Data from: Trade-offs between age-related breeding improvement and survival senescence in highly polygynous elephant seals: dominant males always do better

Life history trade-off theory predicts that current reproduction can negatively affect survival and future reproduction. Few studies have assessed breeding costs for males of polygynous species compared to females, despite substantial variation in breeding success among individual males (e.g. subordinate cf. dominant breeders). Specifically, differentiating between the cost of attending breeding seasons, and the additional cost of successfully securing and mating females is lacking. We investigated whether trade-offs are present in the highly polygynous male southern elephant seal (Mirounga leonina) using 34-years of individual-level data. We compare age-specific survival, recruitment and future breeding success probabilities of pre-breeders (males yet to recruit) and breeders (subordinate and dominant social ranks) using multievent models. Pre-breeders and breeders of overlapping ages had similar survival probabilities, suggesting that there was no attendance cost for early recruits. In addition, the probability of recruiting as a dominant breeder never exceeded recruitment probability as a subordinate breeder of the same age. Therefore, older pre-breeders that delayed attendance costs generally did not improve their breeding success (probability of being dominant) at recruitment more than younger recruits. Rather, recruitment age may be a function of individual quality, with lower quality individuals requiring more time to socially mature. When comparing subordinate and dominant breeders, we found clear evidence for survival senescence, with subordinate breeders having a higher baseline mortality. In contrast, age-specific future breeding success (probability of being dominant at t+1) increased with age, with dominant breeders maintaining higher subsequent breeding success than subordinate breeders. The opposite trends in survival and future breeding success for both subordinate and dominant breeders may indicate a lifetime, population-level trade-off. However, we found no evidence to suggest that being a dominant breeder consecutively (and having a higher accumulated breeding cost) accelerated the rate of senescence when compared to individuals that were previously subordinate. Thus, males experienced actuarial senescence regardless of social rank, with dominant (and possibly high quality) breeders showing a reduced trade-off between survival and future breeding success. We make several novel contributions to understanding polygynous male life histories and southern elephant seal demography.

生活史权衡理论（life history trade-off theory）预测，当前繁殖会对存活及未来繁殖产生负面影响。相较于雌性，针对多配制物种（polygynous species）雄性的繁殖成本评估研究寥寥无几，尽管雄性个体间的繁殖成功率存在显著差异（例如次等繁殖者与优势繁殖者）。目前仍缺乏对参与繁殖季的成本，与成功获取配偶并完成交配的额外成本之间的区分研究。
我们依托34年的个体水平监测数据，针对高度多配制的雄性南象海豹（Mirounga leonina）开展权衡效应研究。通过多事件模型（multievent models），对比了预繁殖个体（尚未完成繁殖招募的雄性）与繁殖个体（包含从属与优势社会等级的雄性）的年龄特异性存活率、繁殖补充率及未来繁殖成功概率。
年龄重叠的预繁殖个体与繁殖个体存活率相近，表明早期完成繁殖招募的个体并未因参与繁殖付出代价。此外，同年龄段个体以优势繁殖者身份完成招募的概率，始终未超过以从属繁殖者身份招募的概率。因此，推迟参与繁殖、规避成本的年长预繁殖个体，其招募时成为优势繁殖者的概率并未优于年轻个体。换言之，繁殖招募年龄或取决于个体品质，品质较低的个体需要更长时间完成社会成熟。
对比从属与优势繁殖者时，我们发现了明确的生存衰老（survival senescence）证据：从属繁殖者的基础死亡率更高。与之相对，年龄特异性未来繁殖成功率（即t+1时刻成为优势繁殖者的概率）随年龄增长而提升，优势繁殖者后续的繁殖成功率始终高于从属繁殖者。从属与优势繁殖者在存活与未来繁殖成功率上呈现的相反趋势，或暗示了种群水平的终生权衡效应。
不过，相较于曾为从属繁殖者的个体，连续以优势繁殖者身份参与繁殖（且累积繁殖成本更高）的个体并未表现出衰老速率加快的迹象。由此可见，无论社会等级如何，雄性南象海豹均会出现种群统计衰老（actuarial senescence）；而优势繁殖者（或为高品质个体）在存活与未来繁殖成功率之间的权衡效应有所减弱。
本研究为理解多配制雄性的生活史特征与南象海豹种群动态学（demography），做出了多项创新性贡献。