February 26, 2014 | 41
There are lots of reasons for liking wildebeest… or gnus. For me, the main one comes from the fact that they are insanely flamboyant in appearance. Check out all the stuff we have going on in the best known and most widespread of the two (read on) species, the Brindled or Blue wildebeest Connochaetes taurinus: both the dorsal mane and shaggy tail are black are hence well demarcated from the rest of the body, the muzzle is also black, the legs are sometimes brown and thus also differentiated from the grey body, vertical streaks on the neck and body form obvious pseudo-stripes, and there’s a beard and throat mane as well (formed of black, white or cream hairs, depending on the population). Slender-tipped horns curve outwards and upwards. [Adjacent photo by Chris Eason.]
As for the other species – the White-tailed or Black wildebeest (or gnu) C. gnou – why, it’s even more flamboyant. The long, luxuriant tail is brilliant white, the dorsal mane is erect, with white hairs tipped with black, and there’s a black beard, muzzle crest and chest tuft. Slender, strongly curved horns sweep upwards and forwards, and highly active preorbital glands discharge liquid secretion into the muzzle tuft and beard (Kingdon 1997). Pedal, interdigital glands secrete scent as well (in both species), and Black wildebeest also increase their pungency during the rutting season by rolling in their own urine and dung. Incidentally, hybrids between the two kinds are known (von Richter 1974), though they only seem to have occurred in semi-captive settings. They’re fertile.
Rutting male wildebeest show off their flamboyant bodies by posing, head-shaking and cavorting. In fact, a repertoire of about 30 actions are available when one wildebeest challenges another; one of the two makes mock-grazing actions, may rub his neck on the opponent’s rump, present his body in profile, perform the head-up, so-called ‘alarm display’, or drop down onto his ‘knees’ (wrists, of course) to do more mock-grazing or initiate a fight (Spinage 1986). They also make noisy exclamations. During the rutt, wildebeest defend marked-out territorial patches, the aim being to get oestrous females to stay in their company.
‘Male-mimic’ females – why?
However, lest we think that all the remarkable features of wildebeest are display ornaments specifically designed by evolution to lure in female attention, they’re present in females too, so – what gives? Because females need to defend themselves and their calves from the males whose territories they have to pass through, from predators, and from competing females, one popular suggestion for these antelopes is that females are ‘male mimics’ that require all that ornament and armament for self-defence, both from other wildebeest and from predators (Stankovich & Caro 2009). If this is true, some aspects of female wildebeest appearance are driven more by the pressures of natural selection than sexual selection. [Adjacent image by Yathin S Krishnappa.]
But while this might explain body size and horn form, why do females have the showy manes, tassels and flowing tails and so on? This hasn’t really been explained – after all, ornaments of this sort are lacking in the females of some other bovid species where the males have them. Maybe they persist through natural selection because they’re crucial whatever sex you are, or maybe they’re the product of genetic correlation between the sexes. Or maybe there’s something really interesting going on. Estes (2000) has proposed the so-called andromimicry hypothesis, suggesting that females resemble males such that their maturing sons are disguised within the herd, thus avoiding the aggressive, territorial attention of sexually mature males, and thus not being driven out of the herd and subjected to the high rates of mortality that would result. It’s certainly an intriguing hypothesis.
Flexed faces and the wildebeest of the past
Wildebeest are anatomically interesting for other reasons. Despite looking like cattle, they’re antelopes, and specifically alcelaphines. That is, they’re members of the same long-snouted bovid group as the Hirola Beatragus hunteri, Bontebok Damaliscus dorcas, Topi D. lunatus and the hartebeests (Alcelaphus). But whereas other alcelaphines have a fairly slender neck and a head usually held well above the level of the shoulders, wildebeest have a bull-necked, low-slung appearance. Does this relate specifically to any aspect of their skeletal anatomy? I don’t think that there any studies that might shed light on this, though the tall shoulders have been linked to the evolution of an efficient (but not especially fast) cantering gait. [Adjacent image by Dilly Lama.]
