Why do Larger Snakes Eat Larger Prey Items?

Abstract

Field collections of snakes often reveal a correlation between the body size of a snake and the size of its prey. Such a correlation could result from biases in (1) encounter rates with prey items of different sizes (especially in aquatic or creviceforaging species, in which smaller prey may be inaccessible to larger snakes), (2) prey choice, (3) ability to capture or handle prey, or (4) prey-swallowing ability. Field data showed that larger diamond pythons (Morelia spilota Lacapede, Boidae) took larger mean and maximum prey sizes, whereas prey mass did not correlate significantly with predator size in blacksnakes (Pseudechis porphyriacus Shaw, Elapidae). Minimum prey sizes increased little, if at all, in larger snakes of either species. Blacksnakes generally took much smaller prey items than did sympatric pythons. Laboratory trials with these two species showed that snakes with larger heads could eat larger prey, and eat prey of a given size more rapidly (and using fewer mandibular movements) than could snakes with smaller heads. Large blacksnakes were unable to enter narrow crevices to obtain prey items. Pythons could ingest much larger prey items than could blacksnakes. The rate of food intake for blacksnakes of all body sizes was highest with small prey items, whereas pythons (especially larger specimens) were more efficient with larger prey items. This fits well with field observations showing that blacksnakes of all body sizes eat small prey items, whereas pythons (especially larger specimens) consume larger prey items. The relationships between prey and predator size observed in the field are probably due to a combination of factors. Differential encounter rates, active selection of prey sizes and gape limitation may all be important influences on dietary composition in snakes. Key-words: Boidae, Elapidae, gape limitation, optimal foraging, predation, prey choice, prey size, python, snake