State-dependent male mating tactics in the grey seal: the importance of body size

The purpose of this study was to examine the importance of body size and body composition as determinants of conditional mating tactics exhibited in male grey seals. We combined behavioral observations with measures of energy expenditure and success on 42 known-age individuals during the breeding seasons of 1997-2001 at Sable Island, Canada. Males with a large body mass arrived at the breeding grounds with more body fat and body energy and catabolized less body protein than smaller males. Males consumed 1.9 +/- 0.2 MJ day(-1), and those with a smaller percentage of body fat had higher rates of food energy intake. The amount of body energy on arrival was positively correlated with the duration of the breeding period. Males that exhibited the primary mating tactic of consortship were heavier, had absolutely more body fat and body energy, and sustained breeding longer than those males that did not exhibit the primary tactic. Amongst all males, body mass showed a quadratic relationship with the number of female consorts mated and the estimated number of pups sired. Thus, intermediate-sized males mated with the most consorts and achieved the highest success. Intermediate body size may be optimal during breeding due to greater agility in male combat. Body size was an important determinant of mating tactics used by male grey seals. A large body size provided an energetic advantage of greater endurance while an intermediate body size may provide greater competitive ability in acquiring consortships.