Seasonal sex ratio and unbalanced investment sex ratio in the Banksia bee Hylaeus alcyoneus

Abstract 1. Hylaeus alcyoneus is an endemic solitary bee common on coastal heaths of Western Australia. The bee is unusual in that males are larger than females. This size dimorphism presents an opportunity to test the theory of resource-dependent sex allocation, in which theory predicts that when resources are low the sex ratio should be biased towards the smaller sex. In most bees, females are larger than males and, in line with theoretical prediction, sex ratios are male biased when resources are scarce.
2. The emerging sex ratio and brood mass from a natural population of H. alcyoneus using trap nests was studied over two seasons (1999, 2000). A switch from a male- to a female-biased sex ratio through the season was found, which was related to a reduced floral resource.
3. Fisherian sex ratio theory predicts that total investment in each sex throughout a season should be equal and that the sex ratio should be biased towards the smaller sex. By measuring the mass of the emerging progeny, the total investment was found to favour males. Possible explanations for this bias in investment are discussed.