Ontogeny of cortisol reaction norms in wild bonobos (Pan paniscus)

Phenotypic plasticity enables animals to adjust physiology, behaviour, morphology and life-history traits in response to changing conditions, either reversibly or through irreversible developmental shifts. In long-lived species, early-life phenotypic changes can have profound consequences if they persist into adulthood. Understanding the balance between stable and flexible trait expression across ontogeny is therefore key. Glucocorticoids (GCs) are central to physiological regulation and known to exhibit plasticity, but little is known about the consistency of GC phenotypes across development. We examined whether bonobos (Pan paniscus), a long-lived species, show consistent GC phenotypes as they mature, focusing on individual differences in average urinary cortisol phenotypes (reaction-norm intercepts) and plasticity (reaction-norm slopes) in response to time of day. We applied a reaction-norm approach to assess individual variation in GC intercepts and slopes across ontogeny, using random regression mixed-effects models. Trait repeatability of urinary cortisol was low across and within years, indicating high within-individual variation relative to between-individual variation. Reaction-norm intercepts were moderately repeatable, suggesting stable individual average GC phenotypes across development. By contrast, slopes were weakly repeatable, reflecting flexibility in how individuals modulate GC output across the day. This dual regulatory structure may support adaptive physiological responses to changing demands in a long-lived species.