Sex differences in the seasonal patterns of energy storage and expenditure in a phocid seal

1. Sex differences in the timing and magnitude of energy expenditure for reproduction may result in sex-specific seasonal patterns of energy storage and utilization, particularly among capital-breeding species. We studied the seasonal patterns of energy storage in adult grey seals, Halichoerus grypus , a capital-breeding, marine carnivore, to test the prediction that males and females differ in their seasonal pattern of energy storage and expenditure. 2. We measured body mass and composition in 135 (67 males, 68 females) adult grey seals at five key points throughout the annual cycle. Longitudinal changes in body composition were also measured in 73 individuals (35 males, 38 females) to examine individual variability. 3. There were significant sex differences in seasonal patterns of total body energy, with females exhibiting greater energy content (after accounting for body mass) throughout the year. Females gained body energy following both the spring moult and the breeding season, whereas males only gained body energy following an extended period of loss that included both breeding and moulting, a period of some 6 months. 4. Mass loss of females during the breeding season was similar in magnitude and composition to that gained during the 7-month pre-breeding foraging period. In contrast, mass loss of males during the breeding season was more similar to that gained during the last three months of the pre-breeding foraging period. 5. Our results suggest that the balance of costs and benefits of storing body energy for reproduction differs between males and females. Females appear to be risk-averse, committing themselves to, and preparing for, reproduction at a much earlier stage in the annual cycle compared to males. As a result, females accumulate body energy stores earlier and carry a higher level of insurance against environmental uncertainty leading up to the breeding season relative to males.