Reproductive success is influenced by early development: the covariation of natal body size and reproductive performance in grey seals (Halichoerus grypus)

An individuals size when young may be an important source 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 mark-recapture and repeated measures reproductive data on a marked sample of 363 females that were measured for standard length at roughly 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 (the rate at which a female returns to breed), modeled using mixed-effects multistate mark-recapture models. After accounting for female age, experience, and offspring sex, we found a positive association between natal length and our measures of reproductive performance. Mothers with the longest natal lengths produced pups nearly 8 kg heavier and were 20% more likely to breed in a given year than mothers with the shortest natal lengths. Because correlation in individual body length between natal and adult life stages is weak (animals that were longer than average as pups do not always grow to be longer than average adults), the covariation between natal length and future reproductive performance reported here may act more as a carry-over effect from the size advantages afforded in the juvenile stage that allow for greater adult performance, rather than a physical trait that is maintained throughout life.