Small Body Size Is Associated with Increased Aggression in the Solitary Sweat Bee Nomia melanderi (Hymenoptera, Halictidae)

Abstract

Modifying ancestral regulatory mechanisms can be a source of evolutionary novelty. Bees in small-colony, dominance-based societies typically show a link between size and aggression: larger bees are more aggressive. This led to the hypothesis that this size-aggression link is a characteristic of ancestral solitary bees that has acquired a novel function in the evolutionary transition from solitary to social behavior. Here we test the central prediction of this hypothesis, that size is linked to aggression in an ancestrally solitary bee. We use the sweat bee Nomia melanderi (Hymenoptera, Halictidae) in the subfamily Nomiinae, which is sister to the social sweat bees, all of which are in the subfamily Halictinae. We measured aggression using a standardized behavioral assay (circle tube) in which two bees were placed together and allowed to interact. We used three treatments: size matched small bees, size matched large bees, and one large bee paired with one small bee. We found no link between ovary size and aggression, but because we used only reproductively active bees we may not have had sufficient variation to detect such a link. Across treatments, body size negatively correlated with aggression. There were no differences between large and small bees in the matched treatments, but in the mixed treatment, small bees were more aggressive than large bees. This supported our prediction of a size-aggression link, although the link was in the opposite direction seen in social bees. Moreover, our data show that even solitary bees can modulate their aggression in response to social context and highlight the importance of studying related solitary species to understand the evolutionary origins of sociality.