Comparative studies provide one of the most powerful means of assessing the relative roles of selective agents underlying social evolution in insects. Because of the wide variation in social organisation, sex allocation and ecological traits within and between species of allodapine bees, this group provides a wealth of material for such comparative work. Recent studies on Australian allodapine bees are reviewed here and their consequences for understanding social evolution are discussed. Studies to date suggest the following trends: (i) benefits of group living appear to be linked to preventing brood failure rather than to increased brood rearing efficiency; (ii) femalebiased sex allocation, when it occurs, is linked to benefits of group living and kinship among nestmates, and is probably mediated via local fitness enhancement; (iii) female biased sex allocation patterns do not usually coincide with opportunities for sib-rearing and are therefore unlikely to facilitate eusociality; (iv) relatedness within colonies is usually high, but in some species females will nest with unrelated females if kin are not available; and (v) phylogenetic studies suggest that opportunities for sib-rearing, arising from brood development patterns and colony phenology, are plesiomorphic for the exoneurine group, but in at least one phylogenetically distal clade, Exoneura sensu stricto, the evolution of large group size and social complexity coincides with the loss or reduction of opportunities for sib-rearing. Assured fitness return models may be applicable to weakly social allodapine species, but do not predict patterns of eusociality. Instead, Australian studies suggest that the evolution of large group size and marked reproductive skew is linked with the need to defend against enemies at the nest, rather than high levels of relatedness, female biased sex allocation or opportunities to rear siblings.

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Allodapine bees have long been regarded as providing useful material for examining the origins of social behavior. Previous researchers have assumed that sociality arose within the Allodapini and have linked the evolution of sociality to a transition from mass provisioning to progressive provisioning of brood. Early phylogenetic studies of allodapines were based on morphological and life-history data, but critical aspects of these studies relied on small character sets, where the polarity and coding of characters is problematic. We used nucleotide sequence data from one nuclear and two mitochondrial gene fragments to examine phylogenetic structure among nine allodapine genera. Our data set comprised 1506 nucleotide positions, of which 402 were parsimony informative. Maximum parsimony, log determinant, and maximum likelihood analyses produced highly similar phylogenetic topologies, and all analyses indicated that the tropical African genus Macrogalea was the sister group to all other allodapines. This finding conflicts with that of previous studies, in which Compsomelissa + Halterapis formed the most basal group. Changing the basal node of the Allodapini has major consequences for understanding evolution in this tribe. Our results cast doubt on the previous hypotheses that progressive provisioning and castelike social behavior evolved among lineages leading to the extant allodapine taxa. Instead, our results suggest that mass provisioning in Halterapis is a derived feature and that social behavior is an ancestral trait for all allodapine lineages. The forms of social behavior present in extant allodapines are likely to have resulted from a long evolutionary history, which may help explain the complexity of social traits found in many allodapine bees.

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The ¢tness associated with behavioural strategies is usually estimated in terms of o¡spring number and size. However, in group-living animals the reproductive value of o¡spring may also depend on their social rank. We show here that in an allodapine bee Exoneura robusta, dominant mothers can behaviourally in£uence their daughters' reproductive rank by controlling insemination of other potential mothers. In E. robusta, group living is near mandatory and reproductive dominance among female nestmates is determined by order of adult emergence. Nests are single, undivided burrows and the dominant female assumes a guarding position closest to the nest entrance. We show that before the egg-laying period, subordinate females who have been absent from the nest are`screened' by the reproductive guard upon attempted re-entry. Those who have been in contact with foreign males are less likely to be granted access back into the nest than those who have been in contact with foreign females or with no bees at all. We argue that by controlling insemination patterns of their nestmates, dominant females ensure that their own daughters eclose ¢rst and are therefore more likely to assume dominance in the next generation. This presents a situation where dominance is bequeathed to daughters by behavioural means. The ability of mothers to in£uence social hierarchies in subsequent generations introduces a ¢tness component additional to the number and size of o¡spring produced.

In heathland populations of the Australian allodapine bee Exoneura bicolor, the presence of overlapping broods in some colonies creates opportunities for eusocial-like sib rearing. By artificially orphaning colonies in observation nests, we investigated whether sib-rearing by some older daughters occurs in the absence of any potential for coercion by parental females. We found strong evidence for sib-rearing in the absence of parental females in 40% of our nests and weaker evidence in another 40%. Of those colonies with strong evidence of sib-rearing, 65% of such rearing occurred in nests where there was also no opportunity for coercion by adult siblings. These results indicate that sib-rearing in E. bicolor does not require coercion by adult nestmates. A variety of factors that may make sib-rearing a preferred strategy in the absence of coercion are discussed.