LAUSR

Two dramatic large scale patterns characterize body size in mammalian herbivores. One is Bergmann's rule that notes that mammals tend to increase in body size at higher latitudes. The other is the inverse relationship between herbivore body size and diet quality. Here, we present a model that may explain both. We start by noting that searching for and handling resources are fundamental activities for feeding mammals. We note that if with body size, encounter probability increases less favorably and handling time more favorably than metabolic costs, then body size represents a tradeoff between search efficiency (favors smaller body size) and handling efficiency (favors larger). If so, then optimal body size increases with both temperature and the conspicuousness of the food, but decreases with food quality. For this to happen there must be food limitation where the herbivores influence food standing crop. Lower energetic foraging costs (lower latitude, lower seasonality and/or higher temperatures) or higher food quality result in lower standing crops of food. A lower standing crop of food favors searching efficiency and, hence, smaller body sizes. Factors that increase the standing crop of food favor handling efficiency and larger body sizes. Simply maximizing net profit from foraging or foraging efficiency that are often assumed to help explain Bergmann's rule do not predict either Bergmann's rule nor the inverse relationship between food quality and body size. With the inclusion of consumer–resource dynamics, fitness maximization predicts both. Testing the model's predictions invites empirical research into the allometries of foraging parameters relating to search and handling.