Neonatal personality is associated with sex-specific developmental trajectories in a wild mammal

Animal personality, defined as consistent among-individual differences in behaviour, is an important source of phenotypic variation, but its developmental origins and associations with subsequent development remain poorly understood. We measured neonatal personalities, early-life growth, and yearling male antler length in wild fallow deer (Dama dama). Using eight years of data, we estimated repeatability and among-individual correlations among neonatal personalities and growth and applied structural equation modelling to examine associations with yearling male morphology. Heart rate and behavioural reaction measures showed repeatable among-individual variation, demonstrating that stable differences were detectable shortly after birth. Behavioural reaction was negatively correlated with growth in males but not in females. Heart rate was not associated with growth in either sex. In males, the effects of neonatal behavioural reactivity persisted beyond early life: more behaviourally reactive neonates had shorter antlers as yearlings, indirectly through reduced early-life growth. These findings show that stable among-individual differences emerge during the neonatal period and are associated with sex-specific developmental trajectories.