We describe an unusual mating system, observed in a land-breeding colony of Grey seal, Halichoerus grypus, in the western Atlantic. Males and females begin to visit the breeding beach about a week before the season begins, but none stay ashore for long until the first pup is born. The cows are gregarious, probably return to the same part of the beach to give birth from one year to the next, and tend to remain in the general vicinity of the birth site during their two and a half week sojourn ashore. Within these limits, however, they are quite mobile, and the size, location and composition of the temporary aggregations which they form vary from one day to the next. The cows become thinner and more sedentary as oestrus approaches, but otherwise they give no overt signs of receptivity. The males do not defend territories, nor do they form dominance hierarchies. Instead, they compete for tenure, the right to remain within the shifting population of females. Tenured bulls directly test the receptivity of nearby cows from time to time, and they continually manoeuvre in ways which maximize their chances of being next to cows which are either in oestrus or likely to become so in the near future. Bulls which fail to establish themselves amongst females try to intercept cows as they are leaving for the sea at the end of their season, but their reproductive success, estimated in several ways, is significantly lower than that of bulls with tenure.
The system would be adaptive for seals which breed on the pack ice, but it is unique amongst land-breeding pinnipeds. If, as some circumstantial evidence suggests, Grey seals were originally pack-ice breeders, the persistence of such a system in a land-breeding colony raises some interesting questions about the plasticity of mating behaviour.