Investigating the role of body size, ecology, and behavior in anuran eye size evolution

Abstract

Vertebrate eye size typically scales hypoallemetrically with body size—as animals grow larger their eyes get relatively smaller. Additionally, eye size is highly variable across species, and such variability often reflects functional adaptations to differences in behavior and/or ecology. The selective pressures underlying the evolution of eye size are especially well studied in birds, mammals, and fishes. However, whether similar scaling rules and selective pressures also underlie the evolution of eye size in amphibians remains enigmatic. Variation in eye size is intimately linked with variation in brain anatomy, as the retina is ontogenetically part of the brain. Eye size may therefore coevolve with brain size. Here we use phylogenetic comparative methods to study interspecific variation in eye volume across 44 species of anurans from 8 families from the Hengduan Mountains, China. We relate this variation to key factors known to impact eye size evolution in other vertebrate taxa such as body mass, habitat use, defense strategy and foraging mobility. We found that also in anurans eyes size scaled hypoallometrically with body mass. However, neither of the behavioral or ecological factors explained any variation in relative eye size in our sample. Whether this is representative for other frog species needs to be clarified. We therefore conclude that eye size in frogs is tightly linked to body mass evolution but that, at least in the species investigated here, none of our tested ecological and behavioral factors have a strong influence on eye size evolution.