Effect of background color and low temperature on skin color and circulating alpha-MSH in two species of leopard frog.

Circulating levels of alpha-melanocyte stimulating hormone (alpha-MSH) in two species of leopard frog, Rana pipiens and R. chiricahuensis, were measured by radioimmunoassay to reveal the correlation between skin color change induced by background color and by low temperature. High levels of alpha-MSH were found in both species of frog on a black background, but R. chiricahuensis had eight times higher levels than R. pipiens, R. chiricahuensis also exhibited the ability to darken its ventral surface, whereas the ventral surface of R. pipiens remained white. Neither skin color nor plasma alpha-MSH of R. pipiens was affected by cold. Low temperature did, however, darken dorsal and ventral skin of R. chiricahuensis in vivo, which corresponded to increased levels of plasma alpha-MSH. Dorsal and ventral skin of R. chiricahuensis, in vitro, darken in a dose-dependent manner to alpha-MSH, but not to cold.