Data from: Cohort variation in individual body mass dissipates with age in large herbivores

Environmental conditions experienced during early growth and development markedly shape phenotypic traits. Consequently, individuals of the same cohort may show similar life-history tactics throughout life. Conditions experienced later in life, however, could fine-tune these initial differences, either increasing (cumulative effect) or decreasing (compensatory effect) the magnitude of cohort variation with increasing age. Our novel comparative analysis that quantifies cohort variation in individual body size trajectories shows that initial cohort variation dissipates throughout life, and that lifetime patterns change both across species with different paces of life and between sexes. We used longitudinal data on body size (mostly assessed using mass) from 11 populations of large herbivores spread along the “slow-fast” continuum of life histories. We first quantified cohort variation using mixture models to identify clusters of cohorts with similar initial size. We identified clear cohort clusters in all species except the one with the slowest pace of life, revealing that variation in early size is structured among cohorts and highlighting typological differences among cohorts. Growth trajectories differed among cohort clusters, highlighting how early size is a fundamental determinant of lifetime growth patterns. In all species, among-cohort variation in size peaked at the start of life, then quickly decreased with age and stabilized around mid-life. Cohort variation was lower in species with a slower than a faster pace of life, and vanished at prime age in species with the slowest pace of life. After accounting for viability selection, compensatory/catch-up growth in early life explained much of the decrease in cohort variation. Females showed less phenotypic variability and stronger compensatory/catch-up growth than males early in life, whereas males showed more progressive changes throughout life. These results confirm that stronger selective pressures for rapid growth make males more vulnerable to poor environmental conditions early in life and less able to recover after a poor start. Our comparative analysis illustrates how variability in growth changes over time in closely related species that span a wide range on the “slow-fast” continuum, the main axis of variation in life-history strategies of vertebrates.

生物个体在早期生长发育阶段所经历的环境条件，会对其表型性状（phenotypic traits）产生显著的塑造作用。因此，同一同生群（cohort）的个体，在其一生中可能表现出相似的生活史策略。然而，个体在生命后期经历的环境条件，可能会对这些初始差异进行微调：随着年龄增长，既可能通过累积效应（cumulative effect）放大同生群变异的幅度，也可能通过补偿效应（compensatory effect）缩小其幅度。

我们开展了一项创新性的比较分析，通过量化个体身体大小轨迹中的同生群变异，发现初始同生群变异会随生命进程逐渐消散，且这种终生变化模式既在具有不同生活史节奏的物种之间存在差异，也在不同性别之间有所不同。本研究使用了沿生活史“慢-快”连续体（slow-fast continuum of life histories）分布的11个大型草食动物种群的身体大小纵向数据（longitudinal data），其中体型多通过体重进行评估。

首先，我们采用混合模型（mixture models）量化同生群变异，以识别初始体型相似的同生群簇（cohort clusters）。除生活史节奏最慢的物种外，我们在所有物种中均发现了清晰的同生群簇，这表明早期体型的变异在同生群之间存在结构化差异，同时凸显了不同同生群之间的表型分型差异。不同同生群簇的生长轨迹存在显著差异，这表明早期体型是决定终生生长模式的核心决定因素。

所有物种的体型同生群变异均在生命初期达到峰值，随后随年龄增长迅速下降，并在中年阶段趋于稳定。生活史节奏较慢的物种，其同生群变异幅度低于节奏较快的物种；而在生活史节奏最慢的物种中，同生群变异在壮年阶段完全消失。在控制存活性选择（viability selection）的影响后，生命早期的补偿性/追赶生长（catch-up growth）可解释同生群变异下降的绝大部分原因。

雌性个体在生命早期表现出更低的表型变异度和更强的补偿/追赶生长能力，而雄性个体在一生中的生长轨迹变化则更为显著。上述结果证实，对快速生长的更强选择压力，使得雄性在生命早期更易受到不良环境条件的影响，且在初始生长状况不佳时更难实现恢复。

我们的比较分析阐明了，在以“慢-快”连续体为脊椎动物生活史策略变异核心轴线的各类群中，近缘物种的生长变异如何随时间发生变化。