The influence of reproductive experience on lactation performance in a capital breeding mammal, the grey seal (Halichoerus grypus)

In mammals, the most significant determinant of offspring growth during lactation is the ability of females to efficiently transfer milk energy to their neonates. Although evidence from domestic and laboratory species suggests that reproductive experience plays a critical role in the development of mammary gland capacity and maternal behaviours, whether reproductive experience may have a significant influence on milk energy transfer to neonates in wild populations has never been directly investigated. In this study I examined how reproductive experience influences aspects of lactation performance and, therefore, maternal reproductive success in free-ranging grey seals ( Halichoerus grypus). Longitudinal data collected from fully-grown, multiparous grey seal females demonstrated that the overall physiological capacity to deliver milk energy to their pups was characteristic of individuals. Comparisons of maternal behaviours, energy expenditures and milk transfer demonstrated that, although primiparous grey seal females had a lower physiological capacity for milk secretion than multiparous females, they effectively compensated through an increased nursing effort and, thus, achieved the same relative rate of milk energy transfer to their pups as multiparous females. While primiparous females were also more active than multiparous females throughout lactation this did not affect their efficiency of energy transfer to pups. Thus, despite a lower capacity for milk production, primiparous females weaned pups of the same relative size and condition as multiparous females without expending proportionally more energy. Comparisons of the mammary gland ultrastructure of primiparous and multiparous females demonstrated that reproductive experience has a significant influence on both the rate and pattern of mammary gland alveolar development suggesting that reproductive experience may have a significant influence on the relative storage capacity of the mammary gland. Although these results demonstrate that reproductive experience does not significantly influence the overall lactation performance of grey seals they do suggest that a reduced physiological capacity for milk secretion in primiparous females may be common among mammals. This work provides an important comparison to studies of domestic and laboratory species and highlights the need for further studies in free-ranging species with different lactation strategies.