Environmental constraints have been shown to influence the relative advantages of solitary and cooperative breeding at small spatial scales. However, their contribution to variation in the frequency of cooperative breeding across broad geographic scales is not well understood. We present a latitudinal survey of four Polistes species in the eastern United States. We built and competed models including two climate PCs per species based on a PCA of thirteen climate variables, nearest neighbor distance, and threshold activity time: summer length and day length, the days per year and hours per day, respectively, that the wasps can be physiologically active, based on the temperature threshold below which their flight is impaired. We analyzed the effect of these ecological factors on the frequency of cooperative breeding in our survey, as well as data from a meta-analysis by Sheehan et al. (2015).
We found that in addition to species-specific climate PCs, threshold activity time had a pronounced species-specific effect on cooperation in P. dominula, P. exclamans, P. fuscatus, and P. metricus (>2 dAIC from the null model). This is the first time that threshold activity time has been shown to influence intraspecific variation in cooperative behavior. Threshold activity time is a climate variable based on the physiology of the organism and can be easily adapted to other social animals with different temperature thresholds of activity.