Understanding how behavioral variation emerges during early development is essential for interpreting individual differences and their ecological relevance. Early-life behavioral development can involve substantial individual variation shaped by biological and social contexts, resulting in differing levels of repeatability across behaviors. In captive-breeding programs, identifying these developmental patterns and their causes can provide a valuable reference for conservation-relevant species. We studied the ontogeny, individual variation, and repeatability of behavior in 138 captive-born Arctic foxes (Vulpes lagopus) in a reintroduction program using repeated focal observations during early and late juvenile periods. Using multivariate Poisson mixed models, we quantified individual- and litter-level variance, assessed temporal and socio-biological influences (i.e., month, sex, sex ratio, litter size), and estimated among- and within-individual behavioral correlations. Juveniles exhibited clear developmental shifts, including reduced play and resting and increased agonistic interactions. Among socio-biological factors, only litter size had a significant effect, with juveniles from larger litters showing elevated vigilance. Crucially, overall repeatability, which incorporates both individual and litter identity, was consistently higher than individual-only repeatability. This divergence indicates that shared early-life environments account for a substantial portion of consistent behavioral variation, whereas individual-level consistency remains comparatively low during the juvenile period. Among-individual correlations were weak, whereas within-individual correlations were stronger for specific behavior combinations. These findings show how early-life behavioral traits are jointly shaped by individual and social factors, clarifying the sources of behavioral variation during development.

Life history theory predicts that individuals will differ in their risk-taking behavior according to their expected future fitness. Understanding consequences of such individual variation within a behavioral trait is crucial in explaining potential trade-offs between different traits and in predicting future dynamics in changing environments. Here, we studied individuals in a wild arctic fox population to explore if (1) individual variation in risk-taking behaviors of adult arctic foxes and in stress-dealing behaviors of their juveniles exist and are consistent over time to verify the existence of personality traits; (2) those behavioral traits in adults and juveniles are correlated; (3) they can explain fitness-related components (i.e., juvenile physical condition, mortality rate). We presented simple field experiments assessing behavioral traits by observing adult reactions toward approaching observers, and juvenile behaviors while trapping. Through the experiments, we found highly consistent individual variation of adults in vigilance and boldness levels, and more flexible juvenile behavioral traits categorized as investigating, passive, and escaping. The offspring of bolder adults exhibited more investigating behaviors and were less passive than the offspring of shy adults. Juvenile physical condition was not related to their mortality nor any behavioral traits of either parents or themselves. Lastly, highly investigating and active juveniles with bold parents had significantly lower mortality rates. This shows that interactions between parent personality and juvenile behavioral traits affect a fitness-related component in the life history of individuals. Significance statement The recent surge of interest in consistent individual difference in behavior, also called as animal personality, has already focused on its fitness consequences, but few studies have investigated the interactions between parent and offspring personality, and their ecological consequences. Moreover, this has rarely been studied in wild canids. The arctic fox is a charismatic species showing wide individual variation in behaviors. They live in highly fluctuating tundra ecosystems providing different selection regimes, making it even more eco-evolutionarily intriguing. Yet, few studies looked into behavioral traits and their importance in this system. While introducing simple methods to improve personality research in the wild, we provide a unique example of how variation in both parents and their juveniles collectively works for group dynamics in a cyclic population. This provides a firm basic for understanding behavior-mediated dynamics and opens up broader questions on how fluctuating environments exert varying pressures on individual differences.