Large offspring have enhanced lifetime reproductive success: long-term carry-over effects of natal size in grey seals (Halichoerus grypus)

An individual’s size in early stages of life may be an important source of individual variation in lifetime reproductive performance, as size effects on ontogenetic development can have cascading physiological and behavioral consequences throughout life. Here, we explored how natal size influences subsequent reproductive performance in grey seals (Halichoerus grypus) using repeated encounter and reproductive data on a marked sample of 363 females that were measured for length at ~4 weeks of age and eventually recruited to the Sable Island breeding colony. Two reproductive traits were considered: provisioning performance (mass of weaned offspring), modeled using linear mixed effects models; and reproductive frequency (rate at which a female returns to breed), modeled using mixed-effects multistate mark-recapture models. Mothers with the longest natal lengths produced pups 8 kg heavier and were 20% more likely to breed in a given year than mothers with the shortest lengths. Correlation in body lengths between natal and adult life stages, however, is weak: longer pups do not grow to be longer than average adults. Thus covariation between natal length and future reproductive performance appears to be a carry-over effect, where the size advantages afforded in early juvenile stages may allow enhanced long-term performance in adulthood.