The Effect of Maternal Size and Milk Energy Output on Pup Growth in Grey Seals (Halichoerus grypus)

Abstract

In phocid species in which females fast throughout lactation, maternal energy reserves at parturition must support lactation. Thus, differences in body size and energy stores may influence the magnitude of maternal energy transfer. We examined variation in milk energy transfer and its consequence for pup growth in grey seals over a wide range of maternal body sizes. Milk samples (n = 42) were collected from 28 females over the 16-d lactation period. Milk yields were measured in nine mother-pup pairs by deuterium oxide dilution. Milk fat and energy content increased dramatically over the first half of lactation. At mid-to late lactation milk composition averaged 71.1% dry matter, 59.8% fat, 9.2% protein, and 24.8 kJ/g (bomb calorimetry). Initial maternal mass (149-256 kg) and daily mass loss (3-9 kg/d) were correlated with daily milk output (1.3-4.1 kg/d). Milk energy output (29-92 MJ/d) in turn explained pup mass gain (0.8-2.8 kg/d, r² = 0.997). Pups with greater energy intakes had a greater ratio of fat to protein deposition and thus a greater efficiency of energy storage, increasing from 40% to 79%. Daily milk intake did not change over lactation, but growth rate was greater during the second half of lactation because milk energy intake was greater. Results of this study indicate that large females lose mass more rapidly, produce more milk, and wean larger, fatter pups than do small females. We conclude that differences in maternal mass and available energy stores may have substantial consequences for the pup and may account for much of the reported variation in pup growth both within and between populations of phocids such as the grey seal.