Patterns of colonisation and breeding behaviour were observed in part of the Grey seal rookery at North Rona during the entire breeding season in 1972.
Bulls came ashore when pupping started and joined cows grouped around pools. Numbers of cows and bulls increased to maxima in three and five weeks respectively. Limited access to the rookery and the presence of pools and waterlogged ground influenced site selection by cows.
In the study area the breeding season lasted for 74 days, with individual bulls staying for periods varying from 6–57 days. Daily ratios of bulls to cows varied, but the average ratio was 1:7-5. It was estimated that 275 cows participated in mating, compared with 37 bulls, during the entire breeding season. The interval of 18 days between peak numbers of cows and bulls ashore corresponded closely with the average time that cows spent ashore.
The aggressive responses of cows in mating behaviour were noted during observations of 114 copulations. It was concluded that a bull requires a very strong sexual drive in order to overcome a cow's aggressive responses, and this reinforces the selective advantage of aggressive dominance in bulls. The stimulus inducing a bull to attempt mating seemed to be the presence of oestrous cows in the rookery rather than an individual cow being in oestrus, because the number of attempted copulations increased even when the number of cows in oestrus temporarily declined during the season. Direct observations and calculations using an index of copulation suggested that the copulation frequency for individual cows ranged from 2.5-3.6. A bull's participation in mating, and therefore, its genetic influence, was related to the length of its stay in the rookery, and from this evidence it was deduced that a social hierarchy existed amongst the bulls.
Appraisal of the results led to the conclusion that the Grey seal bull maintains a high level of sexual activity so as to mate with as many cows as possible, rather than ensuring exclusive access to cows through territorial fighting and boundary display.