The role of increased prey spectrum and reduced prey size in the evolution of sociality in Cerceris wasps

Two recent hypotheses have linked resource usage to the evolution of sociality in Hymenoptera. One hypothesis (Expanded Food Choice hypothesis) states that the spectrum width of food resources collected by adults for the brood positively correlates with the degree of vespid sociality. The other hypothesis (Reduced Food Size hypothesis) states that a reduction of size of resource exploited (from prey to pollen) favoured cooperation in Apoidea. I tested, using data available in the literature, if enlarging prey spectrum and/or reducing prey size are linked to social evolution in the genus Cerceris, which includes both solitary and primitively social beetle-hunting digger wasps. I found that social species of Cerceris hunt, on average, about twice (up to 6 times) the number of beetle families hunted by solitary species. Moreover, interestingly, the composition of beetle prey families seems to depend more on social organization than on phylogenetic relationships among wasp species. Social species are smaller than solitary ones. Despite the fact that the data did not allow the direct comparison of the relative prey size between solitary and social species, a positive correlation between the number of prey families and the number of prey/cell indirectly suggests that social Cerceris tend to fill the brood cells with smaller prey. In support of this, I also found a marginal difference in the maximum number of prey/cell between the two groups. Thus, enlarging prey spectrum seems to parallel social evolution in Cerceris wasps, while new studies are necessary to confirm the preliminary suggestion that the relative prey size correlates with the social organization.