Ontogenetic and seasonal variation in the diets of a Costa Rican leaf-litter herpetofauna

Abstract

Ontogenetic and seasonal variation in diet was examined for 11 species of insectivorous forest-floor frogs and lizards from a lowland wet forest in north-eastern Costa Rica. Specimens were collected systematically over an entire seasonal cycle and represented individuals of all sizes. Individual prey items were removed from stomachs of preserved specimens, measured and identified. Ontogenetic shifts in prey size were pervasive. Ontogenetic shifts in prey composition were limited to four species; these were not the species with greatest range in body size, nor the species with the broadest diets. Small prey types (ants, mites, collembolans) decreased in representation and large prey types (roaches, orthopterans, millipedes) increased in importance over ontogeny; this could be because prey selection is based primarily on prey size or because of different prey preferences among age classes. There is little evidence for size-structure in this assemblage. There is no evidence that total availability of arthropod prey varies among seasons, but some evidence that preferred prey are less common in the wet season. Diet was similar between lizards and frogs. Lizards were more likely to have empty stomachs, but also greater stomach volume, than frogs; this indicates a difference in food-gathering strategies. Our study indicates strong similarity between frogs and lizards in diet despite enormous differences in physiology and behaviour.