Data from: Phenotypic selection and covariation in the life‐history traits of elephant seals: heavier offspring gain a double selective advantage

Early developmental conditions contribute to individual heterogeneity of both phenotypic traits and fitness components, ultimately affecting population dynamics. Although the demographic consequences of ontogenic growth are best quantified using an integrated measure of fitness, most analyses to date have instead studied individual fitness components in isolation. Here, wWe estimated phenotypic selection on weaning mass in female southern elephant seals (Mirounga leonina) by analyzing individual-based data collected between 1986 and 2016 with capture-recapture and matrix projection models. In support of a hypothesis predicting a gradual decrease of weaning mass effects with time since weaning (the replacement hypothesis), we found that the estimated effects of weaning mass on future survival and recruitment probability was of intermediate duration (rather than transient or permanent). Heavier female offspring had improved odds of survival in early life and a higher probability to recruit at an early age. The positive link between weaning mass and recruitment age is noteworthy, considering that pre-recruitment mortality already imposed a strong selective filter on the population, leaving only the most ‘robust’ individuals to reproduce. The selection gradient on asymptotic population growth rate, a measure of mean absolute fitness, was weaker than selection on first-year survival and recruitment probabilities. Weaker selection on mean fitness occurs because weaning mass has little impact on adult survival, the fitness component to which the growth of long-lived species is most sensitive. These results highlight the need to interpret individual variation in phenotypic traits in a context that considers the demographic pathways between the trait and an inclusive proxy of individual fitness. Although variation in weaning mass do not translate to permanent survival differences among individuals in adulthood, it explains heterogeneity and positive covariation between survival and breeding in early life, which contribute to between-individual variation in fitness.

早期发育条件可引发表型性状（phenotypic traits）与适合度组分（fitness components）产生个体异质性，最终对种群动态（population dynamics）产生影响。尽管个体发育生长（ontogenic growth）的种群动态效应最佳量化方式为采用综合适合度度量（integrated measure of fitness），但迄今为止绝大多数分析均单独探究个体适合度组分。本研究通过分析1986年至2016年收集的个体水平数据，并结合标记重捕法（capture-recapture）与矩阵投影模型（matrix projection models），估算了雌性南象海豹（Mirounga leonina）断乳体重（weaning mass）的表型选择强度。为验证‘断乳后随时间推移，断乳体重的效应会逐步减弱’的更替假说（replacement hypothesis），本研究发现断乳体重对个体后续存活与招募概率（recruitment probability）的估算效应持续时长处于中间状态（既非短暂也非永久）。体重更大的雌性幼崽在生命早期的存活概率更高，且更早具备繁殖招募的能力。断乳体重与招募年龄间的正相关关系值得关注：繁殖前死亡（pre-recruitment mortality）已对种群形成了极强的选择过滤（selective filter），仅留存最为‘健壮’的个体参与繁殖。作为平均绝对适合度（mean absolute fitness）度量指标的渐近种群增长率（asymptotic population growth rate），其选择梯度（selection gradient）弱于针对第一年存活与招募概率的选择强度。对平均适合度的选择强度较弱，原因在于断乳体重对成体存活几乎无影响，而成体存活是长寿物种最为敏感的适合度组分。上述结果凸显了一个研究需求：解读表型性状的个体变异时，需结合该性状与个体适合度综合代理指标（inclusive proxy of individual fitness）间的种群动态路径（demographic pathways）开展分析。尽管断乳体重的变异不会导致成体个体间产生永久性的存活差异，但它可解释生命早期存活与繁殖间的异质性及正协变关系（positive covariation），而这类关系正是适合度个体间变异的重要成因。