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Social butterflies find safety in numbers
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.
Field work

Finkbeiner and colleagues observed nine Heliconius erato roosts for three months in Panama, and three Heliconius sara roosts for two months in Costa Rica.

They got up before dawn to observe what happened as the Sun rose. But after following the butterflies as they flew off, they found no evidence that they followed each other to foraging sites, suggesting the "information sharing" hypothesis was not relevant.

The researchers then observed what happened when they made fake butterflies and put them out in the field individually or in groups of different sizes - altogether they used 920 artificial roosts.

"We found there was a lot more predation on the single butterflies than on the groups," says Finkbeiner, adding this supported the idea that the grouping was to fend off predators.

"Predators are avoiding the aggregation," she says. "It's like a giant stop sign saying: 'Hey, look at us. Now you can see there's a group of us - you can see what kind of animal we are, you don't want to eat us.'"

Finkbeiner says Heliconius probably developed this system of fending off predators because they live for 6 months, which is relatively long for butterflies.

Also, it is practical because they tend to forage over a relatively small area (up to a 1 square kilometre), and have a great memory for locations.

Finally, their bright colouring and toxicity gives them a basis for developing the anti-predatory system, says Finkbeiner.
A childhood fascination

Finkbeiner says she's been fascinated by butterflies since she was four years old.

"I used to always follow them and watch them," she says. "I specifically got interested in Heliconius roosting when I studied abroad in Costa Rica when I was a junior at college in Cornell University."

Finkbeiner found that medium-sized roosts were the best at deterring predators and thinks this may be because small roosts are not conspicuous enough but large ones are conspicuous, attracting naive predators that don't normally attack these kinds of butterflies.

She now wants to look further at why the butterflies always roost with the same roost mates and what number of roost mates they prefer.

"Maybe we'll find out that butterflies can count," she muses.

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