Water-borne cortisol levels show individuality and predict bold/shy behaviors in the self-fertilizing fish Kryptolebias marmoratus

Differences in personality traits can have important consequences for ecological and evolutionary processes because they can either provide fitness benefits (e.g., better access to food or reproductive success) or lower responsiveness to changes in the environment (i.e., lower behavioral flexibility). Since the last decade, there is a growing interest in studying the mechanisms that generate and maintain consistent individual differences in animal behavior such as inter-individual differences in endocrine traits. In fish, little is known about how repeatable, among-individual variance in endocrine traits such as cortisol levels may predict inter-individual variability in behavioral expression. Based on a common-garden experiment, we investigated whether cortisol could predict expression of behaviors associated with bold/shy personality in three naturally isogenic lineages of the self-fertilizing mangrove rivulus, Kryptolebias marmoratus. First, we explored both inter-individual and inter-lineage variability of water-borne cortisol levels over time. Cortisol levels were different between lineages: fish that originate from Emerson Point Preserve population (EPP) had significantly lower cortisol levels than individuals from the Dove Creek population. Moreover, while fish cortisol levels can vary over time, we observed repeatability in inter-individual variation within each lineage. We also obtain evidence for a slight but significant effect of interaction between cortisol levels and lineages on fish probability of exiting from a shelter and proportion of time before exiting from a shelter. Mangrove rivulus from the EPP lineage had lower probability of exiting from shelter and spent more time before exiting from shelter when they had low cortisol levels, compared to DC4 and DC11 individuals. Our study supports that cortisol levels have a genetic basis in the mangrove rivulus and suggests that water-borne cortisol levels predict behaviors associated with fish personality traits.