Split sex ratios provide broad insights into how reproductive strategies evolve, and historically have special relevance to the evolution of eusociality. Yet almost no attention has been directed to situations where split sex ratios may potentially decrease the payoffs for worker-like behaviour, increasing selective thresholds for eusociality. We examined sex ratios in a facultatively social colletid bee, Amphylaeus morosus. Sex ratios in this bee vary strongly with the presence of a nest guard and in a pattern that does not conform to assumptions of previous models in which split sex ratios facilitate altruism. While the production of daughters was constant across social and solitary nests, mothers produced more brood when a non-reproductive guard was present, but these extra brood were all male. This leads to split sex ratios, vicariously driven by guards that are unable to manipulate sex ratios in their favour. Importantly, if guarding becomes more common in a population this would lead to an excess of males and lower the genetic value of these extra males to guards, effectively putting a brake on selection for worker-like behaviour.
Keywords: Hymenoptera; natal philopatry; sex allocation; social evolution; split sex ratios.