Doomed from the start: Consequences of a harsh early life on life-history traits of a wild Arctic ungulate.

Environmental conditions in early life can generate long-term individual differences in life-history traits such as survival and reproduction. Early-life conditions can challenge developing individuals through distinct environmental mechanisms. Each mechanism can induce long-term consequences, potentially leading to cohort effects in which individuals born in different years exhibit persistent differences in life-history traits. In ungulates, early life spans three sensitive periods-gestation, preweaning, and postweaning-during which distinct environmental mechanisms may exert long-term effects. Most studies, however, have examined only a single period or mechanism, leaving the combined effects of multiple mechanisms across development largely untested, despite evidence that these effects are often multifactorial. Benefiting from over 20 years of life-history data on migratory caribou (Rangifer tarandus) from the Rivière-aux-Feuilles and Rivière-George herds in northern Québec and Labrador (Canada), we investigated how three environmental mechanisms-environmental energetic constraints, resource availability, and competition-during gestation, preweaning, and postweaning influenced life-history traits. We hypothesized that harsh conditions during gestation and preweaning would most strongly reduce early-life body condition, survival to adult age, and longevity. We first performed a confirmatory path analysis to evaluate effects on early-life body condition and survival to adult age, followed by a time-to-event analysis to assess differences in longevity. Our results indicate that high environmental energetic constraints during gestation and limiting resource availability in late gestation and early preweaning reduced early-life body condition and survival to adult age. Among individuals surviving to adulthood, early-life competition also strongly reduced longevity. Overall, effects on life-history traits were associated with environmental conditions during gestation or early preweaning and stemming from multiple environmental mechanisms. These findings underscore the importance of gestational and early postnatal environments in shaping life-history traits. These results also demonstrate how a multifactor, multi-period approach provides a more complete understanding of how early-life conditions determine individual fitness, with strong implications for population dynamics.