The Minimum Water Requirements of Some East African Bovids

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Several of the wild East African bovids are reported to be capable of surviving in aridregions without drinking. Minimum amounts of water required by eland, oryx andGrant's gazelle, whose ranges include the hot arid sub-Sahara, are compared with theamounts required by buffalo, wildebeest and Thomson's gazelle, whose ranges arelimited by the availability of water. Frequent droughts have provided a severe selection pressure for frugal use of water during the relatively short period in which zebu cattle have been resident in East Africa. Thus the water requirements of these animals provide a yardstick for evaluating the rate at which water conserving mechanisms evolve, while the water requirements of the temperate Hereford steer serve as a baseline against which to judge the efficiency of these mechanisms in other species. Studies are carried out with and without an intermittent heat load, enabling one to evaluate adaptations for dealing with both heat and aridity. Oryx, Grant's gazelle, Thomson's gazelle and zebu cattle are found to be specialized for conserving water and can maintain a constant weight on a total water input of approximately 2 % of their body weight/day (in an environment of 22°C). Hereford steers require more than twice this amount of water_ Zebu cattle are more specialized physiologically for coping with arid conditions than the eland, wildebeest or buffalo. The eland, however, is able to gain independence of surface water by behavioural and physiological means other than conservation of water. In contrast, the Thomson's gazelle, which requires extremely small amounts of water, is unable to inhabit the hot arid regions of East Africa for reasons yet to be explained. The development of water sparing mechanisms is only one aspect of the complex of adaptations which determines the limits of a species' range.
Systematic Zoology (No 21), p. 195-206