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Birth timing after the long feeding migration in northern elephant seals


Condit, Richard et al. (2021), Birth timing after the long feeding migration in northern elephant seals, Dryad, Dataset,


A prominent phase of the annual cyle in migratory animals is the transition between migration and reproduction. The transition is a small part of the annual cycle, but details of its timing deserve attention. From a distant location, animals must initiate a long migration so that they arrive at the breeding ground on a precise schedule. Here we take advantage of a sample of female northern elephant seals that were tracked by satellite during their migration prior to parturition. In these animals, we could estimate the time interval between arrival and birth, allowing tests of the following hypotheses: 1) More experienced mothers could time arrival more precisely and thus reduce the pre-parturition interval. 2) Mothers in poor body condition were forced to forage longer and thus shorten the interval. 3) Late-arriving mothers had a shorter interval because they foraged longer. We also calculated the distance traveled in the last two weeks of the migration to examine females' ability to control arrival time, hypothesizing that animals further from the colony traveled back at a higher speed.


Seals were identified by numbered plastic tags attached to their flippers, allowing individuals to be tracked throughout their 22-year lifespans. ARGOS satellite transmitters, GPS, and Time-Depth Recorders were deployed on individuals to document movements while at sea. The instruments were attached prior to foraging migrations and removed immediately thereafter. While the animals were sedated, detailed morphometric measurements were collected, allowing percent body fat to be calculated; percent fat was assumed to reflect female condition. When satellite records showed animals near the colony, daily searches were carried out in order to observe them on the breeding beaches and find their newborn pups. The table below documents 106 cases in which there was no gap between the last day a female was seen without a pup and the first with a pup. In all, the time of birth was known with a precision of 24 hours, while the time of arrival was known within a few minutes from satellite and depth tracks, so the delay between arrival and birth could be estimated with a precision of one day.

Usage Notes

The table includes observations from 106 female migrations ending at the Año Nuevo colony in California. It is tab-delimited ascii. The columns are:

  • animalID: Unique individual identifier
  • season: Year of birth (arrival dates were in Dec of previous year in several cases)
  • age: Age of female in years (85 of the records have age)
  • adate: Arrival date, Year-month-day format
  • aday: Arrival day (days since 1 December of the year before season)
  • firstwith: First day with a pup (days since 1 December)
  • dist15: Distance from colony (km) 15 days prior to birth (99 of the records have distances)
  • dist10: Distance from colony (km) 10 days prior to birth
  • dist5: Distance from colony (km) 5 days prior to birth
  • distmax: Maximum distance from colony (km) during entire migration prior to birth


Office of Academic Research, U.S. Naval Academy, Award: N00014-00-1-0880, N00014-03-1-0651, N00014-08-1-1195, N00014-10-1-0356

Ocean Partnership Programme, Award: N00014-02-1-1012