REPRODUCTIVE SKEW AND GENETIC RELATEDNESS IN LEPTOTHORAX ANTS

Optimization models for the evolution of high reproductive skew in ani mal societies predict that high skew is evolutionarily stable only if group members are closely related. In the ant genus Leptothorax, nestm ate queens differ remarkably in their behaviour: whereas in some speci es, queens fight and form social and reproductive dominance hierarchie s ('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 ind icates that: (i) worker and queen relatedness are surprisingly high ev en in most polygynous species; and (ii) reproductive skew as estimated from egg laying rates or the ovarian development of nestmate queens d oes not strongly affect the genetic structure of the colonies. In func tionally monogynous species, a frequent exchange of the dominant queen apparently lowers the average relatedness among adult nestmates. In s ome polygynous species, on the other hand, mechanisms other than aggre ssive competition among queens keep relatedness from decreasing. Cycli cal changes in queen number per colony and a negative influence of hig h queen number on the production of female sexuals may be important in this context. Preliminary relatedness estimates from virgin female se xuals from polygynous colonies of three Leptothorax species seem to in dicate an association of high skew and high queen relatedness as predi cted by the skew models. This result, however, does not necessarily me an that high relatedness causes high skew: as young queens are readopt ed mostly into their native colonies after mating, high relatedness an d high skew may be mutually reinforcing.