Limited intra-specific variation in male grey seal (Halichoerus grypus) dominance relationships in relation to variation in male mating success and female availability

Abstract

Inter-male aggressive encounters were examined at three grey seal breeding colonies that differed in topography, adult dispersion patterns and operational sex ratios; North Rona and the Monach Isles (Scotland), and Sable Island (Nova Scotia). At North Rona, known males were observed over three successive breeding seasons, with a total of 275 males being individually identified. A single season's observations at both the Monach Isles and Sable Island provided data on 53 and 80 individual males respectively. Cardinal dominance ranks were computed for 68, 92 and 112 males on North Rona in each season respectively, 37 males on the Monach Isles and 68 males on Sable Island. All sites showed close approximation to dominance hierarchies when considering only interactions resulting in a clear outcome (wins), with less than 10% of interactions resulting in reversals. Measures of variance in male mating success showed no significant differences between colonies. These results show the existence of male dominance hierarchies in colonies from both east and west Atlantic populations and at breeding sites with widely differing habitat structure, seal dispersion patterns and levels of inter-male competition, suggesting limited plasticity in the form of grey seal mating systems and limited variation in the degree of polygyny attained at these sites.