Within-individual canalization contributes to age-related increases in trait repeatability: a longitudinal experiment in red knots

Age-related increases in the repeatable expression of labile phenotypic traits are often assumed to arise from an increase in among-individual variance due to differences in developmental plasticity or by means of state-behavior feedbacks. However, age-related increases in repeatability could also arise from a decrease in within-individual variance as a result of stabilizing trait expression, that is, canalization. Here we describe age-related changes in within-individual and among-individual variance components in two correlated traits-gizzard mass and exploration behavior-in a medium-sized shorebird, the red knot (Calidris canutus). Increased repeatability of gizzard mass came about due to an increase in among-individual variance, unrelated to differences in developmental plasticity, together with decreases in within-individual variance consistent with canalization. We also found canalization of exploration but no age-related increase in overall repeatability, which suggests that showing predictable expression of exploration behavior may be advantageous from a very young age onward. Contrasts between juveniles and adults in the first year after their capture provide support for the idea that environmental conditions play a key role in generating among-individual variation in both gizzard mass and exploration behavior. Our study shows that stabilization of traits occurs under constant conditions: with increased exposure to predictable cues, individuals may become more certain in their assessment of the environment allowing traits to become canalized.