Consequently, male pups in those species often receive greater maternal investment, particularly when resources are abundant. Several pinniped species exhibit sexual dimorphism and a polygamous reproductive strategy. Colony density and maternal age are both important factors in northern elephant seal reproductive output: older females have greater success at weaning pups than younger animals, and the difference between age groups is exaggerated in high density breeding aggregations. Higher weaning mass increases a pup's chance of survival, as they must rely on their body stores to sustain them for weeks between weaning and departing the colony to attempt foraging for themselves. They produce high energy content milk while fasting over a 27-day lactation period, allowing their pup to rapidly put on mass until they are abruptly weaned. Prior to parturition, female northern elephant seals spend eight months foraging in the mesopelagic northeast Pacific Ocean, accumulating body stores to sustain gestation and lactation. The colony at Año Nuevo State Park, CA, has been studied continuously since its inception nearly 60 years ago, documenting the population as it grew rapidly, peaked, and plateaued after 2002. The northern species ( Mirounga angustirostris) has been recovering from near extinction since the early 1900s. They are colonial, highly polygynous, sexually dimorphic, marine mammals that give birth to and nurse a single pup each year while fasting on land. ) are excellent systems for investigating the dynamics of population growth, environmental variation, and reproduction. This study disentangles the effects of environmental variation and colony size on reproductive success in a large carnivore.Įlephant seals ( Mirounga sp. Long-term datasets of expanding populations are critical to examining density dependent feedback, how density interacts with environmental variation, and mechanisms other than food competition that limit reproduction as populations increase. Animals that forage over large temporal and spatial scales are more buffered against food limitation, but their synchronous, aggregated breeding system creates other mechanisms by which density can act to limit reproduction. Population size, resource availability, and maternal traits all affect reproductive success, but the interaction between population density and environmental variability is understudied, particularly for wide-ranging species that separate food resources from their reproductive sites. Many non-threatened mammalian populations exist at or near carrying capacity and are limited through competition for food and habitat resources. While all organisms face some form of resource limitation that ultimately restricts population growth, the controlling mechanisms vary among ecosystems, taxa, and reproductive strategies. 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.ĭensity-dependent feedback on population growth is one of the most critical ecological controls on a species. We observed density-dependent effects on weaning mass, and maternal age (experience) was more important than oceanographic conditions or maternal mass in determining offspring weaning mass. This strategy may have evolved, in part, to confer resilience to short-term environmental variability. Capital breeders accrue resources over large temporal and spatial scales for use during an abbreviated reproductive period. This species provides a unique opportunity to examine the relative importance of resource acquisition and density-dependence on breeding success. 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). All organisms face resource limitations that will ultimately restrict population growth, but the controlling mechanisms vary across ecosystems, taxa, and reproductive strategies.
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