Social competition but not subfertility leads to a division of labour in the facultatively social sweat bee Megalopta genalis (Hymenoptera: Halictidae)

Abstract

Insects with facultative social behaviour permit direct examination of factors associated with the expression of division of labour: why do some females remain in their natal nest as nonreproductive foragers, while others disperse? The facultatively social halictid bee Megalopta genalis shows strong reproductive division of labour, associated with body size (foragers tend to be smaller than queens and dispersers). We used M. genalis to test two hypotheses for the expression of worker behaviour: (1) queens suppress reproduction by subordinates, which then forage, and (2) small-bodied females are handicapped as reproductives, and therefore take on a foraging role to assist a more fertile relative (the ‘subfertility’ hypothesis). We removed queens from 19 nests and found that the remaining foragers enlarged their ovaries and reproduced at the same rate as solitary reproductives from unmanipulated (nonremoval) nests. This observation suggests that queen dominance limited reproduction by subordinates, and that foragers were not handicapped reproductives. To investigate the effect of body size variation on reproductive rate in the absence of social interactions, we placed single, newly eclosed females into 31 observation nests. Body size was not correlated with reproductive output or with the females' tenure in the observation nests. Nor was there any correlation between body size and number of brood cells in 21 solitary-female nonremoval nests. Taken together these data show that small females were not inherently poor reproductives. We also found that ovaries of reproductive females from social groups were larger than those of solitary reproductives, suggesting that social structure shapes ovary development.