Linear dominance hierarchies and conditional reproductive strategies in a facultatively social carpenter bee

In social groups, dominance rank may have important fitness consequences, as higher ranking individuals tend to have higher overall fitness. In social nests of the eastern carpenter bee, Xylocopa virginica, females in social nests demonstrate a complete division of labour where one female is the dominant egg layer and forager while other females in the nest are non-reproductive. We investigated the nature of reproductive queues in this species by performing removal experiments across 3 years to observe how females respond to new reproductive opportunities in the nest. When a primary female was removed, a secondary female always assumed her position as replacement primary and reproductive queues formed in a linear fashion. A third type of female in the nest, the tertiary female, did not become reproductive, even if she was the only female remaining. In delaying reproduction, tertiary females were able to overwinter a second time and were often successful at becoming reproductive in their second summer. Tertiary females were smaller than primary or secondary females, had higher fat stores and lower ovarian development. When all other females in the nest were removed, tertiary females were observed ejecting the offspring of previous dominant females in the nest. Tertiary females appear to represent a novel reproductive strategy among the Hymenoptera who can drastically alter their physiology and behaviour, essentially doubling their life span to maximize reproductive potential.