Abstract:
Correlations between early- and late-life performance are a major prediction of
life-history theory. Negative early–late correlations can emerge because biological processes
are optimized for early but not late life (e.g., rapid development may accelerate the onset of
senescence; “developmental theory of aging”) or because allocation to early-life performance
comes at a cost in terms of late-life performance (as in the disposable soma theory). But variation
in genetic and environmental challenges that each individual has to cope with during early
life may also lead to positive early–late life-history trait correlations (the “fixed heterogeneity”
or “individual quality” hypothesis). We analyzed individual life-history trajectories of 7,420
known-age female southern elephant seals (Mirounga leonina) monitored over 36 yr to determine
how actuarial senescence (a proxy for late-life performance) correlate with age at first
reproduction (a proxy for early-life performance). As some breeding events may not be
detected in this field study, we used a custom “multievent” hierarchical model to estimate the
age at first reproduction and correlate it to other life-history traits. The probability of first
reproduction was 0.34 at age 3, with most females breeding for the first time at age 4, and comparatively
few at older ages. Females with an early age of first reproduction outperformed
delayed breeders in all aspects we considered (survival, rate of senescence, net reproductive
output) but one: early breeders appeared to have an onset of actuarial senescence 1 yr earlier
compared to late breeders. Genetics and environmental conditions during early life likely
explain the positive correlation between early- and late-life performance. Our results provide
the first evidence of actuarial senescence in female southern elephant seals.
