Early-life cortisol exposure modulates habituation to human disturbance in fallow deer fawns.

The survival of vulnerable juveniles depends on how they respond to threat, yet the escape behaviour of individuals often differs despite similar exposure to stimuli. These differences may arise from interactions between experience, early-life physiology and developing behaviours such as play. Using flight initiation distance (FID) trials from fallow deer fawns (Dama dama) in an urban parkland, we investigated whether FID was associated with pedestrian exposure, locomotor play and cortisol extracted from neonatal hair. Fawns with lower cortisol showed shorter FIDs as prior pedestrian exposure increased, consistent with habituation to repeated low-risk human presence. By contrast, fawns with higher neonatal hair cortisol showed longer FIDs with increasing pedestrian exposure, suggesting maintained or increased sensitivity to disturbance. Accumulated locomotor play was not associated with FID, indicating that any benefits of play may be expressed in escape performance rather than the decision of when to flee. These results indicate that early-life physiological state influences how juveniles respond to repeated human exposure, with consequences for individual development of tolerance and escape behaviour. Understanding the physiological basis of habituation in young animals may help predict which individuals are most vulnerable to disturbance in human-dominated landscapes.