Mineral nutrition and seasonal movements of African migratory ungulates

Abstract

TWO distinct foraging behaviours differentiate Africa's grazing ungulates^1–3: over the wet–dry season cycle resident herds occupy seasonally stable and spatially restricted home ranges and migratory herds move over vast geographical regions, using different areas in different seasons. Migratory herds dominate animal biomass wherever they occur⁴. Some migrants congregate around permanent water sources in the dry season and disperse to drier habitats in the wet season⁵; others travel along rainfall gradients, concentrating in areas of low annual rainfall in the wet season and moving to areas of higher rainfall as the dry season advances⁴. The Serengeti National Park in Tanzania is typical of the latter type of ecosystem⁶; migratory grazers there select drier localities in the wet season, despite the fact that forage production rates and drinking water supplies are high everywhere. These migrations could be explained by the animals' avoidance of muddy, sticky soils⁷, general forage-quality properties⁸, by calcium requirementsof lactating females⁹ and by avoidance of woodland habitats with greater predation risk¹⁰. Resident grazers are concentrated in localities supporting grasses with mineral contents more capable of meeting dietary requirements than forages in nearby, little-used grasslands¹¹. I now present evidence that seasonal movements of migratory grazers in the Serengeti ecosystem are also related to grass mineral content. Particularly important, based on forage concentrations and animal requirements¹², are Ca, Cu, N, Na, Zn and also, for lactating females and growing young, Mg, P and Ca/P balance.