Active and effective nest defence by males in a social apoid wasp

Altruism in the social Hymenoptera is generally considered to be a feature of females rather than males. A popular explanation for this is that in the solitary ancestors of today's social species, males provided little brood care. Males might therefore lack the preadaptations necessary to evolve altruism in social contexts. While anecdotal observations of male contributions to colony life have been reported, there are few reports of male participation in nest defence. In apoid wasps, there have been several reports of male nest-guarding behaviour in solitary species, potentially setting the evolutionary stage for similar behaviours in social lineages. Here, we present evidence of active and effective nest defence in males of the social apoid wasp Microstigmus nigrophthalmus. Males were observed chasing intruders away from the nest, and the presence of males had a significant effect on nest survival when females were removed. Males potentially obtained direct benefits through defence, so that defence may not represent male altruism. However, our results do show that males can perform acts that benefit their colony.