A spontaneous depressive pattern in adult female rhesus macaques

Non-human primates offer unique opportunities to study the development of depression rooted in behavioral and physiological abnormalities. This study observed adult female rhesus macaques within social hierarchies and aimed to characterize the physiological and brain abnormalities accompanying depressive-like behavior. The behaviors of 31 female rhesus macaques from 14 different breeding groups were video recorded, and the footage was analyzed using the focal animal technique. There were 13 monkeys who never displayed huddling behavior (non-huddlers). The remaining 18 monkeys were divided into two groups according the mean time spent in the huddle posture. Four monkeys were designated as high huddlers, whereas the other 14 monkeys were low huddlers. An inverse relationship was discovered between social rank and depression. High huddlers spent more time engaging in physical contact and in close proximity to other monkeys, as well as less time spontaneously and reactively locomoting, than low huddlers and/or non-huddlers. Cortisol levels measured from the hair were elevated significantly in high huddlers compared with low huddlers and non-huddlers, and the measured cortisol levels were specifically higher in high huddlers than subordinate or dominant control monkeys. Regional cerebral blood flow data revealed significant and widespread decreases in high huddlers compared with non-huddlers.