Cooperative nesting and complex female-biased sex allocation in a tropical allodapine bee

Patterns of sociality and sex allocation are well known for many allodapine bee genera, but have only been described for one species in the genus Macrogalea. Macrogalea is the sister group to the remaining allodapines and is important for inferring plesiomorphic features for the tribe. The present study describes the broad elements of social behaviour and the patterns of sex allocation in an undescribed Macrogalea species from Malawi, and provides the most detailed description of sociality and sex allocation in this genus. This species has large maximum colony sizes (13 adult females) compared to most other allodapines and there is no evidence of reproductive castes. Alloparental care is common and probably unavoidable because brood are progressively reared in a communal burrow where parental care cannot be restricted to filial brood. Cooperative nesting confers substantial benefits in avoiding brood loss, and via enhancement of both per capita brood production and mean pupal weight, but per capita benefits decline after colony sizes exceed three females per nest and total brood size saturates once colony sizes exceed five or six females. All newly eclosed females lay eggs, regardless of body size, but reproduction by older females depends on their relative body size. This suggests the existence of staying incentives for young females, who are potentially valuable alloparents, but that there is reproductive competition among older females who would have little future value as alloparents. The sex ratios are among the most strongly female biased in any bee studied to date, with a colony mean of r ~ 0.08. There is almost no production of males in colonies with five or fewer adult nestmates, but bias declines very rapidly once colonies exceed this size. This indicates that sex allocation strategies are based on the ability to assess colony size quite accurately, an ability that occurs in several other allodapine genera, and which is likely to be ancestral for the tribe. The present study strengthens recent suggestions that complex sociality and sex allocation patterns are plesiomorphic traits for the allodapines, despite the ubiquity of small colony sizes.