Data from: A dynamic framework for the study of optimal birth intervals reveals the importance of sibling competition and mortality risks

Human reproductive patterns have been well studied, but the mechanisms by which physiology, ecology and existing kin interact to affect the life history need quantification. Here, we create a model to investigate how age-specific interbirth intervals adapt to environmental and intrinsic mortality, and how birth patterns can be shaped by competition and help between siblings. The model provides a flexible framework for studying the processes underlying human reproductive scheduling. We developed a state-based optimality model to determine age-dependent and family-dependent sets of reproductive strategies, including the state of the mother and her offspring. We parameterized the model with realistic mortality curves derived from five human populations. Overall, optimal birth intervals increase until the age of 30 after which they remain relatively constant until the end of the reproductive lifespan. Offspring helping each other does not have much effect on birth intervals. Increasing infant and senescent mortality in different populations decreases interbirth intervals. We show that sibling competition and infant mortality interact to lengthen interbirth intervals. In lower-mortality populations, intense sibling competition pushes births further apart. Varying the adult risk of mortality alone has no effect on birth intervals between populations; competition between offspring drives the differences in birth intervals only when infant mortality is low. These results are relevant to understanding the demographic transition, because our model predicts that sibling competition becomes an important determinant of optimal interbirth intervals only when mortality is low, as in post-transition societies. We do not predict that these effects alone can select for menopause.

人类生殖模式已得到广泛研究，但生理、生态与现存亲属关系的交互作用对生活史的影响机制仍有待量化解析。本研究构建模型，旨在探究特定年龄的出生间隔（interbirth intervals）如何适配环境与内在死亡风险，以及同胞间的竞争与互助如何塑造生育模式。该模型为解析人类生殖调度的内在过程提供了灵活的研究框架。我们开发了基于状态的最优模型，以确定依赖年龄与家庭结构的生殖策略集合，其中涵盖母亲及其后代的状态。我们利用源自五个人类群体的真实死亡曲线对模型进行参数化设定。总体而言，最优出生间隔会随年龄增长至30岁时达到峰值，此后在整个生殖生命周期内基本保持稳定。后代间的互助行为对出生间隔的影响微乎其微。不同群体中婴儿死亡率与衰老死亡率的上升会缩短出生间隔。研究表明，同胞竞争与婴儿死亡率的交互作用会延长出生间隔。在低死亡率群体中，激烈的同胞竞争会进一步拉大生育间隔。仅改变成年死亡风险并不会导致群体间出生间隔出现差异；仅当婴儿死亡率处于较低水平时，后代间的竞争才会驱动群体间出生间隔的差异。这些结果有助于理解人口转型，因为本模型预测，仅当死亡率处于低水平时（如转型后社会），同胞竞争才会成为最优出生间隔的重要决定因素。我们并未预测仅靠这些效应即可通过自然选择催生绝经现象。