Four distinct behavioral profiles observed in male monkey rhesus in response to stressful housing conditions

Investigations of the pathophysiology of mental disorders have dramatically increased in the last decade. Such studies have almost exclusively been performed in rodents submitted to environmental, pharmacological, surgical or genetic manipulations. Those models can be seen as induced as opposed to spontaneously occurring disorders, therefore carrying poor construct validity. Moreover, the diversity of behavioral changes induced is quite limited, conferring poor face validity. Finally, mimicking such complex disorders in species phylogenetically and behaviourally closer to Humans seems more appropriate. Thus, we investigated the existence of spontaneous atypical behaviors displayed by non-human primates (NHP) housed in stressful conditions and their possible similarity with symptoms of human mental disorders. Stressful conditions, imposed by the breeding process, are defined by a peer-rearing from 5-months to 3-years old followed by housing in single cages; as opposed to large wild-like enclosure in social group. Forty males Macaca mulatta housed in single cage were observed using a scan sampling method, commonly used in Ethology. The parameters assessed were behaviors, body posture and orientation, gaze- direction and location in cage. Factor and cluster analysis were used to study inter-individual differences and revealed 4 distinct profiles. The inactive “depressive-like” profile contains males displaying mainly inactivity, a few changes of behaviors between the scans, a body facing the wall and located at the upper back corners of the cage. The self-centered “anxious-like” profile contains males displaying the higher levels of itching, yawning and selfgrooming. The “active-cautious” profile contains males quite active, exploring the cage and handling the feeding tray a lot, facing the wall as much as the front of the cage but standing at the back down corners of their cage. Finally the “active-aggressive” profile contains males very active but displaying a lot of threatening faces and vocalizations toward the observer and standing at the front of the cage. Motor stereotypic behaviors were observed among the 3 last profiles. These results suggest that NHP differ in their ways of coping with stress. As Humans, some individuals seem to be more severely affected by stressful events than others. The use of behavioral observations might allow us to find spontaneous model animals and forecast new perspectives in the study of mental disorders.