Coexistence and adaptive shifts in gazelles

Gazelles are antilopin bovids belonging to the genera Gazella, Eudorcas, Nanger, Litocranius, Ammodorcas, Antidorcas, Antilope and Procapra. In general, only one species of gazelle occurs in any given habitat. Where coexistence is achieved, this tends to be by means of regional shifts in body-size and -shape which correspond partly to differentiation into subspecies.

The best-known example is in the Serengeti ecosystem. Thomson's gazelle (Eudorcas thomsoni nasalis) and Grant's gazelle (Nanger granti granti) coexist here (see https://www.inaturalist.org/observations/33111207 and https://www.youtube.com/watch?v=LBLUtuw964I) by means of divergent body masses, adults of the former subspecies weighing less than half the average for the latter subspecies in the case of females, and about a third in the case of males. The local form of Thomson's gazelle is the smallest of its genus while the local form of Grant's gazelle is the largest of its species. There is also divergence in water-dependence and movement patterns, with the two forms migrating in opposite directions and the smaller form being unusually dependent on drinking for a gazelle.

The dorcas gazelle (Gazella dorcas, see https://www.gettyimages.com.au/detail/video/dorcas-gazelle-walking-in-arid-desert-stock-video-footage/1070908712) and the dama gazelle (Nanger dama, see https://www.pond5.com/stock-footage/item/111832882-dama-or-mhorr-gazelle-al-ain-zoo-nanger-dama-mhorr) coexisted until recently on the southern and western fringes of the Sahara. This was achieved mainly by means of an extreme difference in body sizes and the maximum height of foraging. The former species is the smallest gazelle In Africa while the latter is the largest of all antilopin bovids. In Arabia, the particularly diminutive local form of the dorcas gazelle (Gazella saudiya) has been hunted to extinction while the sand gazelle (Gazella marica, see https://www.biolib.cz/en/taxonimage/id171996/?taxonid=509466&type=1) has survived. These forms differ little in body sizes, suggesting that the Saudi gazelle was more restricted in substrate type and vegetation type than was the case in North Africa, limiting its population even before persecution by humans.

Bearing these patterns in mind, it is interesting to consider how coexistence has been achieved among certain poorly-known gazelles on the Horn of Africa (e.g. see https://www.inaturalist.org/observations/55092041). The gerenuk (Litocranius walleri) coexists as a large northern subspecies with Soemmerring's gazelle (Nanger soemmerringi) and as a small southern subspecies with Grant's gazelle (see https://www.inaturalist.org/observations/44453752). Notwithstanding the greater specialisation of the gerenuk for bipedal foraging, it is puzzling that, in e.g. Tsavo National Park, Peter's subspecies of Grant's gazelle (Nanger granti petersi) has similar body mass (a possible average of 30-35 kg for adult females) to the local form of the gerenuk. A similar puzzle, at larger body sizes, may apply to Soemmerring's gazelle and the gerenuk in Somaliland, a destination for adventurous naturalists.

Publicado por milewski milewski, 13 de junio de 2021

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