Sex allocation in a facultatively polygynous ant: between-population and between-colony variation

Abstract

We investigated sex allocation in three U.K. populations of the facultatively polygynous ant Leptothorax acervorum over 1–3 years. The first main finding was that, across sites, the population sex-investment ratio changed from significantly female biased to significantly male biased with increasing polygyny. This was consistent with workers controlling sex allocation and reacting to changes in their population-level relatedness asymmetry. It was also consistent with local resource competition due to reproduction by colony budding under polygyny. Worker control was supported by the finding that queen number had no effect on sex allocation among polygynous colonies. The second main result was that monogynous colonies consistently produced more female-biased sex-investment ratios than polygynous colonies in one site only (Santon). The results from Santon supported both the relative relatedness asymmetry hypothesis and the idea of sex ratio compensation due to colony budding. The workers’ response to their population-level relatedness asymmetry reinforced the case for relatedness asymmetry being influential at the colony level. The other populations could have lacked split sex ratios because polygynous colonies were either comparatively rare or common, making them behave as almost entirely monogynous (Aberfoyle) or polygynous (Roydon) populations. In Roydon, this was consistent with the inference from allozyme data that monogynous and polygynous colonies did not differ in their worker relatedness asymmetries. The final principal finding was that, of hypotheses linking the colony sex-investment ratio with sexual productivity, there was support for the constant female hypothesis but not for the constant male, cost variation, or multifaceted parental investment hypotheses.