Age and reproduction in Northern Elephant Seals

Maternal age can influence reproductive success and offspring fitness, but
the timing, magnitude, and direction of those impacts are not well
understood. Evolutionary theory predicts that selection on fertility
senescence is stronger than maternal effect senescence, and therefore the
rate of maternal effect senescence will be faster than fertility
senescence.We used a 36-year study of northern elephant seals (Mirounga
angustirostris) to investigate reproductive senescence. Our dataset
included 103,746 sightings of 1,203 known-age female northern elephant
seals.We hypothesized that fertility (maternal reproductive success),
offspring survival and recruitment into the breeding population, and male
offspring production would decline with advanced maternal age.
Furthermore, we hypothesized that older females would shorten their
molting haul out to allow for more time spent foraging. We found
evidence for both fertility and maternal effect senescence, but no
evidence for senescence impacting offspring recruitment or sex ratio.
Breeding probability declined from 96.4% (95% CI: 94.8 - 97.5%) at 11
years old to 89.7% (81.9 - 94.3%) at 19 years old, and the probability of
offspring survival declined from 30.3% (23.6 - 38.0%) at 11 years old to
9.1% (3.2 - 22.9%) at 19 years old.The rates of decline for fertility and
maternal effect senescence were not different from each other. However,
maternal effect senescence had a substantially greater impact on the
number of offspring surviving to age one compared to fertility senescence.
Compared to a hypothetical non-senescent population, maternal effect
senescence resulted in 5.3% fewer surviving pups, whereas fertility
senescence resulted in only 0.3% fewer pups produced per year. These
results are consistent with evolutionary theory predicting weaker
selection on maternal effect than fertility senescence. Maternal effect
senescence may therefore be more influential on population dynamics than
fertility senescence in some systems.