Reproductive skew and genetic relatedness inLeptothoraxants

Abstract

Optimization models for the evolution of high reproductive skew in animal societies predict that high skew is evolutionarily stable only if group members are closely related. In the ant genus Leptothorax, nestmate queens differ remarkably in their behaviour: whereas in some species, queens fight and form social and reproductive dominance hierarchies (\`functional monogyny'), in others they appear to ignore each other and lay eggs at more or less similar rates (\`polygyny'). A comparative analysis of the sociogenetic organization of Leptothorax colonies indicates that: (i) worker and queen relatedness are surprisingly high even in most polygynous species; and (ii) reproductive skew as estimated from egg laying rates or the ovarian development of nestmate queens does not strongly affect the genetic structure of the colonies. In functionally monogynous species, a frequent exchange of the dominant queen apparently lowers the average relatedness among adult nestmates. In some polygynous species, on the other hand, mechanisms other than aggressive competition among queens keep relatedness from decreasing. Cyclical changes in queen number per colony and a negative influence of high queen number on the production of female sexuals may be important in this context. Preliminary relatedness estimates from virgin female sexuals from polygynous colonies of three Leptothorax species seem to indicate an association of high skew and high queen relatedness as predicted by the skew models. This result, however, does not necessarily mean that high relatedness causes high skew: as young queens are readopted mostly into their native colonies after mating, high relatedness and high skew may be mutually reinforcing.