Sources of Pre- and Postnatal Maternal Energy Allocation to Offspring in a Long-Lived, Capital Breeder: The Grey Seal (Halichoerus grypus)

Parental energy allocation impacts the fitness of offspring and parents alike and should provide insight into the sources of variation in life history traits. Nevertheless, few studies have simultaneously assessed multiple sources of variation in maternal energy allocation and how allocation might vary over a female's lifetime. We used 20 years of cross-sectional (females sampled once) and longitudinal (same female sampled more than once) data on 222 known-age grey seals (Halichoerus grypus), a long-lived capital breeder and 324 of their offspring to examine the influence on maternal energy allocation during lactation of the following variables: maternal postpartum mass (MPPM), total maternal mass loss (TMML), relative maternal mass loss (RMML = TMML:MPPM), mass transfer efficiency, lactation duration and pup birth and weaning mass. Mixed-effect general linear models were used to examine the influence of predictors on response variables, with maternal identity and study year included as random effects. Our findings indicate that during the 17-day lactation period, females expended about 37% (73 kg) of their MPPM to support their own metabolism requirements and milk production. Maternal age and MPPM had significant effects on multiple aspects of maternal energy allocation, including TMML, lactation duration, pup birth mass and pup weaning mass. Heavier females lost more body mass over lactation than lighter females. Maternal allocation increased throughout early life, plateaued in slow-growing prime-age females, and then declined in older females. Parity also affected maternal allocation, but the effect was limited to young females, with influences on pup birth mass, proportional pup mass gain and pup weaning mass. Females that lactated longer had greater RMML and produced pups with greater weaning mass. The effect of pup sex differed depending on MPPM, with lighter females experiencing greater TMML when nursing female pups than male pups and heavier females experiencing the reverse. Despite the substantial loss of body mass during lactation, 33 females measured in two consecutive years generally recovered fully from this energy expenditure, with no evidence of reduced allocation in the next year compared with the previous year. Prenatal allocation (pup birth mass) was a quadratic function of maternal age, increasing parity and pup sex, with female pups born lighter than males. Postnatal allocation (offspring weaning mass) also varied with maternal age, parity and pup sex, but TMML, lactation duration and pup birth mass were also influential.