Behavioural and Energetic Determinants of Individual Mating Success in Male Grey Seals (Halichoerus grypus, Fabricius 1791)

(1) Potential behavioural and energetic determinants of male grey seal mating success were examined over three successive breeding seasons (1987 to 1989) on North Rona (Scotland). A total of 275 males were individually identified. Records of individual male mating success, inter-male aggression (to yield dominance indices), colony attendance patterns and detailed activity budgets were obtained. Selected individuals were caught and weighed and rates of weight loss (both absolute and proportional) and weights on date of arrival to and departure from the colony were calculated. Standard measurements were taken and a single incisor tooth extracted for age determination. (2) Length of stay on the breeding colony was the primary correlate of mating success. (3) Using records of inter-male aggressive encounters a clear dominance hierarchy was found amongst the males. Dominant males were not necessarily heavier or bigger, nor were dominant males older than subordinates on average. There were low levels of intermale aggression on the colony, and evidence of prior knowledge of relative status amongst I males. It is proposed that male dominance relationships are predominantly established in the water prior to arrival on the colony. Thus, size may not be an important determinant of male competitive ability in grey seals. (4) Mating success was also highly correlated with dominance in all three seasons. Dominant individuals were able to establish positions amongst the groups of females and maintain these for significantly longer than more subordinate males, thus giving access to more oestrus females. (5) Dominant males experienced lower daily rates of aggression and did not incur greater rates of weight loss (either absolute or proportional). (6) No evidence of energetic constraints on male mating success could be found. Due to the topography of North Rona, dominant males were able to exclude subordinates from the breeding grounds (the average sex ratio on the colony during the breeding season was approximately 1 male : 7 females). Relatively few males were therefore, able to remain on the colony for the length of time that their energy reserves permitted, thus masking any potential energetic limits on length of stay. (7) It is suggested that grey seals are sexually size dimorphic primarily due to the differing energy storage requirements of the two sexes determined by the different strategies adopted by males and females in order to maximise individual reproductive success. (8) Males that returned in successive seasons were the more dominant individuals of the previous season, therefore remained ashore for longer and gained greater mating success in the previous year. Also, returning males were younger on average than those that failed to return, were not necessarily the heavier males but were those that incurred greater proportional rates of weight loss in the previous season. (9) One male, present in all three seasons, was conspicuously successful in all three years and had by far the greatest mating success of all males observed during the course of the study. This male had been present on North Rona since at least 1980. (10) A comparative study was conducted with one field season on Sable Island (Nova Scotia, Canada). Unlike North Rona, dominance was the primary correlate of mating success. Considerable differences in the apparent sex ratios were observed between Sable (1 male : 2 females) and Rona (1:7). Also, oestrus females on Sable were spatially more evenly distributed and temporally more aggregated than on Rona. Thus, one would predict a lower environmental polygamy potential on Sable Island. However, there were no significant differences in measures of the "degree of polygyny" at the two sites. Thus, although relatively more males gained positions ashore on Sable (access was not restricted by topography), dominant individuals still monopolised mating opportunities. These results suggest that plasticity of the form of mating system shown amongst grey seal populations is limited