Correlated evolution of colony defence and social structure: A comparativeanalysis in eusocial wasps (Hymenoptera : Vespidae)

Animal societies depend on effective defence of group resources. Defensive mechanisms can be costly and may constrain the evolution of social structur e. We analysed how exocrine mechanisms of colony defence were affected by t he evolution of social complexity and of nest architecture in paper wasps ( Vespidae). Eusocial paper wasp species exhibit two discrete grades of eusoc iality, with new colonies founded either by queens or by coordinated swarms of queens and workers. Swarm-founding shows multiple evolutionary origins from independent-founding ancestors within the Vespidae. Nest architecture also varies among paper wasps. Nests with covering envelopes evolved from n aked combs several times. We hypothesized that: (1) evolutionary transition s from independent- to swarm-founding would obviate the need for chemical d efence against ants and (2) transitions from naked combs to enveloped nests would have a similar effect on chemical defence. In support of the first h ypothesis, we found that all independent-founding species possess ant-repel lent glands (Van der Vecht's gland), while many swarm-founders do not. Furt hermore, phylogenetic analysis suggested that evolutionary loss of this gla nd was statistically more likely to follow transitions to swarm-founding. E volution of nest envelopes was less strongly associated with losses of the ant repellent gland. These patterns suggest that maintenance of defensive e xocrine glands is costly. The patterns also suggest that group behavioural defence against ants is a key adaptive feature associated with the evolutio n of swarm-founding. The hypothesis that the evolution of nest envelopes ob viated chemical defence against ants was not as well supported.