Rethinking the thermal melanism hypothesis: rearing temperature and coloration in pygmy grasshoppers |
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Authors: | Anders Forsman |
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Institution: | (1) School of Natural Sciences, Linnaeus University, 391 82 Kalmar, Sweden |
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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-ravaged 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 survival 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 individuals 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 pressures on thermal capacity and camouflage as well as the influence of correlated responses to selection
on traits associated with coloration. |
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