Latitudinal variation in the sociality of allodapine bees (Hymenoptera: Apidae):: sex ratios, relatedness and reproductive differentiation

Variation in sociality of two Australian allodapine bees was investigated over a latitudinal range of 10 degrees, spanning subtropical to cool temperate forests in eastern Australia. Exoneura robusta is univoltine and singly brooded in southern populations but is able to produce two broods per season in northern populations, leading to opportunities for sib rearing by older brood members. In contrast, sympatric colonies of E. angophorae show no such variation with latitude, and all populations exhibit some colonies where opportunities for sib rearing arise. Patterns of ovarian differentiation, relatedness and sex allocation were examined over a one-year period for these two sympatric species in four populations. Within each species, the timing of ovarian development and the degree of reproductive skew were similar at all sites, but the onset of egg-laying and brood development was earlier in E. angophorae than in E. robusta. Relatedness and sex allocation patterns in northern populations of these species were not markedly different from southern populations. Selection for female-biased sex allocation and high reproductive skew in Exoneura probably stems from strong benefits from cooperative nesting, and our results suggest that opportunities for sib rearing (and eusociality) in these species may depend on latitudinally mediated brood development rates, and are unrelated to levels of sex bias and relatedness.