Gazelles are surprisingly variable in sexual dimorphism

Most gazelles (genera Gazella, Eudorcas, Nanger, Litocranius, Antilope, Ammodorcas and Antidorcas) are only moderately sexually dimorphic. The adult male is not strikingly larger than the adult female but possesses horns, or at least larger horns.

However, the blackbuck (Antilope cervicapra) presents an intriguing comparison with the gerenuk (Litocranius walleri). In both, the adult female is hornless and weighs about 30 kg, and the female colouration is the least conspicuous among gazelles. The dark flank-band typical of gazelles has been lost; there is a pale horizontal stripe along the upper flank; there is pale around the eye but the pale facial stripe and dark malar stipe, both typical of gazelles, are minimal; and on the hindquarters the bold effect of most gazelles is lost because the pygal band is minimal, the white on posterior haunch is restricted, and the tail-tassel is small/nondescript.

Despite this uniformity of females and juveniles, the mature males could hardly differ more. In the gerenuk the male has modest horns and feminine colouration except on the crown of the head. In the blackbuck the male grows extremely long, corkscrew horns; and his colouration is so converted into a whole-body dark-and-pale 'beacon' that little remains of the pattern of gazelles. Even the face becomes so showily dark-and-pale that it looks categorically different from that of the gerenuk and most gazelles.

This correlates with the ecological and social differences. The blackbuck is a specialised grazer which drinks frequently and concentrates in large groups, whereas the gerenuk is a specialised browser which can forgo drinking for years and is often solitary. Both species are territorial, but in divergent ways.

In the blackbuck, territories are so small, crowded and hectically defended that the competing males show off to each other for most of the time. They alternate this with visual appeasement, because they can forage only by trespassing their way to nearby, untrampled pastures, excusing themselves gesturally along the way there and back. In the gerenuk, the territories are so large that males seldom even see each other. Not only do they not need to trespass, but they only ever patrol a limited central part of the territory - using smell rather than sight.

What this means is that - despite females being so similar - males have social modes so different that the male blackbuck makes no attempt to hide from predators, whereas in the gerenuk the male remains thoroughly secretive.

And the male genitalia differ too, despite the fact that in both species the courting male following the female by walking bipedally, unsupported by the forefeet. In the blackbuck, the penis extends far forward; in the gerenuk it is the scrotum that instead seems shifted forward. It is understandable that the gerenuk avoids pressing the testicles between the hind legs while foraging upright. But the penile aberration in the blackbuck is only partly explained by the fact that this species is unusually touch-averse among gazelles -even in sexual ardour.

Posted by milewski milewski, April 05, 2021 02:34

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