Maternal effects on offspring growth rate and weaning mass in harbour seals

We studied maternal effects on offspring traits during lactation in harbour seals (Phoca vitulina) on Sable Island, Nova Scotia, from 1988 to 1996. Duration of lactation was correlated with rate of pup mass gain (r = -0.34, n = 116) and weaning mass (r = 0.29, n = 154). Pups that grew faster had shorter nursing periods, whereas those that attained higher weaning masses nursed for a greater number of days. Pup sex did not affect patterns of maternal effects. The pups of young females (4-6 years old) gained mass at a constant but lower rate (0.56 kg/d) than the pups of older females through midlactation (0.74-0.78 kg/d; n = 75). In older females, rates of pup mass gain decelerated between mid and late lactation. Although maternal age did not directly affect weaning mass of pups, path analysis showed that maternal age acted on weaning mass through intermediary traits. Lighter females gave birth to smaller and slower growing pups, but invested relatively more than heavier females (n = 153). Effects of maternal postpartum mass on weaning mass (n = 100) were weaker in harbour seals than in phocids that fast during lactation, but apparently stronger than in otariids that forage during lactation, suggesting that the strength of maternal effects is influenced by lactation strategy.