Neonatal growth patterns, sexual dimorphism and compensatory growth in harbour seal pups

Juvenile survival, a fundamental parameter in the population dynamics of long-lived species, is often tightly linked to early-life body growth. Here, based on repeated mass measurements on 555 harbour seal Phoca vitulina pups marked between 1998 and 2023 in the St. Lawrence Estuary, we first characterized the shape of neonatal growth patterns of the pups using Bayesian hierarchical nonlinear models. Second, we evaluated the effect of sex on neonatal growth to better understand the development of sexual dimorphism in this species and tested for compensatory growth. Finally, we assessed the influence of local variables (wind speed, sea surface temperature, pup abundance and breeding site) during the neonatal stage on pup weaning mass. Our results showed that neonatal growth follows a Gompertz curve, characterized by minimal growth the first few days after birth, followed by a significant increase in mass until a stabilization near weaning (~30 d of age). We found no sex difference in the growth pattern, but males were slightly heavier at birth than females. Larger pups accumulated more mass than smaller pups, suggesting increasing returns for greater body mass at birth but no compensatory growth. Finally, wind speed, sea surface temperature, pup abundance and breeding site did not affect weaning mass. This study provides insight into the characterization of nonlinear growth in harbour seal pups. Monitoring neonatal growth provides a tool to assess potential patterns of juvenile survival, which are important in studies of population dynamics.