Abstract
The tradeoff between investment in current reproduction versus future survival is central to life-history theory, and long-lived, iteroparous mammals disproportionately favor their own survival. Previous work has demonstrated that adjustment of reproductive effort in long-lived mammals often occurs after parturition, owing to the greater cost of lactation relative to gestation. Under the right conditions, however, this difference in the relative costs of reproduction may also facilitate another, arguably less intuitive, strategy. Those conditions, which are relatively common among capital-breeding ungulates, include: (1) Females have the capacity to adjust gestation length; (2) Neonatal mortality occurs mostly during the first month of life and is inversely related to birth mass; and (3) The influence of birth mass on the probability of surviving the first month of life is stronger than the influence of autumn body mass on the probability of surviving the first winter of life. Under these circumstances, a female in poor condition in early spring could potentially increase fitness by delaying parturition and increasing investment in gestation, giving birth to a correspondingly larger neonate that has a higher probability of survival during its first month of life, and subsequently reducing investment in lactation to help rebuild somatic reserves. We developed and empirically parameterized a state-dependent model of maternal resource allocation that reflected this strategy. We tested the prediction that population growth would be faster when resource allocation was state-dependent than when gestation length was decoupled from dam condition and adjustment of reproductive investment was largely post-natal. Our results supported this prediction: state-dependent resource allocation by maternal females increased lambda by an average of 4%, leading to larger population sizes after 30 years. Population growth was consistent across a range of winter severities, suggesting that state-dependent resource allocation could help buffer ungulate populations against climatic variation. Our results reveal a potentially general mechanism underpinning intraspecific variation in life-history strategies of long-lived, capital-breeding mammals, and suggest that such variation at the individual level can influence performance outcomes at the population level.