Elephant seals as sentinels of the ongoing changes affecting the Southern Ocean: Disentangling environmental and maternal effects on pup size and body condition.

The weaning mass of southern elephant seal pups (Mirounga leonina) is a key predictor of their first-year survival probability. However, variations in pup morphometric characteristics (length, mass and body condition i.e. the residual of the allometric relationship between log(mass) and log(length)) are strongly associated with mother's length and body condition. Leveraging a comprehensive long-term dataset (2006-2024) that integrates morphometric, isotopic, and environmental data, we applied a structural equation model to disentangle the direct and indirect drivers of pup phenotype at weaning. This approach reveals that pup length and mass are primarily determined by maternal length, while pup body condition is mainly shaped by the environmental conditions experienced by the mother during her pre-breeding trip. Our long-term dataset highlights a decline in pup weaning mass and length but an increase in their body condition despite an increase in the size of the breeding population. The concurrent decrease in pup blood δC and δN values, reflecting those of their mother, suggests shifts in southern hemisphere food webs and in the trophic ecology of Kerguelen southern elephant seals. We hypothesize that these changes are mainly linked to shifts in the composition of the phytoplankton community at the base of the trophic chains, which could have downstream effects on intermediate trophic levels and top predators, as well as a southward shift in female at-sea distribution confirmed by the tracking data.