Effect of habitat saturation on the number and turnover of queens in the polygynous ant, Myrmica sulcinodis

In polygynous social insects more than one queen reproduces in a colony. In such populations ecological factors affecting survival and reproduction of queens are likely to be of prime importance for social organization. In pa rticular, habitat saturation leading to severe limitations in the availabil ity of nest sites has been suggested to promote high queen number. In this study we examine the social and genetic structure of colonies in the polygy nous ant Myrmica sulcinodis. We investigated a single breeding population i n two adjacent habitats which differed markedly in the availability of nest sites. In the main habitat M. sulcinodis occupied almost all suitable nest sites, whereas in the other (marginal) habitat most sites were unoccupied by ants, due to a recent fire. In support of the habitat saturation hypothe sis, the number of queens per colony which could explain the estimated rela tedness among workers was almost five times higher for the main habitat tha n for the marginal habitat. This is the first demonstration that the kin st ructure of a social insect population is plastic and responds adaptively to short-term changes in ecological constraints such as nest site availabilit y. Based on combined genetic and demographic data we discuss queen reproduc tive strategies and suggest that a special class of queen 'floaters' only s tays ephemerally in the colonies, thus causing a substantial turnover of re producing queens across years.