DOI: 10.5061/dryad.pg4f4qrx1
title: Age and reproduction in Northern Elephant Seals

description: 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.
description: 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. 
description: # Age and Reproduction in Northern Elephant Seals
 [https://doi.org/10.5061/dryad.pg4f4qrx1](https://doi.org/10.5061/dryad.pg4f4qrx1) The .csv file contains processed data (each row is a year-seal combination), the .r file contains the code to create formatting functions, and the .qmd file contains the code to fit the models and reproduce the figures. ## Description of the data and file structure **Headers in the "128L pull 2023_12_05.csv" file are as follows:** animalID: a unique identifier for an individual seal, consistent across years and linked to an alphanumeric flipper tag. year: the calendar year of the observation observed: A binary variable indicating whether an animal was seen with a pup ("B") or not ("1"). firstobsmolt: A date in format YYYY-MM-DD indicating the first time in a given year an animal was observed during the molting season, used as a proxy for molting haulout arrival. NA (not available) indicates that phenological observations were not made for that seal-year combination. lastobsmolt: A date in format YYYY-MM-DD indicating the last time in a given year an animal was observed during the molting season, used as a proxy for molting haulout departure. NA (not available) indicates that phenological observations were not made for that seal-year combination. season: a year indicating the seal's season, which differs from calendar years because a season resets when the animal is more than 50% molted. NA (not available) indicates that phenological observations were not made for that seal-year combination. firstobsbreed: A date in format YYYY-MM-DD indicating the first time in a given year an animal was observed during the breeding season, used as a proxy for breeding haulout arrival. NA (not available) indicates that phenological observations were not made for that seal-year combination. lastobsbreed: A date in format YYYY-MM-DD indicating the last time in a given year an animal was observed during the breeding season, used as a proxy for breeding haulout departure. NA (not available) indicates that phenological observations were not made for that seal-year combination. firstobsbreeddoy: The same dates found in the firstobsbreed column, formatted as Julian day of year. NA (not available) indicates that phenological observations were not made for that seal-year combination. lastobsmoltdoy: The same dates found in the lastobsmolt column, formatted as Julian day of year. NA (not available) indicates that phenological observations were not made for that seal-year combination. breeddur: The duration of the breeding haulout, calculated from the first- and lastobsbreed dates. NA (not available) indicates that phenological observations were not made for that seal-year combination. tripdur: The duration of the post-molting foraging trip, calculated from lastobsmoltdoy and firstobsbreeddoy. NA (not available) indicates that phenological observations were not made for that seal-year combination. moltdur: The duration of the molting haulout, calculated from the first- and lastobsmolt dates. NA (not available) indicates that phenological observations were not made for that seal-year combination. pupID: Unique identifier of the pup a female gave birth to in that year. If a unique identifier was unavailable, the cell has an NA. pupsex: The sex of the pup ("M" or "F"). If observers were unable to determine pup sex, the cell has an NA.  yearborn: The year the seal was born. This refers to the original animal, not her pup.  age: The seal's age, calculated by subtracting the year she was born from the year of the observation. pupyearborn: The year the pup was born. This is only available for pups with a pupID; otherwise the column shows an NA. pupseeneveragain: The number of observations of a pup after weaning. This is only available for pups with a pupID; otherwise the column shows an NA. puprecruited: A binary variable indicating whether or not the pup eventually recruited. A 0 indicates that it never recruited, and a 1 indicates that it did recruit. This is only available for pups with a pupID; otherwise the column shows an NA. ## Code/Software Code is also available on Github: * [https://github.com/allisonpayne/sealaging](https://github.com/allisonpayne/sealaging)