Many animals and plants have species-typical annual cycles, but individuals vary in their timing of life-history events. Individual variation in fur replacement (moult) timing is poorly understood in mammals due to the challenge of repeated observations and longitudinal sampling. We examined factors that influence variation in moult duration and timing among elephant seals (Mirounga angustirostris). We quantified the onset and progression of fur loss in 1178 individuals. We found that an exceptionally rapid visible moult (7 days, the shortest of any mammals or birds), and a wide range of moult start dates (spanning 6–10× the event duration) facilitated high asynchrony across individuals (only 20% of individuals in the population moulting at the same time). Some of the variation was due to reproductive state, as reproductively mature females that skipped a breeding season moulted a week earlier than reproductive females. Moreover, individual variation in timing and duration within age-sex categories far outweighed (76–80%) variation among age-sex categories. Individuals arriving at the end of the moult season spent 50% less time on the beach, which allowed them to catch up in their annual cycles and reduce population-level variance during breeding. These findings underscore the importance of individual variation in annual cycles.

许多动物和植物都有物种典型的年度周期，但个体的生活史事件发生时间各不相同。由于重复观察和纵向采样的挑战，哺乳动物中皮毛更换（换毛）时间的个体差异知之甚少。我们研究了影响象海豹（ Mirounga angustirostris ）换毛持续时间和时间变化的因素。我们对 1178 名个体皮毛脱落的发生和进展进行了量化。我们发现，异常快速的可见换羽（7 天，是所有哺乳动物或鸟类中最短的）和广泛的换羽开始日期（跨越事件持续时间的 6-10 倍）促进了个体之间的高度异步性（只有 20% 的个体种群同时换羽）。部分差异是由于生殖状态造成的，因为跳过繁殖季节的生殖成熟雌性比生殖雌性早一周换毛。此外，年龄-性别类别内时间和持续时间的个体差异远远超过年龄-性别类别之间的差异（76-80%）。在换毛季节结束时到达的个体在海滩上花费的时间减少了 50%，这使它们能够赶上年度周期并减少繁殖期间的种群水平差异。这些发现强调了年度周期中个体差异的重要性。