Demography and Sex Ratio in Social Spiders.

Spiders that live in large cooperative societies are scattered among several taxonomic groups. All quasisocial species appear to have female-biased sex ratios, while congeneric species with less advanced forms of sociality have 1:1 sex ratios. I present two sex-ratio models that examine the interaction between the genetic structure of populations and two aspects of colony demography-the changes as the colony grows larger in fecundity per female and in colony survivorship. In the first model, I assume that all members of the population produce the same sex ratio under all conditions (monomorphic strategy), while in the second model I assume that the sex ratio can be adjusted according to stage in the colony growth cycle (conditional strategy). The results of these models are consistent with the typically observed effects of within-sex competition among relatives; the sex ratio is biased toward the sex with less intense competition. In addition to these effects, a number of interesting interactions are revealed among relatedness, demography, and constraints of the sex-determining mechanism (monomorphic vs. conditional strategies). For example, when survivorship or fecundity increases as the colony grows larger, the predicted sex ratio becomes more female-biased. These demographic factors of changing viability and fecundity with changing colony size interact synergistically with relatedness, and the effect of this interaction on the sex ratio depends on the constraints of the sex-determining mechanism. There are two related roles for the models presented here. First, these models provide some general predictions about how complex variables of demography and population structure interact in shaping the evolution of social spider sex ratios. Second, the development of the models illustrates a theoretical method that provides a simple description of complex interactions between demography and population structure, as well as an example of the synergistic interaction between relatedness and cooperation in the context of a particular life history.