Skip to main content

Elephant seal female breeding histories at Año Nuevo

Cite this dataset

Condit, R. et al. (2021). Elephant seal female breeding histories at Año Nuevo [Dataset]. Dryad.


The elephant seal (Mirounga angustirostris) breeding colony at Año Nuevo, California, was founded in 1961, and since monitored closely until the present. Since 1968 a research group at the University of California, Santa Cruz, has studied it in detail. Individually identified females were monitored during the breeding season, and pupping records of many throughout their lifetimes were assembled. We address here the maximum reproductive success of females using the lifetime breeding histories. Females lived to a maximum of 23 years and were observed breeding in as many as 17 seasons. One female was observed with a pup nearby in 16 consecutive seasons, from age 4 through age 19. We compared females that gave birth to 10 or more pups (Supermoms) with females that bred at least once but produced fewer than 10 pups (Generic moms). Less than 17% of female weanlings bred at least once. Only 0.87% of females in the sample became Supermoms. We conclude that exceptional reproductive success in elephant seals is associated with giving birth annually, living long and weaning pups successfully. Once female elephant seals begin reproducing they are pregnant or nursing throughout life until they die.


Plastic cattle tags were affixed in the hind flippers of pups soon after weaning every since since, and one some occasions in juveniles or adults whose age was not known. Tags lasted throughout the lifetime of many animals, though some were lost. Observations of the colony were carried out every year from 1968-2018, including every month but most thoroughly during the winter breeding season. Numbers were read off the plastic tags whenever possible to identify individual females, and many were then marked with hair dye to facilitate subsequent observations of mother-pup behavior. Our sample in this study was the 7735 female pups tagged over 1963-2005, 1294 of which were observed breeding at least once, and 103 of which became Supermoms and were seen in at least 10 breeding seasons.

An interactive graphical display of female histories is provided at (But it is a personal web site and not guaranteed as a permanent archive.)

Usage notes

The dataset includes a single table with a single row for every breeding event of tagged females at the Año Nuevo colony. All breeding observations of a single female appear consecutively in the table in the default sorting. Full descriptions of the data can be found in the accompanying paper, cited below.

Users interested in publishing results with these data should consult the principal investigators to best understand details of the methods.  Those PIs might request co-authorship on publications based largely on the data.

Column headings and definitions

  • animalID: an arbitrary number identifying individuals
  • year: the breeding season, including December of the previous year plus December-March of the designated year
  • age: the female's age in years, blank for animals tagged as adults and thus of unknown age
  • firstday: the first date observed
  • lastday: the last date observed
  • sightings: the number of days on which the female was observed
  • sightpostpart: the number of days sighted beyond 6 days after the first sighting (assumed parturtion date)
  • pup: the number of days on which the female was observed with one pup (with a pup but not two or more pups)
  • puppostpart: the number of days observed with exactly one pup beyond 6 days after first sighting
  • area: the main breeding beach, defined as one at which the female was seen most, or with a pup (local names at Año Nuevo)
  • allarea: each area observed, with the number of observations at each
  • birthregion: tagging colony for weanlings only (AN=Año Nuevo; all other designations are for colonies at least 100 km distant)
  • birthyr: year of birth
  • pupwt: weight in kilograms of the pup's weight at weaning
  • success: fate of the pup (weaned_pup or lost pup; other designations indicate insufficient sightings)
  • pupsex: observed pup sex


National Science Foundation


SW 37.104151, -122.342377
NW 37.126053, -122.323837