Evidence for Communal Roost Size Regulation in Heliconius erato Butterflies (Nymphalidae)

Abstract

Communal roosting behavior in Heliconius passion-vine butterflies has fascinated scientists for centuries. Despite recent research on the complex dynamics involved with this behavior, little work has been done to examine whether butterflies regulate their communal roost sizes and if there is preference to form roosts of a particular size. Here I present results from two studies, that tested 1) whether wild-captured Heliconius erato butterflies, exposed to different roost sizes in captivity, prefer to join small, medium, or large-sized roosts; and 2) whether established H. erato butterflies in natural wild populations abandon roosts when they become too large. In captivity, butterflies significantly preferred to join small- and medium-sized aggregations, and avoided large aggregations, when introduced to a new environment. In the wild, butterfly attendance decreased significantly as roost size increased, showing that butterflies leave natural roosts when they become too large or crowded. Although a number of potential explanations exist for the adaptive significance of communal roost size regulation in H. erato, one of the most striking details about these results are that preferred roost size by individual butterflies corresponds with the average number of butterflies found in natural roosts. This preferred roost size also overlaps with previous studies showing an experimentally optimal roost size with the same number of butterflies (for anti-predatory benefits), in this species in this same field location. These results combine multiple sources of evidence from old and new data suggesting that optimal communal roost sizes exist in Heliconius and that the butterflies regulate their roost sizes in their home ranges. Further studies are welcome to help understand the decision-making process used by these butterflies for joining – or abandoning – communal roosts.