When to bee social: interactions among environmental constraints, incentives, guarding, and relatedness in a facultatively social carpenter bee

In the facultatively social carpenter bee, Xylocopa pubescens, foundresses usually establish nests solitarily. However, nests may become social if a second foundress (referred to as alpha) successfully usurps the nest, with the original foundress (referred to as beta) remaining as a guard. Reproductive skew theory predicts that beta foundresses should remain as helpers only if alpha usurpers allow them a share of reproduction. Because alpha females destroy much of beta's brood and beta females do not lay eggs after takeovers, studies have concluded that usurpers offer no staying incentives or concessions in return for helping behavior. This conclusion is paradoxical, and we suggest that by refraining from destroying all of beta's brood, alpha females do indeed offer concessions to beta females. We constructed a model to examine the conditions under which social nesting is favored by both alpha and beta females. Female preference for social versus solitary nesting is proportional to expected fitness in either setting and is affected by current environmental conditions, the value of guarding behavior in protecting brood from pollen robbery, the size of the concession offered by alpha, and the degree of genetic relatedness between the foundresses. Our model shows that at a minimum, establishing sociality after unrelated usurpations always requires a concession, whereas in related usurpations, a concession is not always required. Generally, agreement between alpha and beta is difficult because alpha requires a much higher premium in pollen robbery protection than beta in order for sociality to be advantageous. Alpha females prefer social nesting only under the most severe environmental conditions because usually they gain less by the presence of a guard than by replacing beta brood with their own. In contrast, beta females always strongly prefer social nesting because the chances of successful renesting elsewhere are low and rarely outweigh the advantages of guarding their own brood that survive usurpation. Effects of relatedness between foundresses on preference for social nesting are nonintuitive: first, alpha's preference increases with relatedness, whereas beta's preference declines, and second, unrelated beta females prefer sociality more strongly than related ones. This is because replacement of beta's offspring with related alpha offspring partially compensates her for the loss of her own brood, even should she leave the nest. Copyright 2003.