Cage Position and Response to Humans in Singly-housed Rhesus Macaques (Macaca mulatta)

Traditional laboratory caging for nonhuman primates is typically configured in a 2-tiered manner, with caging arranged in 2 horizontal rows stacked vertically. Studies of the effects of cage row have yielded inconsistent results with respect to impacts on psychological well-being. This study tests whether rhesus macaques (Macaca mulatta) housed in the bottom tier of caging display poorer responses to attempted positive human interaction than those in the upper tier, suggesting that humans are a greater stressor for animals housed in bottom rows. The attempted positive social interaction took the form of offering a food treat by hand. This study involved 270 male and female singly-housed rhesus macaques, ranging in age from 2.4 to 27.4 y of age. Cage position was characterized not only with respect to tier, but also with respect to proximity of the cage rack to the room door. A single technician recorded whether the animal retrieved the treat within 10 s and also recorded all social behaviors directed toward the technician during the test. No effects of cage tier were detected, nor were effects of proximity to the door found. However, significant contrasts were detected with respect to subjects' sex, age, and lifetime tenure in indoor caging. Females were less likely than males to take treats from a human's hand, and were more likely to show fear. Both increased age and tenure in caging were associated with an increased probability of taking the treat. These findings may have implications for programs aiming to monitor and address fearful behavior.