The presence of a guard vicariously drives split sex ratios in a facultatively social bee

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 le..., Nests of Amphyaleus morosus (n = 298) were collected from the Dandenong Ranges, Victoria, Australia throughout the reproductive season. Nests were sampled across five consecutive years (2017-2021; equating to four reproductive seasons) and seven separate collections.
To assess patterns of investment sex ratios in colonies of A. morosus, brood sex, wet weight, and brood cell position were recorded. Pupae were weighed on a Thermoline precision balance to ± 0.1 mg. The numerical sex ratio (NSR) was calculated as the number of male brood divided by the total number of brood that reached pupation to a point that they could be reliably sexed (NSR = Σmale brood/ Σmale brood + female brood). Investment sex ratio (ISR) was calculated as a product of the numerical sex ratio and the pupal weight ratio was calculated from mean brood sex pupal weight and was used to test whether the observed numerical sex ratio deviated from the investment a mother allocates to each sex. Brood that had died before n...,