Clearly, wildebeest are strongly specialised for grazing on short, ground-hugging grasses. They actually struggle to feel on tall grasses and Kingdon (1997) suggested that the widespread presence of such in otherwise suitable habitat across equatorial Africa explains their absence in these regions. By the way, while we typically think of Brindled wildebeest as migratory, this is not true in places where climate, soil fertility and so on promotes the perpetual, reliable presence of a year-round supply of healthy, short grasses. So wildebeest are sedentary in Ngorongoro Crater in Tanzania and a few other places.
Brindled wildebeest were more widespread in the recent past than they are today. We know that they were present in far northern Africa during the Late Pleistocene (Gentry 1978) and perhaps during part of the Holocene, but then this was true of most of the megafauna now generally restricted to sub-Saharan regions. An extinct Pleistocene wildebeest, C. antiquus, has been said to be similar to the Black wildebeest in the form of its horn bases, but the more distal parts of the horns are intermediate between those of Black and Brindled wildebeest (von Richter 1974). Until recently, Black wildebeest were famously restricted to managed game reserves and ranches in Orange Free State and the Transvaal where they were essentially saved from extinction by farmers. Hunting and habitat loss made them extinct in the wild by about 1900. They’ve now been reintroduced throughout South Africa, Lesotho and Swaziland, and also introduced into Kenya and Namibia. Fossils show that the Black wildebeest once occurred as far south as Hermanus in the southern Cape (von Richter 1974).
The wildebeest muzzle and mouth are broad and squared-off. Large, hair-lined flaps cover the nostrils and the long axis of the face is strongly down-turned relative to the axis of the braincase. The flexed form of the wildebeest face isn’t just there because the animals are standing with their noses pointing downwards. Rather, the face really is pointing downwards instead of forwards (this isn’t a unique feature of wildebeests: it’s true of most of the other alcelaphines as well). Wildebeest are also unusual in having proportionally large lateral hooves (the two smaller digits on either side of the central pair).
We think of wildebeest as a ‘poor’ group in taxonomic terms, since there are only two generally recognised named species, both included in the same genus. However, the two have been considered distinct enough at times that they’ve been given their own subgenera. In 1850, John Gray created the name Gorgon for the Brindled wildebeest (the Black wildebeest is the type species for the name Connochaetes Lichtenstein, 1814). Also of interest is the fact that the name Catoblepas was suggested for wildebeest at one point – a name based on a heavy-headed, sluggish, scaly mythical beast of ‘Ethiopia’, mentioned by Pliny the Elder and said to have the power to turn viewers to stone with its gaze. Needless to say, wildebeest might have had something to do with the origin of this creature.
The Brindled wildebeest: a species complex?
As for that low number of species… Well, as with so many big, widespread mammals, variation observed across the different populations has been reflected in numerous proposals of new species and subspecies. Four ‘subspecies’ have been suggested for the Black wildebeest (none of which are considered currently valid), and 15 for the Brindled wildebeest, four or five of which are still in use. According to Kingdon (1997), these are the nominal Blue wildebeest C. t. taurinus, the Nyassa or Cookson’s wildebeest C. t. cooksoni of the Luangwa Valley, the Mozambique wildebeest C. t. johnstoni of the south-east [image below by Nevit Dilmen], the White-bearded wildebeest C. t. albojubatus of southern Kenya and northern Tanzania, and Heck’s wildebeest C. t. hecki of the western part of the Rift Valley.
