Social evolution in small Carpenter bees (Hymenoptera: Xylocopinae): a phylogenetic perspective

Small carpenter bees are ideal for the study on the origin of sociality because behavior of their member species ranges widely from solitary to highly eusocial. Taxa like the carpenter bees that straddle on the eusocially threshold also respond more to ecological conditions, thereby allowing one to measure the effects of extrinsic factors on social behavior. Moreover, phylogenetic studies have been done at different levels in xylocopinae: tribes, genera, and species. Knowledge of genealogical relationships is essential for comparative evolutionary biology because with a phylogenetic framework, one can disentangle convergent responses from shared ancestry and process theories can be adequately addressed and independently tested. Reyes (1998) proposed a phylogenetic hypothesis based on morphological data that Halterapis + Compsomelissa formed the basal clade of the tribe Allodapini implying that mass provisioning and subsocial behavior are ancestral traits for the tribe. Recently, Schwarz et al. (2003), based on molecular data, argued that Macrogalea forms the basal clade for tribe Allodapini. This phylogenetic hypothesis implies that the origin of social behavior preceded the divergence and diversification of extant lineages and therefore it is not possible to use comparisons among allodapines to explore selective factors underlying the initial origins of eusocialityand the forms of sociality in extant allodapine lineages were shaped by a very long evolutionary history, predating the divergence between Macrogalea and the rest of the tribe. Recent studies on the social organization of some Philippine bee population showed that group living is not fixed trait of the population. In the allodapine genus, Braubsapis, for example, both single- and multi-female nests were observed. Ant predation may play an important role in facilitating group living in tropical environment and this needs to be assessed empirically. The Philippine Ceratinini, represented by the genus Ceratina and close relative of allodapine bees, was also studied. Prereproductive assemblages were observed but females established their nests singly. Abundance of food resource and nesting substrate accounts for solitary nesting strategy of these bees. Future studies should be geared towards elucidating additional social traits for Ceratina and other Braunsapis species. Critical information for future studies include: 1) foundress dispersal2) voltinism3) effects of colony size on brood rearing efficiencyand 4) predator pressure.