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

An animal society based on kin competition, not kin cooperation

“…As a result, secondary females in ranks 2 and 3 often achieved breeder status partway through the brood-provisioning season, although females in ranks 4 to 6 never did. Moreover, in our study population, primary and second females virtually never survive to a second breeding season (14), so their fitness is completely predicated upon raising brood in the current breeding season. Thus, the significantly longer reproductive queues of 2012 would have led to significantly lower average direct fitness of subordinate females that year.…”

“…In high density conditions (2012), 96% of overwintered females nested socially (in groups) and average group size was significantly higher than in 2013 when population density was much lower and only 70% of females nested socially (Table 1) While population-level competition for available nest sites is the critical factor driving group formation in X. virginica, it also intensifies within-group competition for breeding opportunities. In eastern carpenter bees, only the dominant female in a social group forages and lays eggs, whereas subordinates queue for the opportunity to replace her when she dies or becomes moribund (14). Whereas group size was significantly higher in 2012 than in 2013, the duration of the brood provisioning phase of the colony cycle was the same (Table 1).…”

“…Most females breed in old nests, remaining in their natal nests or relocating to neighbouring nests containing groups of unrelated females; much less frequently, a female initiates construction of a new nest. When nesting in groups, female carpenter bees form linear reproductive queues in which the dominant female monopolizes reproduction, while subordinates wait for opportunities to replace her (13,14).…”

“…Group size is usually 2-4 females, but may be as high as 8; refs (14,15) and below. Thus, females experience intense breeding competition at the population level for access to nest sites, and within social groups for egg-laying opportunities.…”

“…The second phase is a brood provisioning phase (BPP) in which the dominant female provisions brood cells in which only she lays eggs. If the dominant female dies, disappears or becomes moribund, the next female in line replaces her as the principal forager and egg-layer (13,14). During NMP and as late as BPP, secondary females often leave their natal nests to join neighbouring nests in the same aggregation, other aggregations, or leave the area altogether to nest elsewhere.…”