Mass transfer during lactation of an ice‐breeding pinniped, the grey seal (Halichoerus grypus), in Nova Scotia, Canada

Mass transfer during lactation was examined by serial weighing of mother‐pup pairs in an ice‐breeding population of grey seals, in the southern Gulf of St. Lawrence (c. 46° N‐ c . 63° W), Canada. Male pups (171 ± 1–3kg, n = 4) were significantly heavier at birth ( P = 001) than females (14.6 ± 1.4 kg, n = 9), and grew significantly faster until weaning (males: 2.7 ± 0.6kg/d, n = 45; females: 2.4 ± 0–5kg/d, n = 63, P = 004). At weaning, males (56.2 ± 6.7 kg, n = 24) were significantly heavier ( P = 0‐01) than females (51.6 ± 53kg, n = 25). Lactation for both sexes lasted 14.9 ± 1.4 days ( n = 13). At parturition, adult females weighed 227 ± 26 kg ( n = 26). During lactation they lost 5.6 ± 1.lkg/d ( n = 35). An analysis of carcass composition of six females collected during different stages of lactation showed that they utilized 61% of their fat reserves during lactation. Efficiency of mass transfer varied from 25–3% to 70–4% (49.3 ± 10.8%, n = 35). Examination of stomach contents indicated that some ice‐breeding females consumed food during lactation. Our results in this ice‐breeding population show that females invest more in sons than in daughters.