Within many animal taxa there is a trend for the species of larger body size to eat food of lower caloric value. For example, most large extant lizards are herbivorous. Reasonable arguments based on energetic considerations are often invoked to explain this trend, yet, while these factors set limits to feasible body size, they do not in themselves mathematically produce optimum body sizes. A simple optimization model is developed here which considers food search, capture, and eating rates and the metabolic cost of these activities for animals of different sizes. The optimization criterion is defined as the net calorie gain a consumer accrues per day. This model does produce an optimum intermediate body size which increases with food quality--not the reverse. This discrepancy is accounted for, however, because the model also predicts that body size should be even more sensitive to increases in food abundance. In nature, many poor quality foods are also relatively abundant foods, hence the consumers eating them may maximize their daily energetic profit by evolving a relatively large body size. Optimum consumer body size also decreases with increases in consumer metabolic rate and "prey" speed.

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