The allometry of patch selection in ruminants

An axiomatic feature of food consumption by animals is that intake rate and prey abundance are positively related. While this has been demonstrated ri gorously for large herbivores, it is apparent from patch selection trials t hat grazers paradoxically tend to prefer short, sparse swards to tall, dens e swards. Indeed, migratory herbivores often shift from areas of high to lo w sward biomass during the growing season. As nutritional quality is an inv erse function of grass abundance. herbivores appear to sacrifice short-term intake for nutritional gains obtainable by earing sparse forage of higher quality. Explicit models of this trade-off suggest that individual ruminant s maximize daily rates of energy gain by choosing immature swards of interm ediate biomass. As body mass is related positively to both ruminant croppin g rates and digestibility, there should be an allometric link between grass abundance and energy maximization, providing a tool for predicting pattern s of herbivore habitat selection. We used previously published studies to d evelop a synthetic model of trade-offs between forage abundance and quality , predicting that optimal sward biomass should scale allometrically with bo dy size. The model predicts size-related variation in habitat selection obs erved in a guild of grazing ungulates in the Serengeti ecosystem.