Resource abundance and sex allocation by queen and workers in the harvester ant, Messor pergandei

Abstract

The degree to which queens and workers control how limiting resources are split between reproductive males and females is central to the study of sex allocation in the eusocial Hymenoptera. We investigated the effect of resource availability on sex allocation decisions by both queens and workers in the ant Messor pergandei. We conducted the following field manipulations of resource availability: food supplementation while queens were laying reproductive brood (early-fed treatment), removal of workers while queens were laying reproductive brood (worker-removal treatment), food supplementation of colonies while workers were tending reproductive brood (late-fed treatment), and unmanipulated colonies (control). Early-fed colonies produced more alates and exhibited more strongly female-biased sex ratios than other treatments. Worker-removal colonies produced the fewest alates and the least female-biased sex ratios. Late-fed colonies yielded individual alates with the heaviest fresh masses (males and females) and dry masses (only females). Aside from worker-removal colonies, the sex and investment ratios of colonies in this study were significantly more female-biased than the relatedness asymmetries hypothesis under worker control would predict. Consistent with the multifaceted parental investment hypothesis, the timing of food supplementation relative to the reproductive cycle of the colony plays a prominent role in influencing sex allocation by both queens and workers. Early food supplements resulted in increased number of females, whereas late food supplements resulted in heavier individual females. Temporal dynamics of food availability may explain part of the tremendous inter-population variation in colony sex ratios seen in this and other ant species. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00265-002-0462-6.