The marginal male hypothesis explains only small amounts of spatial variation in density in pinnipeds.

The marginal male hypothesis is a mechanism proposed to explain gregariousness in pinnipeds. Here, we explore whether this mechanism, combined with density-dependent pup survival, can also account for heterogeneity in density across colonies, as observed in Antarctic fur seals (Arctocephalus gazella). We built a discrete-time matrix model inspired by the Antarctic fur seal to investigate how differences in density between two colonies can emerge through mate choice and density-dependent pup mortality. Our model assumes a heritable male colony preference that is coupled to competitive ability, i.e. a local siring advantage. Female colony preference is modelled as an independent trait that is allowed to evolve separately. Pup survival decreases with local density. We observe slight differences in density between the colonies at equilibrium. When siring advantage is high and the survival penalty for males with siring advantage is low, the colony that is preferred by the high-quality males becomes the less dense colony. Our model serves as a proof of concept that such density differences can emerge, although the predicted density differences are much smaller than those observed in the field. We expect that by including additional environmental factors, future models will explain more of the observed variation in density.