An ambush bug with a darker-coloured body is better at snagging a sexual partner than its brighter counterpart when it is chilly. Darker males can warm up more easily in the early mornings, and therefore get busy while everybody else is still warming up.
This is one of the many examples of how temperature affects colouring 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 breakdown is raising temperatures to unprecedented levels.
“On the one hand, we could be rejoicing, saying: how are the insects? They are responding to climate change. We don’t have to worry about them,” said Mariella Herberstein, a behavioural ecologist at Macquarie University in Sydney, Australia, who is one of the authors of the study.
“And then we could wake up the next day going: Oh, damn – they can’t find each other any more because they have lost really important identification colours that help them find a mate.”
The prevailing theory among scientists, says Herberstein, is that when temperatures increase, insects largely evolve to produce less of the melanin pigment that regulates their hue, becoming lighter and brighter in colour. That is because darker objects absorb more heat and heat up quicker, while lighter objects reflect more incoming irradiation and can stay cooler for longer.
For example, the wing colours of the Mead’s sulphur butterflies of the North American mountains have faded over time as temperatures have risen – their shimmery, sulphur yellow wings paler, according to a 2016 study. Between the 1980s and the 2000s, it became increasingly 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 similarly patterned subarctic leaf beetle have also decreased as springs get warmer.
But Herberstein’s team have found the pattern is not always so straightforward. A follow-up study on the Mead’s sulphur butterflies that looked at more than 800 butterflies collected for museum samples between 1953 and 2013 found that in some areas, their pale yellow wings actually got richer and darker in colour over time. One species of the walking stick insect got greener and darker over time as temperatures got warmer, according to a study from 2018, as did one species of planthopper, as researchers sampled it higher and higher up the mountain.
“The mechanism isn’t so clearcut – it’s confusing,” said 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 that little data collected comes from similar studies with similar insects in similar locations, he says. It is probably also because melanin does not just have a heat-related function, but is involved in immunological defences and helps protect against ultraviolet radiation from the sun.
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But colour is also involved in attracting mates, in camouflage from predators or from prey, and in allowing one member of a species to easily recognise others – and all of this could be altered by rising temperatures. “If we’re affecting their reproduction, we’re seriously impacting their population viability,” said Herberstein. “It is just one of those pieces that we need to figure out.”
Cracking this conundrum could play a crucial role in figuring out exactly how insects might be able to weather climate breakdown, said Michael Moore, an integrative biologist at the University of Colorado Denver. Moore was not involved in the latest research but in 2021 spotted that male dragonflies were losing their wing colour patterns where the climate is hotter, and is trying to establish whether that makes it harder for the dragonflies to find their mates.
“One thing that really sticks out to me is that there isn’t a one-size-fits-all rule,” said Moore. “We have a lot more work to do – we haven’t solved this one yet.”