Data from: Early reproductive investment, senescence and lifetime reproductive success in female Asian elephants

The evolutionary theory of senescence posits that as the probability of extrinsic mortality increases with age, selection should favour early-life over late-life reproduction. Studies on natural vertebrate populations show early reproduction may impair later-life performance, but the consequences for lifetime fitness have rarely been determined, and little is known of whether similar patterns apply to mammals which typically live for several decades. We used a longitudinal dataset on Asian elephants (Elephas maximus) to investigate associations between early-life reproduction and female age-specific survival, fecundity and offspring survival to independence, as well as lifetime breeding success (lifetime number of calves produced). Females showed low fecundity following sexual maturity, followed by a rapid increase to a peak at age 19 and a subsequent decline. High early life reproductive output (before the peak of performance) was positively associated with subsequent age-specific fecundity and offspring survival, but significantly impaired a female's own later-life survival. Despite the negative effects of early reproduction on late-life survival, early reproduction is under positive selection through a positive association with lifetime breeding success. Our results suggest a trade-off between early reproduction and later survival which is maintained by strong selection for high early fecundity, and thus support the prediction from life history theory that high investment in reproductive success in early life is favoured by selection through lifetime fitness despite costs to later-life survival. That maternal survival in elephants depends on previous reproductive investment also has implications for the success of (semi-)captive breeding programmes of this endangered species.

衰老演化理论（evolutionary theory of senescence）提出，随着外源性死亡概率随年龄增长而升高，自然选择应更青睐早期繁殖而非晚期繁殖。针对天然脊椎动物种群的研究表明，早期繁殖可能损害个体后期生存表现，但目前极少有研究明确其对终身适合度的影响，且对于通常可存活数十年的哺乳动物是否存在类似模式，人们所知甚少。我们采用亚洲象（Elephas maximus）的纵向数据集（longitudinal dataset），探究早期繁殖与雌性年龄特异性存活率、繁殖力、后代独立前存活率，以及终身繁殖成功率（终身产犊总数）之间的关联。雌性个体在性成熟后繁殖力较低，随后快速上升并在19岁时达到峰值，之后逐渐下降。早期（繁殖性能达到峰值前）高繁殖产出与后续的年龄特异性繁殖力及后代存活率呈正相关，但会显著损害雌性个体后期的自身存活率。尽管早期繁殖对后期存活存在负面影响，但早期繁殖仍处于正选择之下，这是因为其与终身繁殖成功率呈正相关。我们的研究结果表明，早期繁殖与后期存活之间存在权衡关系，这种权衡因强烈选择青睐高早期繁殖力而得以维持，从而支持了生活史理论（life history theory）的预测：尽管会付出后期存活的代价，自然选择仍会通过终身适合度青睐对早期繁殖成功的高投入。大象的母体存活依赖于此前的繁殖投入，这一发现对于该濒危物种的（半）圈养繁殖计划的成功也具有启示意义。