Maternal effects on offspring mass and stage of development at birth in the harbor seal, Phoca vitulina

We studied effects of maternal age, body mass, and parturition date on birth mass and stage of development at birth in harbor seals (Phoca vitulina) on Sable Island, Nova Scotia, over a 10-year period. As predicted, effects of maternal mass and age on pup traits varied with maternal age. Premature pups, characterized by extensive fetal pelage termed lanugo, weighed 20‰ less than newborns without lanugo and were born early in the season to low-body-mass females 4-6 years of age. Among intermediate-age females (7-10 years old), percentage of lanugo on pups was correlated negatively with female postpartum body mass but not with female age. Thus, maternal condition rather than age had stronger effects on stage of development of offspring at birth. Overall, maternal age explained 54‰ of the variance in birth mass, whereas postpartum mass of females explained only 20‰ of the variance. These relationships did not differ with sex of the pup, although male pups were 3.7‰ heavier than female pups. Effects of maternal age were strongest among young females; however, reproductive parity was more important than age itself in determining offspring birth mass.