiker's range identified them as a new species. This decision was made on the basis of clear differences in the skull shape (most notably, a narrower nose) but was also supported by genetic evidence that has since been confirmed. As a result, we recognise Walter's duiker (Philantomba walteri) as distinct from Maxwell's, despite its similar appearance. It is slightly smaller, but otherwise hard to distinguish without detailed skull measurements or genetic data.

It lives in Togo, Benin, and western Nigeria, although the full extent of its range is unclear, as so few individuals have been definitively identified - a 2016 review was only able to find mention of 41. For the same reason, and because it has only been so recently identified as a species, we know little about it as distinct from its more westerly relative, although one 2022 study may indicate a greater willingness to eat leaves rather than fruit. This may be because the area it lives in is a wide stretch of savannah and scrubland between denser forests, so suitable fruit trees may simply be less available.

The other three species I have mentioned are all common and at least reasonably widespread. With so few individuals located, we don't really know about the conservation status of Walter's species, although the fact that most of them were found in bushmeat markets is hardly promising.

There are, however, as many as 17 other species of duiker, and next time, I will be taking a look at some of them.

[Photos by "Chuckupd", Derek Keats, and Charles J. Sharp, from Wikimedia Commons. Cladogram adapted from Ghassemi-Khademi and Hamidi, 2019.]