Physiological adaptation in savanna ungulates.

Savannas lie between the equatorial rain forests and the semi-deserts of the southern continents. From the herbivore’s viewpoint, the savanna environment is characterized by a mixture of herbaceous and woody vegetation, varying in quantity and growth stage according to the seasonal cycle of rainfall. C4 grasses (i.e. those with the C4 photosynthetic pathway) dominate the herbaceous layer, which otherwise comprises dicotyledenous forbs; the woody component consists of fire-tolerant trees and shrubs. The intake of grasses is constrained by their low abundance early in the growing season and by lignification and low digestibility subsequently. The other vegetation components of savanna commonly contain secondary metabolites, such as tannins, in their currently-growing tissue, or are heavily lignified in structural tissue. The phenology of grasses is strongly associated with the incidence of the rainy season, whereas woody browse bears green leaf both before and after the rains. The N concentration in grasses is generally lower than that in browse, especially in leguminous browse (Fig. l), and may be lower in the stem and dead tissue of grasses than the effective rumen degradable protein : fermentable metabolizable energy required by rumen microbes (Agricultural and Food Research Council, 1993). The present paper reviews the problems of the ruminant fauna of the African savannas in extracting energy and nutrients from their food sources. It aims to set the hypothetical physiological adaptations in an ecological context. The contrasts between foregut and hindgut digestive strategies, respectively those of ruminant and non-ruminant ungulates, are discussed elsewhere (Illius & Gordon, 1992).