Water location, piospheres and a review of evolution in African ruminants

Abstract

The main radiation of large mammalian herbivores in Africa took place in the Pliocene–Pleistocene, when a long-term trend towards aridification promoted grasslands and the diversification of ruminant grazers. Traditional models of this evolution identify habitat fragmentation in response to climate change as the primary cause of speciation and diversification. However, with their adaptation to poorer and drier diets, these animals incurred a cost: an increase in water requirements to aid digestion and to thermoregulate their larger body size. Water requirements are included in the current model, relating this foremost physiological need to an environmental resource: habitat fragments would have contained sources of drinking water for the persistence of water-dependent species. However, the location of drinking water and its influences are subsumed within the extent of those fragments; their roles in habitat fragmentation and the subsequent evolutionary processes are less clear. The current model neglects to consider the interaction of water dependency with the location of drinking water; increased aridification would not only have involved shifts in the spatial distribution and species composition of forage resources, but it would also have increased the patchiness of surface water distributions. Animal foraging distributions would have been constrained within species-specific distances of their drinking water over an evolutionary timescale, distances that were not necessarily in accord with the extent of their potential habitat as solely defined by forage distributions, particularly as animals became more dependent on water with the increase in grazing and its associated traits. Drinking-water location exerts a dominant influence on animal survival and reproduction, suggesting that these roles of drinking-water location need to be incorporated into existing models of large mammalian herbivore evolution in Africa.

Keywords: bovid speciation and diversification; climate change; grazer evolution; niche construction; utilisation gradients

African Journal of Range & Forage Science 2008, 25(2): 79–92