influenced by soil type, soil fertility, vegetation cover, and distribution of water
(Parer 1987).High temperatures are often combined with high solar radiation and restricted water
supplies. In high-rainfall areas the last factor is important for restricting distribution;
in arid regions all three have interrelated effects on animals. These effects are
expressed as heat loads built up in the body, and there are various adaptations to
overcome them.
Adaptations to high temperatures include behavioral responses such as using
shade in the middle of the day and restricting feeding to the hours of darkness. Both
eland (Taurotragus oryx) and impala (Aepyceros melampus) reduce heat stress by feed-
ing at night in East Africa (Taylor 1968a). At the driest times of year both species
boost water intake by switching from grazing grasses and forbs to browsing on suc-
culent shrubs (Taylor 1969; Jarman 1973).
Solar radiation restricts the movements of animals that are large and that have dark
coats. Elephant and buffalo are examples where they seek shade in the heat of the
day to cool off (Sinclair 1977). Coat color and structure can reduce heat loads. The
lighter tan-colored coat of hartebeest (Alcelaphus buselaphus) reflects 42% of short-
wave solar radiation as against only 22% for the darker coat of eland. In both species
re-radiation of long-wave thermal radiation is greater than that absorbed, and this
represented 75% of total heat loss (Finch 1972).
High heat loads can be avoided by sweating when water is abundant. African
buffalo, eland, and waterbuck use sweating for evaporative cooling (Taylor 1968a;
Taylor et al. 1969b). Buffalo keep body temperature in the range 37.4–39.3°C and
allow body temperature to rise to 40°C only when water is restricted. They cannot
reduce water loss from sweating when water is restricted (Taylor 1970a,b).
Waterbuck show similar physiological adaptations. When water is restricted for
12 hours at 40°C ambient (environmental) temperature they lose 12% of their body
weight compared with the 2% for beisa oryx (Oryx beisa) which is a desert-adapted
species (Taylor et al. 1969b). As a consequence both buffalo and waterbuck must
remain within a day’s walk of surface water.
Large animals can afford to lose water by sweating but smaller animals such as the
gazelles cannot. They employ panting instead, as do species in arid areas (e.g. the
beisa oryx) or species on open plains with high solar radiation, such as wildebeest
(Robertshaw and Taylor 1969; Taylor et al. 1969a; Maloiy 1973).
Some species can adapt to extreme arid conditions by allowing their body tem-
perature to rise before they start panting: up to 43°C for Thomson’s gazelle (Gazella
thomsonii) and 46°C for Grant’s gazelle (G.granti) (Taylor 1972). Other adaptations
for water conservation include restriction of urine output, concentrating the urine,
and reabsorbing water from the feces. Dikdik, a very small antelope that lives in semi-
arid scrub away from water, had the lowest fecal water content and the highest urine
concentration of all antelopes, followed by hartebeest, impala, and eland (Maloiy 1973).
Grazing ungulates in Africa are restricted to areas within reach of surface water
and all show behavioral adaptations such as night feeding or migration (Sinclair 1983).
Those that can do without water are all browsers (Western 1975). Beisa oryx and
Grant’s gazelle select hygroscopic shrubs (Dispermaspecies). They eat them at night
because these shrubs contain only 1% free water in the day but absorb water from
the air at night to boost the water content of the leaves to 43% (Taylor 1968b).96 Chapter 7
7.4.2Range limited
by water loss and
heat stress