However, those of you interested in hoofed mammals will know that Colin Groves and Peter Grubb have recently made an enormous number of suggested taxonomic recommendations in their book Ungulate Taxonomy (Groves & Grubb 2011). Therein, they argue that the Brindled wildebeest is actually a complex containing four species that can be reliably distinguished on the basis of cranial morphometrics. They recognised C. taurinus and C. johnstoni in the south, and C. albojubatus and C. mearnsi in the north. The two southern species, they say, have far wider horn-spans than the northern ones, with C. mearnsi being especially narrow across the horns. C. mearnsi is also especially small, dark and long-tailed compared to the others. C. johnstoni is also unusual in having especially long nasal bones and in sometimes possessing a white facial crescent between or below the eyes (Groves & Grubb 2011) [adjacent image by Nevit Dilmen]. These forms also differ in whether their manes are erect or drooping, and they differ in various aspects of their colouration, too. [Image below from German Federal Archive.]
As of yet, I don’t see any indication that these taxonomic recommendations have caught on, and it’s difficult to say if they will (when you do find specific comments on the Groves and Grubb taxonomy – as you do in Lorenzen et al. (2012) – it’s usually something along the lines of “we have not followed their species delimitations”!). And, as is usually the case with suggestions of this sort, arguments over whether these clearly distinct populations really are ‘distinct enough’ to be regarded as ‘species’ is generally subjective and dependent on the philosophy of the scientist concerned. It doesn’t really make any difference either way. I do think that we should try to be consistent across groups of animals, however. [Map below by Cephas.]
So – are these different forms of wildebeest similar in their degree of distinction (I mean, in molecular or morphometric terms) to other bovid taxa conventionally regarded as species? Arctander et al. (1999) found a distinct population structure in the Brindled wildebeest, with the northern (= east African) populations showing evidence of having descended from the southern ones. This pattern probably shows that living Brindled wildebeest are ancestrally southern animals that invaded eastern Africa relatively recently (within the last million years). However, the complication with this is that the oldest Brindled wildebeest fossils (dating to about 1.5 million years ago) are from east Africa meaning that – if the genetic results from the modern populations have been interpreted correctly – the old east African population(s) became extinct, the species (or species complex!) then persisting only in a southern refugium, and only later re-invading the old part of its range (as well as new ones, like the far north of Africa). Patterns of this sort have been suggested for several African bovid lineages (Arctander et al. 1999, Flagstaff et al. 2001), so this is actually familiar stuff.
And that, my friends, is how a couple of picture and text saying “Look: wildebeest!” turns into a Tet Zoo article. Grrr.
For previous Tet Zoo articles on bovids, see…
Refs – -
Arctander, P., Johansen, C. & Coutellec-Vreto, M.-A. 1999. Phylogeography of three closely related African bovids (tribe Alcelaphini). Molecular Biology and Evolution 16, 1724-1739.
Estes, R. D. 2000. The evolution of conspicuous coloration in the family Bovidae: female mimicry of male secondary characters as catalyst. In Vrba, E. & Schaller, G. S. (eds) Antelopes, Deer, and Relatives. Yale University Press, New Haven, pp. 234-246.
Flagstad, Ø., Syvertsen, P. O., Stenseth, N. C. & Jakobsen, K. S. 2001. Environmental change and rates of evolution: the phylogeographic pattern within the hartebeest complex as related to climatic variation. Proceedings of the Royal Society of London B 268, 667-677.
Gentry, A. W. 1978. Bovidae. In Maglio, V. J. & Cooke, H. B. S. (eds). Evolution of African Mammals. Harvard University Press, Cambridge, pp. 54-572.
Groves, C. & Grubb, P. 2011. Ungulate Taxonomy. The John Hopkins University Press, Baltimore.
Kingdon, J. 1997. The Kingdon Field Guide to African Mammals. Academic Press, San Diego.
Lorenzen, E. D., Heller, R. & Siegismund, H. R. 2012. Comparative phylogeography of African savannah ungulates. Molecular Ecology 21, 3656-3670.
Spinage, C. A. 1986. The Natural History of Antelopes. Christopher Helm, London.
Stankovich, T. & Caro, T. 2009. Evolution of weaponry in female bovids. Proceedings of the Royal Society B 276, 4329-433.
von Richter, W. von 1974. Connochaetes gnou. Mammalian Species 50, 1-6.