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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.