Eels' individual migratory behavior stems from a complex syndrome involving cognition, behavior, physiology, and life...

Variability within species is key for adaptability and biological evolution. To understand individualities in the context of animal movement, we focused on one of the most remarkable migrations-the journey of the endangered European eel from their birthplace in the Sargasso Sea to freshwater environments. Laboratory observations unveiled a continuum of diverse phenotypes of migrating eels: Some displayed a heightened tendency to swim against a constant water flow, while others a greater propensity to climb obstacles. Looking for the biological underpinnings of this migratory diversity, we characterized the eels' individual differences in traits of four key domains: life history, physiology, behavior, and cognition, among which we found significant variance and interconnectedness. Upon reducing this variance to its primary multivariate axes, we found that these predict the migratory types. Eels with 1) low exploration, high activity, low boldness, and high lateralization; 2) strong lateralization, enhanced quantitative abilities, short problem-solving time, high boldness, and low growth rates; or 3) enhanced problem-solving, reduced spatial learning, high cognitive flexibility, and shorter time to solve the cognitive tasks were more likely to display the climbing migratory type. Field sampling revealed how specific traits' combinations seemed to influence the distribution of eels once they begin to settle in the freshwater environment. Our study underscores the impressive diversity of individuals during this critical migration, emphasizing an intrinsic connection to multidomain trait variance. Preserving this diversity becomes paramount, as it likely contributes to the resilience and adaptability of endangered migratory species.