Correlated trait response: comparing amphibian defense strategies across a stress gradient

Abstract

Animals inhabiting complex environments often contend with multiple stressors that can select for conflicting responses. Individuals can mediate these conflicts by utilizing correlated responses across multiple traits. In aquatic habitats, larval amphibians often face conflicting, simultaneous pressures, such as ultraviolet-B (UV-B) radiation and predators. UV-B radiation and predation risk influence behavior and body color in many amphibian species, altering activity rates, refuge use, and coloration. When both UV-B and predators are present, individuals can avoid conflicts by coupling behavior with body color to form a correlated response. UV-B exposure rates vary along an elevation gradient, thus trait combinations may also vary. We quantified changes in activity rates and body color in two anuran species, the red-legged frog ( Rana aurora Baird and Girard, 1852) (low elevations) and the cascades frog ( Rana cascadae Slater, 1939) (high elevations), during exposure to predator chemical cues (rough-skinned newt, Taricha granulosa (Skilton, 1849)) and UV-B radiation. Rana aurora decreased activity in response to UV-B and became more cryptic over time, while R. cascadae coupled decreased activity rates in response to predators with dark body coloration to screen out UV-B. Both species responded with a correlated trait response, yet employed opposite strategies. This observed species difference may be reflective of differences in stress across habitats and availability of alternative defenses.