Data from: Individual quality and age but not environmental or social conditions modulate costs of reproduction in a capital breeder

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.

繁殖相关成本已被广泛证实会影响繁殖策略的演化，并对种群动态及生态-演化动力学产生连锁效应。针对野生物种评估此类繁殖成本，仍是进化生态学的重要研究目标之一。个体异质性（包括个体质量差异——即存活与繁殖相关性状的个体间差异——或个体状态差异），以及环境与社会条件的变异，均可调控繁殖成本；但目前鲜有研究同时考量这些因素的综合效应。本研究借助加拿大新斯科舍省萨布尔岛野化马种群的长期精细化数据集，探究内在因素（个体质量、年龄）、环境因素（冬季严酷程度、栖息位置）与社会条件（群体规模、群体组成、性比、种群密度）如何影响繁殖成本对后续繁殖的作用。研究通过对四项静态与动态性状的组合开展多变量分析，以此量化个体质量——此类性状可反映个体表现的异质性。雌性个体质量与年龄会同前一年的繁殖状态共同影响当前的繁殖投入，而未观测到社会或环境协变量的显著效应。高质量雌性个体无论前一年繁殖状态如何，其产驹与抚育幼驹至断奶的概率均更高；而低质量雌性个体连续产驹的概率则相对更低。未在前一年繁殖的中年（壮年）雌性个体产驹概率最高，但在前一年完成繁殖的雌性个体中未发现此类年龄相关性，这表明壮年雌性个体需承担更高的繁殖成本。本研究证实，在资本繁殖者类群中，个体质量与年龄是调控繁殖成本的关键因素，而环境与社会条件并非如此，这凸显了在繁殖成本研究中同时考量多类因素的重要性。