The size of prey relative to predators has profound consequences for the ecology of animals, especially for gape-limited predators, in which anatomy imposes an upper limit on the size of prey that can be consumed whole. Despite great potential for being a model system to study the ecology of gape-limited predators, maximal gape of snakes has been quantified in less than 10 of more than 3,500 extant species. The even greater paucity of data for the relative size of prey (prey cross-section area/ maximal gape area) has impeded understanding how often these animals in nature tax a maximal performance that is constrained by anatomy. We studied Regina septemvittata, which feeds exclusively on freshly molted crayfish. This unusual diet facilitated estimating the relative size of intact prey from fragments of the exoskeleton recovered from snakes captured in the field (N = 163 crayfish). For 25 snakes we also determined the scaling relationships between overall snake size and maximal gape. Laboratory feedings (N = 117) with 8 snakes tested the effects of relative prey size on behavior and feeding performance. Finally, we compared our results with those for an unrelated snake clade that convergently evolved a crustacean diet.
Results/Conclusions
For log-transformed data, maximal gape area (mm2) had negative allometry with snout-vent length (mm) (r2±0.26). Hence, adults had smaller values of gape than juveniles compared to the expectation from geometric similarity (slope = 2). For the field-caught snakes, 88% and 39% of the prey consumed had relative sizes of 50% and 90%, respectively. Hence, this species regularly eats large prey in nature. Relative prey size accounted for 63% of the observed variance in a regression predicting total prey handling times for laboratory feedings. In captivity and in the field, snakes occasionally ate crayfish with intact relative areas >100%, either by destroying the integrity of the soft exoskeleton or by only eating a single claw. For three Southeast Asian species of crustacean-eating snakes, the prey handling times are greatest and least for the ones that eat hard-shelled and soft-shelled crabs, respectively, and one species that eats snapping shrimp has intermediate handling times that are nearly identical to those of R. septemvittata. Collectively, these data emphasize the importance of behavior and choice of prey for affecting the foraging ecology of predators by mediating the interaction between predator size and prey size.