Sex allocation shifts towards sons in the final year of life: Terminal reproductive effects in northern elephant seals
Data files
Jun 02, 2026 version files 29.30 MB
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128L_pull_2023_12_05.csv
213.74 KB
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Adrien_Data_Pull_2024_12_06.csv
28.89 MB
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lact_info.csv
160.46 KB
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README.md
5.40 KB
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Terminal_Effects_Share.qmd
27.60 KB
Abstract
Wild animals experience reproductive tradeoffs where they must allocate resources to both their own survival and their offspring. These tradeoffs may shift as animals grow older. Terminal effects occur when tradeoffs are particularly stark in the last year of life (e.g., increased investment comes at the cost of reduced survival or future reproductive success). Northern elephant seals (Mirounga angustirostris) are capital breeders that produce one pup every year and allocate energy-dense milk to their pups for three to four weeks. However, it is unclear how resource allocation, offspring weight at weaning, offspring sex ratio, and offspring survival changes in the animal’s last year of life.
Using a long-term dataset with 63,902 observations of 1,715 adult female seals, we investigated the reproductive strategies and tradeoffs in an adult female’s last year of life.
We hypothesized that in their last year of life, pre-prime (ages 3-6) seals would put more effort into their offspring, resulting in longer lactation durations and producing more male pups (i.e., terminal investment). We also hypothesized that prime (ages 7-14) and post-prime (ages 15+) seals would experience terminal constraints of reproduction, resulting in shorter lactation durations and more female pups produced near the end of life.
We found no evidence for terminal investment for young seals in their last year of life. We did find evidence that older seals experience terminal constraints of reproduction exhibiting shorter lactation durations during their last year of life. However, weanling weights did not decrease due to the shorter duration. Additionally, prime/post-prime terminal mothers produced more energetically expensive male pups, and while age may correlate to offspring survival, terminal status did not affect the chance of offspring survival .
We found evidence supporting the terminal investment hypothesis in elephant seals and describe a basis for how marine mammals may alter reproductive effort during their last year of life. These findings contribute to the broader investigation of how life history trade offs and transgenerational effects vary across the lifetimes of long-lived mammals.
Dataset DOI: 10.5061/dryad.m37pvmdh3
Description of the data and file structure
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 2024.The .csv file contains processed data (each row is a year-seal combination).
Files and variables
Analyses were done using R version 4.2.3 (2023-03-15) with R package brms, rstan and tidyverse.
File: 128L_pull_2023_12_05.csv
Description:
Variables
- 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").
- tagsex: Sex of the animal "M" for male "F" for female.
- 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.
- 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.
File: lact_info.csv
Description:
Variables
- animalID: a unique identifier for an individual seal, consistent across years and linked to an alphanumeric flipper tag.
- 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.
- age: The seal's age, calculated by subtracting the year she was born from the year of the observation.
- max_age: The seal's oldest age they were observed in our database.
- bio_age: The seal's biological age, calculated by subtracting the age from the max_age column to determines years since death.
- terminal: Seal's were assigned a terminal status of 0 or 1. Terminal = 0 means they survived that year and terminal = 1 means this was their last year alive (year of death).
- lact_start: This refers to the first date mothers were observed with a pup which equates to first day of the nursing period.
- lact_end: This refers to the last date mom was observed with her pup signifying the end of the nursing period.
- lact_dur: Total number of days mom was observed nursing her pup. Calculated using lact_end - lact_start.
- date: International Standard format for date of year (YYYY-MM-DD)
File: Adrien_Data_Pull_2024_12_06.csv
Description:
Variables
- 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 or not that year ("0" for not seen, "1" if seen)
- tagsex: Sex of the animal "M" for male "F" for female.
- 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.
- calyear: The calendar year of the observation
- 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.
- Wt: Weight of offspring at weaning in kilograms(kg)
Code/software:
File: Terminal_Effects_Share.qmd
R code for all models and figures 2-6 generated in Bastidas et al., 2026 "Sex allocation shifts towards sons in final year of life: Terminal effects in northern elephant seals. "
Code is also available here on Github:
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 2024. 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. 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.