An ambush louse with a darker colored body is better at catching a mate than its brighter counterpart when it’s cold. Darker males can warm up more easily in the early mornings, and therefore get busy while everyone else is still warming up.
This is one of many examples of how temperature affects coloration in insects, and in turn can affect their ability to mate, according to a new review article published in the journal Ecology and Evolution.
But scientists are still trying to work out what will happen to insects’ sex lives now that human-induced climate disruption is driving temperatures to unprecedented levels.
“On the one hand we can be happy and say: how are the insects doing? They are responding to climate change. We don’t have to worry about them,” said Mariella Herberstein, a behavioral ecologist at Macquarie University in Sydney, Australia, who is one of the authors of the study.
“And then we can wake up the next day and say: Oh, damn – they can’t find each other anymore because they’ve lost very important identification colors that help them find a mate.”
The prevailing theory among scientists, Herberstein says, is that as temperatures rise, insects largely evolve to produce less of the melanin pigment that regulates their coloration, becoming lighter and brighter in color. This is because darker objects absorb more heat and heat up faster, while lighter objects reflect more incoming radiation and can stay cooler longer.
For example, the wing colors of the Mead’s swallowtail butterflies of the North American mountains faded over time as temperatures rose – their shiny, sulphur-yellow wings paler, according to a 2016 study. Between the 1980s and the 2000s, it became less and less likely for the two-spotted ladybug to be black with red spots rather than red with black spots. The dark spots on the back of the subarctic leaf beetle with similar patterns also decreased as spring warms.
But Herberstein’s team found that the pattern is not always so simple. A follow up study on the Mead’s swallowtails who looked at more than 800 butterflies collected for museum specimens between 1953 and 2013, found that in some areas their pale yellow wings became richer and darker in color over time. One species of the walking stick insect grew greener and darker over time as temperatures warmed, according to after a 2018 studyjust like one species of planthopper, as researchers poked it higher and higher the mountain.
“The mechanism is not that clear – it’s confusing,” says another of the study’s authors, Md Tangigul Haque, a PhD student at Macquarie University. This may be because researchers are working with a limited set of data, and because much of what little data has been collected comes from similar studies with similar insects in similar locations, he says. This is probably also because melanin not only has a heat-related function, but is involved in immunological defense and helps protect against ultraviolet radiation from the sun.
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But color is also involved in attracting mates, in camouflage from predators or from prey, and in allowing one member of a species to easily recognize others – all of which can be changed by rising temperatures. “If we affect their reproduction, we seriously affect their viability,” Herberstein said. “It’s just one of those pieces that we have to figure out.”
Cracking this mystery could play a crucial role in figuring out exactly how insects might be able to withstand the climate collapse, said Michael Moore, an integrative biologist at the University of Colorado Denver. Moore was not involved in the latest research but in 2021 saw that male dragonflies have lost their wing color patterns where the climate is warmer, and are trying to determine if this makes it harder for the dragonflies to find their mates.
“One thing that really stands out to me is that there is no one-size-fits-all rule,” Moore said. “We still have a lot of work to do – we haven’t solved this one yet.”
