Desensitization Reduces Fear and Reactivity to Routine Husbandry Procedures in Rhesus Macaques (Macaca mulatta) Relocated Indoors From Outdoors.

Relocation and housing transitions are routine in captive settings, including biomedical facilities, and can negatively affect welfare by increasing restriction and reducing control. This study tested whether desensitization to human activity reduces fear and reactivity in rhesus macaques (Macaca mulatta) during indoor husbandry procedures. Fifty-six macaques were assigned to a treatment group receiving structured desensitization training (mean ± SE: 8.4 ± 3.4 sessions) or to a control group with no training. Fear and behavioral reactivity were assessed using a modified Human Intruder Test (mHIT) at baseline (day 0), day 15, and day 30. The mHIT included five conditions with progressively increasing mild aversive stimuli, with responses scored for each condition. No baseline differences were detected when conditions were combined. Significant Group × Time interactions showed greater improvement from baseline in the treatment group (Day 15: β = 0.59, p < 0.001; Day 30: β = 0.71, p < 0.001), with estimated marginal mean differences of 0.63 and 0.75 mHIT units, indicating moderate to large effects. Condition-specific analyses showed the strongest effects in tasks involving direct human interaction, particularly the Lixit Rod condition, with additional improvement in the Hand Feeding condition. Although a baseline difference was present in Hand Feeding, it did not explain the greater improvement over time in the treatment group. These findings indicate that structured desensitization reduces reactivity to human-related stimuli and may facilitate adaptation to routine procedures following relocation.