Foraging effort, food intake and lactation performance depend on maternal mass in a small phocid seal

1. Female mammals increase energy expenditure during lactation to support t he high cost of milk production. The extent to which lactation in a small p hocid species, the Harbour Seal Phoca vitulina L., was fuelled by food vs b ody stores, how this allocation varied with maternal body mass and the cons equences of maternal expenditure on offspring growth were studied. 2. The proportional body composition of 30 females was independent of initi al postpartum body mass, but larger females had absolutely more stored ener gy than smaller ones. 3. Females lost 32% of postpartum body mass and 62% of body energy by late lactation; 97% of energy loss was derived from body fat. Percentage loss of body energy was independent of initial body mass, indicating that females limit their allocation of body stores to offspring by expending a constant proportion of stores rather than a constant amount. 4. Females spent more time diving and individual dives were deeper and long er as lactation progressed. By late lactation, these characteristics of div ing were inversely proportional with initial postpartum mass. 5. During early lactation, female expenditures were covered mainly by a red uction in body energy stores. By late lactation, food intake increased six- fold but the extent of this increase varied inversely with postpartum mass. 6. Pup growth rate and weaning mass were positively related to postpartum m ass and total daily energy expenditure of females, but were independent of the source of energy used by females during lactation. Pups of heavy female s had higher survival than pups of light females. 7. Our results support the hypothesis that maternal body mass is an importa nt determinant of lactation strategies in pinnipeds.