Why the adaptive radiation of antilopins on the Horn of Africa?

Antilopin bovids range widely in dry climates in Africa and Asia, but their greatest concentration of genera and species occurs in arid to semi-arid northeastern Africa (Somalia, Somaliland, Djibouti, Eritrea and parts of Sudan, Ethiopia and Kenya). In an area the size of France plus Spain located just north of the equator, seven species of gazelles, the beira (Dorcatragus megalotis), and nine species/subspecies of dikdiks (Madoqua) occur. This is more than all the species of bovids, antilocaprids and deer in the whole of the United States of America.

One clue to the reasons for such diversity is the poverty of antilopins in the similarly extensive arid to semi-arid climate in southern Africa, just south of the tropic of Capricorn, where only two species of antilopins occur in the relevant parts of Namibia, Botswana and South Africa. These are the springbok (Antidorcas marsupialis, which is a type of gazelle) and the steenbok (Raphicerus campestris, which is intermediate in size between dikdiks and the beira).

That is a dozen species versus a couple , in ostensibly similar environments on the same continent.

The antilopins of arid to semi-arid northeastern Africa include some of the most peculiar and specialised of ruminants. The gerenuk (Litocranius) has an exceptionally small face, an unrivalled ability to free-stand upright on its hind hooves, and such extreme economy of water that it refuses to drink even when raised in zoos. The smallest species of dikdik is the smallest of all ungulates, worldwide, that live in semi-desert. And Speke's gazelle (Gazella spekei) has an oddly inflatable nose even compared to its closest relatives within the same genus.

Both parts of Africa have similar mean annual rainfall, but a crucial difference is this. Whereas the dry parts of southern Africa have a single rainy season each year, those of northeastern Africa tend to have two rainy seasons each year owing to the East African Monsoon. This means that plant growth tends to be more reliable in the northeast than in the southwest of the continent, allowing greater specialisation of body sizes and shapes, diets, and foraging heights, and thus ecological niches. Whereas in general arid climates mean that 'beggars cannot be choosers' and survival depends on versatility, the antilopins of northeastern Africa include some of the choosiest species of ungulates known with respect to type of terrain, vegetation and diet.

Posted by milewski milewski, April 11, 2021 04:47

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