Can the marginal male hypothesis explain spatial density variation in pinnipeds?

Abstract

The marginal male hypothesis is a mechanism proposed to explain gregariousness, for example 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 for example in Antarctic fur seals (Arctocephalus gazella). We build 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. Based on numerical analysis of the model, we find 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 actually becomes the less dense colony. Our model serves as a proof of concept that such density differences can emerge. We expect that by including additional environmental factors, future models will explain a larger part of the observed variation in density.