Data from: Birth dates vary with fixed and dynamic maternal features, offspring sex, and extreme climatic events in a high-latitude marine mammal

Reproductive synchrony tends to be widespread in diverse species of plants and animals, especially at higher latitudes. However, for long-lived mammals, birth dates for different individuals can vary by weeks within a population. A mother's birth timing can reveal useful information about her reproductive abilities and have important implications for the characteristics and survival of her offspring. Despite this, our current knowledge of factors associated with variation in birth dates is modest. We used long-term data for known-age Weddell seals in Antarctica and a Bayesian hierarchical modeling approach to study how birth dates varied with fixed and temporally varying features of mothers, whether sex allocation varied with birth timing, and annual variation in birth dates. Based on birth dates for 4465 pups born to 1117 mothers aged 4–31, we found that diverse features of mothers were associated with variation in birth dates. Maternal identity was the most important among these. Unlike most studies, which have reported that birth dates occur earlier as mothers age, we found that birth dates progressively occurred earlier in the year in the early part of a mother's reproductive life, reached a minimum at age 16, and then occurred later at later ages. Birth dates were positively related to a mother's age at primiparity and recent reproductive effort. The earliest birth dates were for pups born to prime-age mothers who did not reproduce in the previous year but began reproduction early in life, suggesting that females in the best condition gave birth earlier than others. If so, our finding that male pups tended to be born earlier than females provides support for the Trivers–Willard sex-allocation model. Average birth dates were quite consistent across years, except for 2 years that had notable delays and occurred during the period when massive icebergs were present and disrupted the ecosystem.生殖同步性在各类动植物物种中普遍存在，尤其在高纬度地区更为显著。但对于长寿哺乳动物而言，种群内不同个体的产仔日期可相差数周。母兽的产仔时机能够反映其繁殖能力的相关信息，且对子代的性状与存活具有关键影响。尽管如此，目前学界对产仔日期变异相关影响因素的认知仍较为有限。我们利用南极洲已知年龄个体的威德尔海豹（Weddell seals）长期监测数据，结合贝叶斯分层建模（Bayesian hierarchical modeling）方法，探究了母兽固定特征与随时间变化特征对产仔日期的影响、性分配随产仔时机的变化规律，以及产仔日期的年度变异情况。基于1117头年龄4至31岁的母兽所产4465只幼崽的产仔日期数据，我们发现母兽的多项特征均与产仔日期变异相关，其中母兽个体身份是影响最大的因素。与多数研究报道的“母兽年龄越大产仔越早”不同，我们发现：在母兽繁殖生涯早期，产仔日期随年龄增长逐年提前，于16岁时达到最早，之后随年龄增加反而逐渐延后。产仔日期与母兽的初产年龄及近期繁殖投入呈正相关关系。产仔日期最早的幼崽来自于前一年未繁殖且早年即开始繁殖的适龄繁殖母兽，这表明身体状况最优的雌性个体产仔早于其他个体。若此推论成立，我们发现雄性幼崽产仔早于雌性的结果，则为特里弗斯-威拉德性分配模型（Trivers–Willard sex-allocation model）提供了支持证据。各年度的平均产仔日期整体较为稳定，仅有2个年度的产仔日期出现显著延后，这两个年份均处于巨型冰山入侵并破坏生态系统的时期。