Body Size and Net Energy Gain in Gape-Limited Predators: A Model

Abstract

In snakes and other predators that swallow their food whole, gape size sets an upper limit to ingestible prey size. Individuals that vary in body size and swallowing capacity will therefore differ not only in energy requirements but also in rates of food intake. Here I show that size-frequency distri- bution of available prey may be an important determinant of life-history evolution in snakes and other gape-limited predators. A simple model is presented for the rate of net energy gain of gape-limited predators during food scarcity as a function of body size and swallowing capacity of the predator. Net energy gain is defined as the difference between rates of energy intake and expenditure while searching for prey, and thus represents the energetic scope available for growth, storage, reproduction, and survival. The model suggests that rate of energy intake will increase asymptotically with predator body size until it reaches an asymptote that differs depending on the size-frequency distribution of available prey. Rate of net energy gain will reach a maximum at intermediate or large predator size, but shift towards smaller predator body size if energy demands increase, maximum ingestible prey size increases more rapidly with predator size, or if mean body size of available prey decreases. Variation in swallowing capacity among equal-sized predators results in a difference in net energy gains that is at a maximum in intermediate-sized individuals. The model generates seven testable predictions. (1) Smaller body size does not confer an energetic advantage during periods of prey scarcity. (2) Mortality due to energy shortage sets lower and upper limits to body size. (3) Temporal shifts in size-frequency distribution of prey will result in oscillating survival selection on body size. (4) The proportion of resources allocated to energy storage and reproduction should increase with increasing body size at the expense of energy allocated to growth. (5) The reproductive advantage of large body size in females will increase with prey availability. (6) The intensity of selection on characters determining swallowing capacity will vary with body size and be lowest in larger individuals. (7) Dis- crimination against small prey is more likely among larger than among smaller individuals. Examples from natural populations relating to these predictions are provided.