The allometry of patch selection in ruminants

Abstract

An axiomatic feature of food consumption by animals is that intake rate and prey abundance are positively related.While this has been demonstrated rigorously for large herbivores, it is apparent from patch selection trials that grazers paradoxically tend to prefer short, sparse swards to tall, dense swards. Indeed, migratory herbivores often shift from areas of high to low sward biomass during the growing season. As nutritional quality is an inverse function of grass abundance, herbivores appear to sacri¢ce short-term intake for nutri-tional gains obtainable by eating sparse forage of higher quality. Explicit models of this trade-o ¡ suggest that individual ruminants maximize daily rates of energy gain by choosing immature swards of inter-mediate biomass. As body mass is related positively to both ruminant cropping rates and digestibility, there should be an allometric link between grass abundance and energy maximization, providing a tool for predicting patterns of herbivore habitat selection. We used previously published studies to develop a synthetic model of trade-o¡s between forage abundance and quality, predicting that optimal sward biomass should scale allometrically with body size. The model predicts size-related variation in habitat selection observed in a guild of grazing ungulates in the Serengeti ecosystem