Adaptive significance of alternative social strategies in a facultatively social carpenter bee (Xylocopa varipuncta)
Madeleine Ostwald, Madeleine Ostwald
School of Life Sciences, Arizona State University; School of Life Sciences, Arizona State University
With their frequent evolutions and losses of eusociality and broad diversity of social behaviors, bees represent an ideal taxon in which to ask questions about social evolutionary trajectories. However, comparative studies that address differences between solitary and social bee species necessarily involve problematic comparisons across taxa with different life history strategies. Similarly, because the evolutionary forces that maintain eusociality are not necessarily the same as those that originally promoted it, studies of obligately eusocial bees are less informative about the conditions that favor the transition from solitary to social living. Socially polymorphic species in which solitary and social life history strategies co-occur temporally and geographically present a unique opportunity to overcome to these limitations. The carpenter bees (Xylocopinae) have recently received attention for their considerable variation in social behavior. In this study, I present the first formal description of the social organization of the valley carpenter bee, Xylocopa varipuncta. Using observation nests and CT imaging of natural nesting aggregations, I characterize the co-occurrence of social and solitary nests across seasons. The facultative sociality of this species, along with its non-traditional system of division of labor, make it a particularly rewarding system in which to investigate behavioral mechanisms and ecological constraints that precipitate complex sociality. Finally, I estimate per capita brood production in social and solitary nests and propose mechanisms by which sociality may have evolved in this species, even in the absence of inclusive fitness benefits for workers.