Rethinking the thermal melanism hypothesis: rearing temperature and coloration in pygmy grasshoppers

Abstract

Selection for efficient conversion of solar radiation to body heat has favored the evolution of dark coloration in many ectotherms. The thermal melanism hypothesis posits that dark coloration is beneficial under conditions of low ambient temperatures because it results in faster heating rates and higher body temperatures. Fast heating rates, however, may come at a cost of overheating unless compensated for by thermal physiology or behaviour. Pygmy grasshopper (Orthoptera, Tetrigidae) populations that inhabit fire-rav- aged areas characterized by blackened backgrounds and hot surface temperatures due to high absorbance of solar radiation show an increased frequency of black phenotypes. I raised the progeny of wild-captured Tetrix undulata in cold and hot temperatures and used data on color patterns and survival in a greenhouse to examine whether a cold thermal environment triggered the development of melanic coloration or differently affected sur- vival of melanic versus non-melanic individuals. My results indicate that melanism was not influenced by rearing temperature but by genes or epigenetic maternal effects. Temperature also did not affect survival. However, melanic individuals produced by melanic mothers survived longer than melanic individuals produced by non- melanic mothers, whereas non- melanic individuals produced by non-black mothers survived longer than melanic indi- viduals produced by non-black mothers. This suggests a mismatch between color and physiology in offspring belonging to a different color morph than their mother. Future investigations into the evolution of melanism should consider conflicting selection pres- sures on thermal capacity and camouflage as well as the influence of correlated responses to selection on traits associated with coloration.