Fertility signalling and reproductive skew in queenless ants

Abstract

Social insects often show an extreme reproductive skew. In queenless ants, colonies consist of morphologically identical workers that can all potentially reproduce sexually. Similarly to that in social vertebrates, aggression in these ants functions to select the reproductive(s). We investigated the mechanisms underlying reproductive skew in the monogynous queenless ant Streblognathus peetersi . Behavioural observations of disturbed hierarchies were integrated with physiological measures of fertility (vitellogenin titre in the haemolymph) and chemical analysis of cuticular hydrocarbons, which are putative fertility pheromones. This multifaceted approach revealed that the colony reproductive is determined as a result of aggression between high-ranking workers, but once an alpha is established, chemical signalling is enough to maintain reproductive skew. As already reported in several species of ants and also in a social wasp, egg layers have distinct profiles of cuticular hydrocarbons compared with infertile workers. Importantly, ‘high rankers’ who are unable to lay eggs also have a specific cuticular profile; this is consistent with their intermediate state of fertility indicated by vitellogenin levels. Rather than just ovarian activity, the cuticular hydrocarbon profile thus reveals the individual hormonal state that underlies reproductive activity. We compare the fertility signal in queenless ants with the ‘badge of status’ reported in various birds. We discuss the evolutionary stability of this communication system and give special emphasis to ecological constraints and the high degree of intracolonial relatedness typical of social insects.