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Message: <blockquote data-quote="locustella" data-source="post: 3167601" data-attributes="member: 26257">Butterflies are <a href="https://en.wikipedia.org/wiki/Ectotherm" target="_blank">ectotherms</a> - they use external sources of heat. Black color means that they absorb light, at least visible part. Perhaps they are heating up themselves this way, using large surface of their wings. Mammals use internal, physiological source of heat, they are <a href="https://en.wikipedia.org/wiki/Endothermy" target="_blank">endotherms</a>, so they don't care about absorption and can afford to be white (reflect everything) and be less visible in the snow. Butterflies don't fly when everything is covered by snow. Maybe the hibernate or are dead and their eggs or caterpillars hibernate, I don't know.However white color doesn't mean, that an animal is loosing heat (infrared). Polar bears have such special structure of hair preventing loosing heat in the form of infrared radiation.So probably it is good to distinguish between ectotherms and endotherms here.Squirrels live in forest and maybe this color is a camouflage ? But why such color would be needed more in in the north, but not south ? Or they are too small (the surface of their body is too small in comparison to their volume) and they must take care about catching external heat like butterflies ?I don't understand, why dark color of squirrels is called melanism. This is their normal color.</blockquote>