Temporal and spatial dynamics of carpenter bee sociality revealed by CT imaging

Facultatively social animals adaptively match social strategy to environmental context; as such, they offer unique insights into the ecological factors facilitating social evolution. We investigated temporal (seasonal) and spatial (nest architectural) factors governing flexible social behavior in the carpenter bee Xylocopa varipuncta Patton using repeated, non-destructive computerized tomography scans of nesting logs. We tested the hypothesis that group living is mediated by environmental factors, specifically ecological constraints and phenological parameters. These imaging data support a facultative social organization strongly influenced by seasonal shifts in life-history strategy. Our results also illuminate patterns of structural change associated with nest inheritance and eventual nest abandonment. This dynamic use of space mediates the within-nest interactions that determine social organization. Furthermore, constraints on the usefulness of inherited nest structures compound an existing limitation on nest sites that may underlie the origins of this flexible social strategy. These findings emphasize the importance of including spatial dynamics in considerations of the ecological contexts in which sociality evolved.