| Summary: | Individual variation in life history timing influences predator-prey interactions, information transfer, disease transmission, ecosystem services, and resource subsidies. Unfortunately, precise estimates of individual variability and plasticity in the timing of critical life history events are rare and often rely on cross-sectional observational data. Understanding how life history timing varies across individuals requires longitudinal sampling and more advanced analytical techniques for quantifying onset and progression based on imperfect observations. We utilized a Bayesian hierarchical logistic function to examine variation in timing of fur replacement (molt) across individuals, ages, and sexes of 483 northern elephant seals Mirounga angustirostris to understand the prevalence of population-level synchrony in life history timing and underscore its implications. We found that the visible molt was extremely rapid (6.2 days in adult females and 10.5 days in juveniles) and that the wide range of molt start dates (60-day span even within a single age-sex category) resulted in extreme asynchrony across individuals. Across all life history events associated with the molt, individual variation overwhelmed age and sex differences in timing; 99% of the variation in haul-out duration was among individuals within age-sex categories and only 1% between categories. Similarly, 82% of variation in molt start date and 59% in molt duration was among individuals within age-sex categories. We discovered a strong correlation between molt timing, molt duration, and haul-out duration in adult female seals across consecutive seasons, suggesting that individual differences are repeatable. These findings emphasize that longitudinal sampling approaches are critical for explaining ecological patterns and processes when population-level variation in life history timing is high.
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