Cooperative or not? Big groups of foundresses in a quasisocial parasitoid

In the parasitoid genus Sclerodermus, multiple foundresses communally produce and care for broods. When hosts are large, group members have greater reproductive success than when attempting to reproduce alone. Prior studies evaluated groups of ≤10 foundresses but the natural range of group sizes is undocumented. We create groups of up to 55 foundresses and explore the consequences for the success and timing of cooperative brooding and on brood sex ratios. As group size increases, brood failures increase and per capita success decreases. Most broods fail at the early developmental stages. Foundresses competition for oviposition sites is likely as is determined by reproductive dominance and ovicide. While most foundresses likely produce some progeny, many foundresses do not produce any adult sons. Sex ratios are very biased: 10% of offspring are males. Based on this result, and a recent finding that larger and earlier-arriving females within smaller foundress groups produce most male offspring, we suggest that Sclerodermus brood sex ratios are the result of a combination of local mate competition and the dominance (via suppression or infanticide) of male production by some foundresses within what initially appeared to be cooperatively reproducing groups.