Social consequences of energetically costly nest construction in a facultatively social bee

Social groups form when the costs of breeding independently exceed fitness
costs imposed by group living. The costs of independent breeding can often
be energetic, especially for animals performing expensive behaviors, such
as nest construction. To test the hypothesis that nesting costs can drive
sociality by disincentivizing independent nest founding, we measured the
energetics of nest construction and inheritance in a facultatively social
carpenter bee (Xylocopa sonorina Smith), which bores tunnel nests
in wood. We measured metabolic rates of bees excavating wood and used
computerized tomography (CT) images of nesting logs to measure excavation
volumes. From these data, we demonstrate costly energetic investments in
nest excavation of a minimum 4.3 kJ per offspring provisioned, an expense
equivalent to nearly 7 hours of flight. This high, potentially prohibitive
cost of nest founding may explain why females compete for existing nests
rather than constructing new ones, often leading to the formation of
social groups. Further, we found that nest inheritors varied considerably
in their investment in nest renovation, with costs ranging more than
twelve-fold (from 7.08 kJ – 89.1 kJ energy), likely reflecting
differences in inherited nest quality. These results suggest that females
may join social groups to avoid steep energetic costs, but that the
benefits of this strategy are not experienced equally.