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.

Methods
Seals are tagged with alphanumeric flipper tags at the time of weaning (approximately one month old), allowing for a demographic database of known-age individuals. We made daily attempts to observe tagged individuals during the breeding and molting seasons, approximately January through June of each year, from 1987 to 2023. When observing a tagged breeding female, we collected information about her pup status (present or absent) and pup sex (male or female). Pup sex was determined in the field by visual inspection based on the presence or absence of a penile opening. When possible, we used hair bleach to mark pup fur with a temporary unique identifier. After weaning, this identifier allowed us to find pups, attach permanent unique flipper tags, and link the pups to their mothers in the database. This analysis includes adult female seals observed on four or more days during the breeding season (December 1 - March 15). The threshold number of days was chosen to minimize observation errors regarding individual identifiers of the mothers or presence of offspring. Seals were considered breeders if they were observed with a pup at least once; otherwise they were considered non-breeders. All research procedures were conducted under the National Marine Fisheries Service (NMFS) marine mammal permit numbers 786-1463, 87-143, 14636, 19108, and 23188 and authorized by the University of California, Santa Cruz Institutional Animal Care and Use Committee, the California State Park system, and the University of California Natural Reserve System.