Blackbuck and gerenuk: similar females, extremely different males

Most gazelles (genera Gazella, Eudorcas, Nanger, Litocranius, Antilope, Ammodorcas, Antidorcas and Procapra) are only moderately sexually dimorphic. Adult males are not strikingly larger than adult females but possess horns, or at least larger horns.

However, the blackbuck (Antilope cervicapra, see https://www.gettyimages.com.au/detail/video/female-blackbuck-antelope-walks-through-pool-on-stock-video-footage/918341978?adppopup=true) presents an intriguing comparison with the gerenuk (Litocranius walleri, see https://www.inaturalist.org/observations/62024073).

In both, adult females (compare https://www.alamy.com/stock-photo-female-indian-blackbuck-antelope-antilope-cervicapra-138074045.html with https://www.alamy.com/stock-photo-gerenuk-female-walking-48605348.html) are hornless and weigh about 30 kg, and 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 stripe, both typical of gazelles, are minimal; and on the hindquarters the bold effect of most gazelles is lost because the pygal band is minimal, and the white on the buttocks is restricted (compare https://www.sciencephoto.com/media/389667/view or https://thewolfintelligencer.com/antelope-indian-blackbuck-antilope-cervicapra/#jp-carousel-10113 with https://www.inaturalist.org/observations/58316540).

Despite this uniformity of females and juveniles, the mature males could hardly differ more. In the gerenuk, males have modest horns and feminine colouration except on the crown of the head (see https://www.dreamstime.com/female-male-gerenuk-ong-necked-antelope-samburu-national-park-kenya-image137690107). In the blackbuck males grow extremely long, corkscrew horns; and their colouration is so converted into a whole-body dark-and-pale 'beacon' (see https://www.alamy.com/blackbuck-antelope-antilope-cervicapra-image279138319.html) that little remains of the pattern of gazelles. Even the face becomes so showily dark-and-pale (see https://www.gettyimages.com.au/detail/video/male-blackbuck-antelope-looks-around-on-grassland-stock-video-footage/918471144?adppopup=true) that it seems unrelated to 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 often appears 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 males of the blackbuck make no attempt to hide from predators, whereas those of the gerenuk remain as secretive as possible.

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

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Another difference is that the blackbuck is touch-averse, keeping 'personal distance' even in courtship, whereas the gerenuk is not touch-averse (see https://www.facebook.com/watch/?v=951809542260525).

Posted by milewski 3 months ago (Flag)

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