Female meadow brown butterflies who develop in warmer weather sport fewer spots on their wings, in an unexpected adaptation to global heating.
The discovery was made by University of Exeter scientists who found that females whose chrysalises developed at 11C had six spots on average, while those developing at 15C had just three.
The findings may challenge lepidopterists’ long-held views about why butterflies have varying numbers of spots.
The meadow brown, a common midsummer butterfly found on grasslands across Europe, has large eyespots on its forewings that are believed to startle and alarm predators. The eyespots also encourage potential predators to home in on the wing-edge where the spots are positioned, well away from the butterfly’s vulnerable body, enabling the insect to escape attacks with no more damage than tatty wings.
Meadow browns also have smaller spots on their hind wings, which probably serve as camouflage when the butterfly is resting in grass.
Prof Richard ffrench-Constant, from Exeter’s Centre for Ecology and Conservation and co-author of the study, said: “Our findings show that fewer of these hindwing spots appear when females experience higher temperatures during their pupal stage (in a chrysalis before emerging as a butterfly).
“This suggests the butterflies adapt their camouflage based on the conditions. For example, with fewer spots they may be harder to spot on dry, brown grass that would be more common in hot weather.
“We did not observe such a strong effect in males, possibly because their spots are important for sexual selection.”
Eyespot variation in meadow brown butterflies has been used as an example of genetic polymorphism, whereby multiple genetic forms live together in a single population, in the decades since the classic study of butterflies by the biologist EB Ford, who wrote the bestselling New Naturalists guide in 1948.
The study, published in the journal Ecology and Evolution, shows that eyespot variation in this instance is caused by the butterfly’s ability to react to changing temperatures.
As well as examining male and female meadow browns from the same field in Cornwall every day throughout the flight season, Ffrench-Constant, whose father collected butterflies for the geneticist EB Ford in Cornwall, studied historic collections of dried butterflies from Eton and Buckingham.
The researchers predict that the spots on meadow browns will decrease as the British climate heats up.
Ffrench-Constant added: “This is an unexpected consequence of climate change. We tend to think about species moving north, rather than changing appearance.”
• This article was amended on 18 January 2024 to replace the image as an earlier picture did not show a meadow brown butterfly.
