A novel mobile approach to investigating mating tactics in male grey seals (Halichoerus grypus)

Abstract

Studies of the grey seal Halichoerus grypus mating system, using focal animal sampling constrained by study site location and size, limit the measurement of variation in male mating tactics and success. Using this method, the mating tactics of grey seal males have been classified as either ‘tenured’ or ‘transient’. Preliminary evidence is presented of wider variation in male mating tactics and success using mobile focal-animal techniques that are not constrained by site, and of the effects of age on behaviour when accounting for body mass. The study was conducted during the breeding seasons of 1997 and 1998 at Sable Island, Nova Scotia. Six young (11–12 years) and five old (20–25 years) males were captured at the beginning of the breeding season, weighed and fitted with a VHF transmitter and time-depth recorder. Each male was located daily, and a count of late-lactating females within a 10 m radius recorded as a measure of mating potential. Female guarding and observed copulatory attempts were used as a measure of mating success. Mean initial body mass of young males and old males was similar, 291 ± 13 kg and 298 ± 14 kg, respectively. All males changed location during the study period. Young males changed sites more often and moved shorter distances between sites than old males. Estimated mating success varied from probably no matings to a minimum of seven; there were no differences between young and old males. Maximum duration of stay at a site and the number of late-lactating females at a site significantly influenced mating success. All the males except for two young males departed on diving excursions during the breeding season and engaged in deep diving behaviour. Diving behaviour did not differ between the age classes. Our results indicate that the current classification of mating tactics in grey seal males is an oversimplification. Preliminary data show that age may account for some of the variation in male behaviour on land and mating success, but not for behaviour at sea.