The benefit of being a social butterfly: Communal roosting deters predation

Aposematic passion-vine butterflies from the genus Heliconius form communal roosts on a nightly basis. This behavior has been hypothesized to be beneficial in terms of information-sharing and/or anti-predator defense. To better understand the adaptive value of communal roosting, these two hypotheses were tested in field studies. The information- sharing hypothesis was addressed by examining following behavior of butterflies departing from natural roosts. No evidence was found of roost-mates following one another to resources, thus providing no support for this hypothesis. The anti-predator defense hypothesis was tested using avian-indiscriminable Heliconius erato models placed singly and in aggregations at field sites. A significantly higher number of predation attempts were observed on solitary models versus aggregations of models. This relationship between aggregation size and attack rate suggests that communally roosting butterflies enjoy the benefits of both overall decreased attack frequency as well as a prey dilution effect. Communal roosts likely deter predators through collective aposematism in which aggregations of conspicuous, unpalatable prey communicate a more effective repel signal to predators. These results suggest that predation by birds is a key selective pressure maintaining Heliconius communal roosting behavior.