The Extension of Foundress Life Span and the Evolution of Eusociality in the Hymenoptera.

AbstractThe evolution of effectively sterile workers in the aculeate Hymenoptera (ants, bees, and stinging wasps) requires that a female's life span largely overlap that of her daughters. The evolution of long nest foundress life spans in eusocial species from the short life spans of solitary species is investigated. Analyses that control for phylogeny show for the first time that foundress adult life span increases and first-brood offspring development time decreases with increasing colony size, resulting in the ratio of foundress adult life span to worker total life span increasing with increasing colony size. These patterns support the hypothesis that the reproductive division of labor increases with increasing colony size, explaining the evolution of effectively sterile workers in species with large colonies. However, there is a discrete increase in foundress adult life span in the transition from noneusociality to eusociality that is independent of colony size. An analysis of life history characters suggests that this increase is explained by nests being founded by multiple females and progressive feeding of larvae as they develop. A reduced rate of senescence of a dominant cofoundress may be selected as a plastic response to social status if high-risk tasks performed by subordinate cofoundresses reduce the dominant's extrinsic mortality rate. Multiple-foundress nests in which one female is responsible for most or all of the reproduction (semisociality) and in which foundresses are full sisters are favored by haplodiploidy, perhaps explaining why eusociality is so common in the Hymenoptera.