Abstract
Costs associated with reproduction are widely known to play a role in the evolution of reproductive tactics with consequences to population and eco‐evolutionary dynamics. Evaluating these costs as they pertain to species in the wild remains an important goal of evolutionary ecology. Individual heterogeneity, including differences in individual quality (i.e., among‐individual differences in traits associated with survival and reproduction) or state, and variation in environmental and social conditions can modulate the costs of reproduction; however, few studies have considered effects of these factors simultaneously. Taking advantage of a detailed, long‐term dataset for a population of feral horses (Sable Island, Nova Scotia, Canada), we address the question of how intrinsic (quality, age), environmental (winter severity, location), and social conditions (group size, composition, sex ratio, density) influence the costs of reproduction on subsequent reproduction. Individual quality was measured using a multivariate analysis on a combination of four static and dynamic traits expected to depict heterogeneity in individual performance. Female quality and age interacted with reproductive status of the previous year to determine current reproductive effort, while no effect of social or environmental covariates was found. High‐quality females showed higher probabilities of giving birth and weaning their foal regardless of their reproductive status the previous year, while those of lower quality showed lower probabilities of producing foals in successive years. Middle‐aged (prime) females had the highest probability of giving birth when they had not reproduced the year before, but no such relationship with age was found among females that had reproduced the previous year, indicating that prime‐aged females bear higher costs of reproduction. We show that individual quality and age were key factors modulating the costs of reproduction in a capital breeder but that environmental or social conditions were not, highlighting the importance of considering multiple factors when studying costs of reproduction.
Highlights
Energy acquired by an individual can be allocated to survival, reproduction, or growth
Understanding the costs of reproduction in the wild and their role in the dynamics and evolution of species remains an important topic of evolutionary ecology
Because location on Sable Island is correlated with horse habitat quality (Contasti et al, 2012; Rozen-Rechels et al, 2015), we considered each individual’s median longitude during summer census surveys as a measure of local environmental quality
Summary
Energy acquired by an individual can be allocated to survival, reproduction, or growth. In addition to variation in individual quality, which persists throughout an individual’s lifetime, short-term or local environmental conditions can modulate costs of reproduction (Cam et al, 2002; Hamel, Cote et al, 2010; Robert et al, 2012). While an increasingly large number of studies are reporting evidence for the effect of individual quality and environmental conditions on costs of reproduction (Cam et al, 2002; Clutton-Brock et al, 1996; Hamel, Cote et al, 2009; Hamel, Cote et al, 2010; Lescroel et al, 2009; Robert et al, 2012; Weladji et al, 2008), little attention has been devoted to social effects such as group composition, sex ratio, or local density (Clutton-Brock et al, 1996; Hamel, Cote et al, 2010; Nicolaus et al, 2012). Because horse habitat quality decreases from west to east on Sable Island (forage quality and access to freshwater [Contasti, Tissier, Johnstone, & McLoughlin, 2012; Rozen-Rechels et al, 2015]), we predicted that females in the east would bear greater reproductive costs (P7)
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