Bowen, W.D., S.J. Iverson, J.I. McMillan & D.J. Boness. 2006. Reproductive performance in grey seals: age-related improvement and senescence in a capital breeder. Journal of Animal Ecology 75: 1340-1351.

Extract, from Introduction: “Although the pattern of female reproductive performance over a broad range of ages is known for a number of animals, relatively few studies have clearly demonstrated reproductive senescence (e.g. Packer et al. 1998). Here, we report on a long-term study of age-specific reproductive performance in female grey seals (Halichoerus grypus), ranging from first-time breeders to females approaching the life span of individuals observed in the wild. The grey seal, Family Phocidae, is well suited for studies of age-related changes in reproductive performance. Females are long-lived (~40 years) and large, with an average body mass of postparturient females of about 197 kg (Mellish, Iverson & Bowen 1999). Females are capital breeders, giving birth to a single pup each year, beginning at age 4-5 years and continuing for several decades or more. During a 16-day lactation period females do not feed, relying instead on body energy stores, in the form of blubber, for both maintenance metabolism and milk production (Iverson et al. 1993). Thus, females with low body mass at parturition tend to wean smaller pups or wean pups prematurely (Iverson et al. 1993; Mellish et al. 1999; Pomeroy et al. 1999), increasing the probability of juvenile mortality (Hall, McConnell & Barker 2001). Females end maternal care abruptly by abandoning their pup and returning to sea. This means that female reproductive expenditure can be measured accurately by the duration of expenditure (i.e. lactation length) and the energy allocated to the offspring (i.e. offspring mass at weaning).”