Nesting biology of an African allodapine bee Braunsapis vitrea: female biased sex allocation in the absence of worker-like behavioural castes

Abstract

The life cycle and social behaviour of an African allodapine bee, Braunsapis vitrea, was examined using nest contents data from thorn-nests in Acacia karroo in Limpopo Province, South Africa. The majority of B. vitrea colonies were subsocial, with 34% of nests collected containing more than one adult female. Although brood numbers increased with colony size, per capita brood numbers did not increase. Within multifemale colonies, reproductively active females had greater wing wear than their nestmates, which tended to be uninseminated, and there was no relationship between relative ovary and body size. This suggests that females who were not reproductively active were not workers in the nest, but were instead waiting to either disperse or inherit the nest. There is evidence that these younger females may help rear their mother's brood in the event of orphaning, and the number of brood in multifemale nests was greater than for single–female nests, suggesting that egg layers increase brood production when a potential alloparent is present. Sex allocation was strongly female biased, with a numerical population ratio of r ≈ 0.3. This bias is interesting because our results suggest that it occurs in the absence of behavioural castes, where most colonies are occupied by single females, and where capita brood production does not increase with colony size. We argue that such bias might be selected by the benefits of having potentially alloparental ‘insurance’ daughters, who can help rear their younger siblings in the case of orphaning.