Differential predation drives the geographical divergence in multiple traits in aposematic frogs

Abstract

Prey have evolved a range of traits to enhance their survival against predators. These traits often show geographical variation due to the differences in local predation pressure. To date, there exists ample evidence of the geographical variation in single anti-predator traits such as coloration induced by differential predation. However, predation pressure often induces the shift in a suite of (correlated) prey traits, such as coloration and behavior, rather than a single trait in nature. In this study, we investigated the nature and extent of geographical variation in both color and behavioral traits in the oriental fire-bellied toad, Bombina orientalis, testing whether the frogs experience different predation pressure in different sites and evaluating how this might affect a range of traits associated with predator avoidance. Specifically, we quantified dorsal/ventral coloration, daytime resting place (i.e. the place where each frog was first spotted), and activity levels of natural populations of the frogs in 9 geographically distinct sites distributed across South Korea. We found the clearest differences in these traits between mainland and Jeju Island populations, with Jeju Island frogs exhibiting 1) less bright and less chromatic aposematic ventral coloration than mainland frogs, 2) lower activity during both day and night, and 3) a tendency to rest underwater. Follow-up predation experiments using clay frog models found that frogs in Jeju Island suffer higher predation pressure than mainland frogs, which readily explains the divergence in behavioral and morphological traits among populations.