Some butterflies find it safer to hang out in a gang, collectively creating a big visual 'stop sign' to keep predatory birds at bay, say researchers.
Entomologist Susan Finkbeiner, of the University of California, Irvine, and colleagues, report their findings today in Proceedings of the Royal Society B: Biological Sciences.
"It's the exact same butterfly that roosts with the same individuals in the same place," says Finkbeiner. "Some even roost on the exact same part of the branch every night, as if their name was written on it."
Scientists have known for 150 years that come sundown, many species of butterflies of the genus Heliconius bed down in gangs of about 4 to 15 individuals.
"It's really unique in butterflies because most adult butterflies tend to be solitary. Any social behaviour among butterflies is really rare," says Finkbeiner.
Until now, the reason behind this unique communal roosting behaviour has remained a mystery.
Now, Finkbeiner and colleagues have investigated and found that it's all about safety in numbers.
The researchers investigated two hypotheses: First, that the butterflies were hanging out together so they could share information about foraging sites.
This is known, for example in groups of birds that roost together and follow their roost mates to foraging sites.
Second, that the grouping helped the butterflies to fend off predators, who tend to attack first thing in the morning.
Finkbeiner says the butterflies are actually loaded with cyanide, because of the passion vine they feed on as larvae, and are coloured brightly to warn off bird predators.
It is possible that by joining together they enhance their bright warning signal, she says.
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