Data from: Experimental manipulation of female reproduction demonstrates its fitness costs in kangaroos

1. When resources are scarce, female mammals should face a trade-off between lactation and other life-history traits such as growth, survival and subsequent reproduction. Kangaroos are ideal to test predictions about reproductive costs because they may simultaneously lactate and carry a young, and have indeterminate growth and a long breeding season. 2. An earlier study in three of our five study populations prevented female eastern grey kangaroos (Macropus giganteus) from reproducing during one reproductive season by either inserting contraceptive implants or removing very small pouch young. We explored how individual and environmental variables affect the costs of reproduction over time, combining this experimental reduction of reproductive effort with multi-year monitoring of 270 marked females. Experimental manipulation should control for individual heterogeneity, revealing the costs of reproduction and their likely sources. We also examined the fitness consequences of reproductive effort and offspring sex among unmanipulated individuals to test if sex allocation strategies affected trade-offs. 3. Costs of reproduction included longer inter-birth intervals and lower probability of producing a young that survived to 7 months in the subsequent reproductive event. Weaning success, however, did not differ significantly between manipulated and control females. By reducing reproductive effort, manipulation appeared to increase individual condition and subsequent reproductive success. 4. Effects of offspring sex upon subsequent reproductive success varied according to year and study population. Mothers of sons were generally more likely to have a young that survived to 7 months, compared to mothers of daughters. 5. The fitness costs of reproduction arise from constraints in both acquisition and allocation of resources. To meet these costs, females delay subsequent parturition and may manipulate offspring sex. Reproductive tactics thus vary according to the amount of resource available to each individual, promoting a wide range in reproductive performance within and among individuals and populations.

1. 当资源匮乏时，雌性哺乳动物需要在泌乳（lactation）与生长、存活以及后续繁殖等其他生活史特征（life-history traits）之间进行权衡取舍。袋鼠是检验繁殖成本相关预测的理想实验类群，因为它们可同时开展泌乳与育幼活动，且具备不定生长（indeterminate growth）和较长繁殖季的特性。
2. 我们此前在5个研究种群中的3个开展过一项前期研究，通过植入避孕植入物或移除极小型育儿袋幼崽的方式，使雌性东部灰袋鼠（Macropus giganteus）在一个繁殖季内无法完成繁殖。本研究将这种通过实验降低繁殖投入的操控手段，与对270只佩戴个体标记的雌性个体的多年监测数据相结合，探究了个体与环境变量如何随时间推移影响繁殖成本。实验操控可控制个体异质性（individual heterogeneity），从而揭示繁殖成本及其潜在来源。此外，我们还在未接受实验操控的个体中，分析了繁殖投入与后代性别对适合度（fitness）的影响，以检验性别分配策略是否会对权衡取舍产生调控作用。
3. 繁殖成本包括更长的产仔间隔（inter-birth intervals），以及后续繁殖事件中幼崽存活至7月龄的概率降低。不过，实验处理组与对照组雌性的断乳成功率并无显著差异。通过降低繁殖投入，实验操控似乎提升了个体状态与后续繁殖成功率。
4. 后代性别对后续繁殖成功率的影响会随年份与研究种群的不同而产生差异。相较于育有雌性幼崽的母袋鼠，育有雄性幼崽的母袋鼠其幼崽存活至7月龄的概率普遍更高。
5. 繁殖的适合度成本源于资源获取与分配两方面的限制。为应对这些成本，雌性个体将推迟后续分娩，且可能会对后代性别进行调控。因此，繁殖策略会随个体可获取的资源量发生变化，进而推动个体间、种群间的繁殖表现呈现出广泛的多样性。