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.早期发育条件会塑造表型性状（phenotypic traits）与适合度组分（fitness components）两方面的个体异质性，最终对种群动态产生影响。尽管若要精准量化个体发育生长的种群统计效应，最优方案是采用整合型适合度测度，但截至目前的多数研究均仅单独分析单个适合度组分。本研究基于1986年至2016年间采集的标记重捕（capture-recapture）与矩阵投影模型（matrix projection models）个体水平数据，估算了雌性南象海豹（Mirounga leonina）断奶体重（weaning mass）所承受的表型选择（phenotypic selection）强度。本研究结果支持"断奶体重的效应会随断奶后时长逐渐减弱"这一假说（即替换假说（replacement hypothesis）），结果显示，断奶体重对后续存活与招募概率（recruitment probability）的影响持续时长处于中等水平（既非瞬时也非永久）。体重更大的雌性幼崽在生命早期的存活概率更高，且更有可能在较早的年龄实现种群招募。考虑到招募前死亡（pre-recruitment mortality）已对种群形成了极强的选择过滤，仅留下最"健壮"的个体参与繁殖，因此断奶体重与招募年龄之间的正向关联值得关注。作为平均绝对适合度（mean absolute fitness）的测度指标，渐近种群增长率（asymptotic population growth rate）所对应的选择梯度（selection gradient）强度弱于对第一年存活与招募概率的选择强度。平均适合度所受选择强度较弱，这是因为断奶体重对成体存活几乎无影响——而成体存活是长寿物种生长发育最敏感的适合度组分。本研究结果凸显了一个研究需求：在解读表型性状的个体变异时，需结合该性状与个体适合度的整合性替代指标之间的种群统计通路展开分析。尽管断奶体重的变异并不会导致成体个体间出现永久性的存活差异，但该变异可以解释生命早期存活与繁殖间的异质性及正向协变关系，而这正是个体间适合度差异的重要来源。