DOI: 10.7291/d1k099
title: Data from: Density-dependent effects on reproductive output in a capital breeding carnivore, the northern elephant seal (Mirounga angustirostris)

description: All organisms face resource limitations that will ultimately restrict
 population growth, but the controlling mechanisms vary across ecosystems,
 taxa, and reproductive strategies. Using four decades of data, we examine
 how variation in the environment and population density affect
 reproductive outcomes in a capital-breeding carnivore, the northern
 elephant seal (Mirounga angustirostris). This species provides a unique
 opportunity to examine the relative importance of resource acquisition and
 density-dependence on breeding success. Capital breeders accrue resources
 over large temporal and spatial scales for use during an abbreviated
 reproductive period. This strategy may have evolved, in part, to confer
 resilience to short-term environmental variability. We observed
 density-dependent effects on both weaning mass and male-biased allocation
 of resources to offspring. Furthermore, maternal age (experience) was more
 important than oceanographic conditions or maternal mass in determining
 offspring weaning mass. Together these findings show that the mechanisms
 controlling reproductive output are conserved across terrestrial and
 marine systems and vary with population dynamics, an important
 consideration when assessing the effect of extrinsic changes, such as
 climate change, on a population.
description: Adult female mass gain
 during the gestational (post-moult) foraging trip was measured in
 approximately 20 individuals per year from 2004 – 2019.  Females were
 sedated a few days before departure at the end of the moulting fast and
 again upon return to shore during the breeding season, following standard
 protocols. These individuals were equipped with time-depth recorders
 (TDRs) and satellite transmitters (Wildlife Computers, Redmond, WA, USA or
 Sea Mammal Research Unit, St. Andrews, Scotland) that provided location
 information through the Argos network.  At each handling, the animal was
 weighed in a canvas sling suspended from a hanging scale with a precision
 of ± 1 kg. During the breeding season, instruments were recovered on the
 fifth day following birth. During these procedures, the female’s pup was
 also weighed and flipper tagged. Additionally, adult females were sedated
 for physiological studies in 1991, 1992, 1995-1997, 2001-2006, and 2009. 
 These procedures provided additional data on female arrival mass, pup
 birth mass, and pup weaning mass. All measured masses were
 corrected to account for changes in mass due to fasting and lactation and
 to account for variation in the time spent onshore before and after
 measurements were taken. The measured mass of the pup was corrected to
 birth mass based on an average mass gain of 2.2 kg
 day<sup>-1</sup> during the first few days of lactation
 (Deutsch et al. 1994).  In adult females, days spent fasting without
 lactating (prior to parturition) (<em>df</em> ) were assumed to cost 3.0 kg day<sup>-1</sup> (Worthy et al. 1992), while lactation days (<em>dl)</em>  were assumed to cost the female 7.5 kg day<sup>-1</sup> (Crocker et al. 2001).  Departure from and arrival at the colony was determined from either TDR or satellite records. Animals were resighted daily after arrival to assess whether they had given birth.