Data from: Phenotypic selection and covariation in the life‐history traits of elephant seals: heavier offspring gain a double selective advantage

Early developmental conditions contribute to individual heterogeneity of
both phenotypic traits and fitness components, ultimately affecting
population dynamics. Although the demographic consequences of ontogenic
growth are best quantified using an integrated measure of fitness, most
analyses to date have instead studied individual fitness components in
isolation. Here, wWe estimated phenotypic selection on weaning mass in
female southern elephant seals (Mirounga leonina) by analyzing
individual-based data collected between 1986 and 2016 with
capture-recapture and matrix projection models. In support of a hypothesis
predicting a gradual decrease of weaning mass effects with time since
weaning (the replacement hypothesis), we found that the estimated effects
of weaning mass on future survival and recruitment probability was of
intermediate duration (rather than transient or permanent). Heavier female
offspring had improved odds of survival in early life and a higher
probability to recruit at an early age. The positive link between weaning
mass and recruitment age is noteworthy, considering that pre-recruitment
mortality already imposed a strong selective filter on the population,
leaving only the most ‘robust’ individuals to reproduce. The selection
gradient on asymptotic population growth rate, a measure of mean absolute
fitness, was weaker than selection on first-year survival and recruitment
probabilities. Weaker selection on mean fitness occurs because weaning
mass has little impact on adult survival, the fitness component to which
the growth of long-lived species is most sensitive. These results
highlight the need to interpret individual variation in phenotypic traits
in a context that considers the demographic pathways between the trait and
an inclusive proxy of individual fitness. Although variation in weaning
mass do not translate to permanent survival differences among individuals
in adulthood, it explains heterogeneity and positive covariation between
survival and breeding in early life, which contribute to
between-individual variation in fitness.