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

Abstract

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 behaviours, 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 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 h 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 12-fold (from 7.08 to 89.1 kJ energy), probably reflecting differences in inherited nest quality. On average, renovation costs were lower than estimated new nest construction costs, with some nests providing major savings. 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.

Keywords: energetic costs; group living; metabolic rate; nest building; social strategy.

Figure 1.

(a) Metabolic rates (W) of bees during resting, excavation and flight. The metabolic rate of bees during wood excavation was significantly higher than that of resting bees (p < 0.001), but lower than that of flying bees (p < 0.001). (b) Cost of excavation (J cm⁻³) is significantly higher for excavation of hard wood than for soft wood (p < 0.001).

Figure 2.

(a) CT scan image slice of an occupied nesting log, showing one focal nest, highlighted blue, and one brood cell, highlighted orange. (b) Three-dimensional reconstruction of focal nest prior to the breeding season (January) (c) Three-dimensional reconstruction of the same focal nest after the breeding season (September). The change in tunnel volume from (b) to (c) represents the renovation volume excavated by the female who inherited this nest. Yellow dots indicate the position of nest entrances. (d) Estimates of energy costs (J) to renovate a nest (left axis; n = 8 nests). The axis on the right corresponds to estimated energy costs associated with excavating brood cells (i.e. one brood cell = space to provision a single offspring). The horizontal line and grey region represent the observed mean ± s.e. number of offspring provisioned per nest in this population. (Online version in colour.)