A prey insect with a darker colored body is better at capturing a sexual partner than its lighter counterpart when cold. Darker males can warm up more easily in the early morning, and are therefore busy while everyone else is still warming up.
This is one of many examples of how temperature affects the coloration of insects and can in turn affect their ability to mate, according to a new review article published in the journal Ecology and Evolution.
But scientists are still trying to figure out what will happen to the insects’ sex lives now that human-caused climate change is raising temperatures to unprecedented levels.
“On the one hand, we could be happy saying: how are the insects? 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 study’s authors.
“And then we could wake up the next day going: Oh, hell—they can’t find each other anymore because they’ve lost the really important identifying colors that help them find a mate.”
The prevailing theory among scientists, Herberstein says, is that when temperatures rise, insects primarily evolve to produce less of the melanin pigment that regulates their color, 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 for longer.
For example, the wing colors of Mead’s sulfur butterflies of the North American mountains have faded over time as temperatures have risen — their bright, sulfur-yellow wings paler, according to a 2016 study. Among 1980s and 2000s, it was increasingly less likely that the two-spotted ladybird was black with red spots than red with black spots. The dark spots on the back of the subarctic leaf beetle with similar patterns have also diminished as springs warm.
But Herberstein’s team has found that the model isn’t always so straightforward. A follow-up study of Mead’s brimstone butterflies, which examined more than 800 butterflies collected for museum specimens between 1953 and 2013, found that in some areas, their pale yellow wings actually became richer and become darker in color over time. According to a 2018 study, a species of walking stick insect became greener and darker over time as temperatures warmed, as did a plant species, as researchers took it higher and higher in mountain
“The mechanism is not so clear – it’s confusing,” said another of the study’s authors, Tonmoy Haque, a PhD student at Macquarie University. This may be because the researchers are working with a limited data set, and because much of the little data collected comes from similar studies of similar insects in similar locations, he says. Perhaps also because melanin does not only have 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, camouflaging from predators or prey, and allowing one member of a species to easily recognize others—all of which can be altered by rising temperatures. “If we’re affecting their reproduction, we’re seriously affecting their population viability,” Herberstein said. “It’s just one of those pieces that we have to figure out.”
Cracking this conundrum could play a crucial role in figuring out exactly how insects might be able to cope with climate disruption, said Michael Moore, an integrative biologist at the University of Colorado Denver. Moore was not involved in the latest research, but in 2021 he noticed that male dragonflies were losing wing color patterns where the climate is hotter, and is trying to determine whether this makes it harder for dragonflies to find mates. theirs.
“One thing that stands out to me is that there is no one-size-fits-all rule,” Moore said. “We have a lot more work to do – we haven’t solved that yet.”