Reproductive performance in grey seals: age‐related improvement and senescence in a capital breeder

Abstract

1. Three hypotheses have been advanced to account for age-related improvement in performance: the selection hypothesis predicts improved due to the loss of lower quality phenotypes, the constraint hypothesis predicts individuals improve function, and the restraint hypothesis predicts younger individuals forego or reduce effort because of mortality risks. A decline in age-related performance (i.e. senescence) is predicted by mutation accumulation, antagonistic pleiotropy and disposable soma (wear and tear) hypotheses. 2. Using five measures of performance - birth rate, maternal and pup birth mass, pup weaning mass, weaning success and lactation length - we tested these hypotheses concerning age-related change in reproduction in 279 female grey seals (Halichoerus grypus), ages 4-42 years, over a 23-year period between 1983 and 2005 on Sable Island, Nova Scotia. These females produced 2071 pups. 3. Although body mass of primiparous females increased with age (4-7 years) birth mass of their pups did not, but pup weaning mass did. Second- and third-parity females of the same age as primiparous females gave birth to and weaned heavier pups. However, parity and age were dropped from models when maternal body mass was included. 4. The proportion of females giving birth varied significantly with maternal age, increasing in young females and then declining late in life. Weaning success rate also increased rapidly to about 8 years and subsequently declined in females > 32 years. 5. Generalized additive models indicated nonlinear changes in 3 day body mass (i.e. approximately birth mass) and weaning mass of pups as a function of maternal age, after accounting statistically for the effects of maternal body mass. Mixed-effects, repeated-measures models fitted to longitudinal data further supported the conclusion that pup birth mass and weaning mass vary nonlinearly with maternal age and indicated nonlinear changes in lactation duration. 6. We found some support for the constraint hypothesis, but our findings were not consistent with the selection hypothesis or the restraint hypothesis as the basis for improvement in reproductive performance. 7. Senescence was evident in multiple female and offspring traits, indicating the degeneration in function of several physiological systems as predicted by the disposable soma hypothesis.

Vertical movements of North Sea cod

“…Similarly, there are currently major efforts to measure feeding rates associated with dive types for air-breathing vertebrates such as seals (Bowen et al 2006) and penguins (Wilson et al 2002) using approaches such as stomach temperature recorders, video imaging, and mouth opening sensors. Until the technology has been further miniaturised to permit the measurement of feeding rate in relatively small vertebrates like cod, our inferences about the purpose of vertical movements will remain educated guesses.…”

Age-related reproductive variation in a wild marine mammal population

“…Life history theory predicts a change in age specific reproductive performance as limited resources must be allocated to maximise reproduction not withstanding maternal growth and maintenance (Roff 1992). Three main hypotheses can explain improved performance (1) selection hypothesis, (2) constraint hypothesis and (3) restraint hypothesis (Curios 1983;Bowen et al 2006). …”

Evolution of maternal effect senescence

“…The rapidly growing literature on senescence in wild animal populations provides evidence for divergent aging patterns in female reproductive traits (48). Numerous studies of mammals, birds, and reptiles have presented simultaneous age-dependent estimates of female fecundity as well as estimates of offspring survival in mammals (11,20,24,49,50), birds (21,23,(51)(52)(53)(54), and reptiles (22,55). However, very few have formally compared the patterns of aging between these two measures.…”