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.

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^-1 during the first few days of lactation (Deutsch et al. 1994).  In adult females, days spent fasting without lactating (prior to parturition) (df ) were assumed to cost 3.0 kg day^-1 (Worthy et al. 1992), while lactation days (dl)  were assumed to cost the female 7.5 kg day^-1 (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.