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Weights of northern elephant seal weanlings from Año Nuevo Reserve


Robinson, Patrick et al. (2021), Weights of northern elephant seal weanlings from Año Nuevo Reserve, Dryad, Dataset,


Long-term observations of foraging success at the population scale are key to understanding demographic and ecological patterns.  Northern elephant seals, Mirounga angustirostris, are capital breeders and the resources delivered to the pup during the 4-week lactation period are related to both the past foraging success of the mother and the future survival of the pup post-weaning.  We collected weight and basic morphometric data from more than 7,000 recently weaned elephant seals at the Año Nuevo colony in central California, USA beginning in 1976 and extending through the present.  This archive will be updated periodically as new data are collected.

The PIs would like to be informed of manuscripts intended for publication that use this dataset. Depending on our level of interest and how much a paper depends on the data, co-authorship might be requested. Funding for this project was provided by a variety of public and private organizations including:  NSF, ONR, JIP, and the Moore, Packard and Sloan foundations (please consult PI's for a complete list).  Funding was also provided by generous donations to Año Nuevo Reserve and UC Santa Cruz.


Recently weaned northern elephant seals (Mirounga angustirostris) from the colony at UC Santa Cruz's Año Nuevo Reserve (San Mateo County, California, USA) were captured shortly after the departure of their mothers using a custom cone-shaped bag.  To obtain a direct weight measurement, each weanling was suspended from a scale hanging from a tripod.  Standard length and axillary girth were collected while the seal was physically restrained.  Sex was determined by rolling the weanling and observing the presence/absence of a penile opening.  The date of weaning for each seal was estimated from routine observations on the colony based on the presence/absence of the mother and the unique bleach mark on the pup/weanling.  If available, the mother's age was determined by querying the unique flipper tag of the mother in a separate database.

Data collection was conducted by a large number of dedicated volunteers, undergraduates, graduate students, postdocs, and researchers; their efforts are greatly appreciated.

Usage Notes

The table includes data starting in 1976.  Sample sizes varied widely across years and not all variables were collected in all years.

Variables include:

animalID: a unique animal identification number
sex: direct observation ("M" = male, "F" = female, and blank = unknown)
weighingdate:  date the weanling was weighed (YYYY-MM-DD format)
weandate: estimated date the seal was weaned (YYYY-MM-DD format).
Wt: weight of the weanling (kg).
SL: standard length (cm).  The straight-line length from the tip of the nose to the tip of the tail
AG: axillary girth (cm).
momage: age of the weanling's mother (years)