The timing of life-history traits may have strong influences on the evolution of life cycles and on population demography. This is especially true of the age at which females first reproduce (Cole's principle). We examined whether the age at which females first reproduce influences fitness in Columbian ground squirrels (Urocitellus columbianus), for which females varied in the age at which they initially produce weaned offspring, from ages 1 through 5 years. With 148 females with complete known life spans in a 28-year data set, we examined four fitness measures: individual fitness (λind), individual fitness relative to the pattern of growth of the population (λrel), lifetime reproductive success (LRS), and LRS relative to the total LRS for each female's cohort (LRSrel). These metrics were calculated for offspring produced at the time of weaning and offspring that survived to emerge after their first hibernation period. Individual fitness (λind) was significantly associated with population growth during a female's lifetime (λLeslie; R2 = 0.523, P < 0.0001), indicating the need to adjust individual fitness for demonstrated changes in population growth and thus producing a relative individual fitness index (λrel). We regressed λrel on age at first reproduction, and found significant selection favoring earlier reproductive success (β ± SE = –0.20 ± 0.06; R2 = 0.306, P < 0.0001). When using an earlier (offspring at weaning) versus later (those that survived their first hibernation) measure of fecundity, we found that the latter introduced considerable variation, likely environmental, into the estimate of selection. This greatly weakened the regression of relative fitness on the age at first successful reproduction. LRS and LRSrel exhibited nonsignificant changes with age at first reproduction. Finally, those females that reproduced successfully at younger ages had similar litter sizes but significantly shorter life spans than females that matured when older, perhaps reflecting costs to early reproduction.

中文翻译：

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比较哥伦比亚地松鼠的健康措施和第一次繁殖年龄的影响

生活史特征的时间可能对生命周期的演变和人口统计有很大的影响。对于雌性首次繁殖的年龄尤其如此（科尔原则）。我们检查了雌性首次繁殖的年龄是否会影响哥伦比亚地松鼠 (Urocitellus columbianus) 的健康，因为雌性在最初生产断奶后代的年龄从 1 岁到 5 岁不等。在 28 年的数据集中有 148 名具有完整已知寿命的女性，我们检查了四种适应度测量：个体适应度 (λind)、个体适应度相对于人口增长模式 (λrel)、终生繁殖成功 (LRS)、和 LRS 相对于每个女性队列的总 LRS (LRSrel)。这些指标是针对断奶时产生的后代和在第一个冬眠期后存活下来的后代计算的。个体适应度 (λind) 与女性一生中的种群增长显着相关 (λLeslie; R2 = 0.523, P < 0.0001)，表明需要根据种群增长的显着变化调整个体适应度，从而产生相对个体适应度指数 (λrel ）。我们对第一次繁殖时的年龄回归 λrel，发现有利于早期繁殖成功的显着选择（β ± SE = –0.20 ± 0.06；R2 = 0.306，P < 0.0001）。当使用较早（断奶时的后代）与较晚（在第一次冬眠中幸存下来的后代）的繁殖力测量时，我们发现后者引入了相当大的变化，可能是环境、进入估计选择。这大大削弱了相对适应度对首次成功繁殖年龄的回归。LRS 和 LRSrel 在第一次繁殖时随年龄的变化不明显。最后，那些在年轻时成功繁殖的雌性与成年后成熟的雌性相比，产仔数相似，但寿命明显更短，这可能反映了早期繁殖的成本。