Widespread 'hilltopping' in Acraea butterflies and the origin of sex-role-reversed swarming in Acraea encedon and A. encedana

In some populations of the butterflies Acraea encedon and A. encedana, most females are infected with a bacterium that kills their sons. The resulting shortage of males is associated with females adopting a sex‐role‐reversed mating system, in which females swarm at landmarks such as hilltops and compete for males. We have observed the mating behaviour of Acraea species that are not known to be infected with the male‐killer. In over half of these species, males were found to aggregate on hilltops. It is likely that this behaviour was ancestral to the sex‐role‐reversed swarms of Acraea encedon and A. encedana, and we discuss how the spread of the male‐killing infection may have converted this mating system into sex‐role‐reversed swarming.