Self-injurious behavior in male rhesus macaques: association with aggression and stress as measured by salivary cortisol

Abstract

Self-injurious behavior (SIB) occurs in populations of socially-reared captive primates and can be a serious problem. The purpose of the first study was to survey a large colony of rhesus macaques for the presence of SIB and to identify potential risk factors. Almost 25% of the subjects were assessed to be self-biters, and over 10% wounded themselves. Subjects with SIB tended to be male and over seven years of age. Environmental risk factors included age when individually housed and the number of blood samples drawn. Although SIB has obvious deleterious effects such as tissue damage, it may also have benefits such as stress reduction. Because of the disruptive nature of obtaining physiological samples, few studies have measured stress concurrent with bouts of SIB. The focus of the second study was to develop new techniques for saliva collection to measure cortisol. These techniques were then used to gauge stress in rhesus subjects at the time of a biting event in comparison to control periods. No differences in cortisol were detected, suggesting that either there was no association between stress and spontaneous biting or the association was not detected due to variability in the data, timing of sample collection, or ameliorated stress by the biting itself. It was then proposed that self-injury is associated with social aggression and occurs in this context when physical contact is prevented. To test this hypothesis, subjects were first presented with videotapes containing either conspecifics or scenery. Levels of cortisol did not increase during the video presentation. Aggression increased during the conspecific videos, but self-biting remained constant, suggesting that the two behaviors were not associated. The subjects were then placed in more stressful situations as determined by cortisol levels: an empty room or a room containing an unfamiliar conspecific. Although these two situations were equally stressful, levels of aggression again increased in the conspecific situation, while levels of self-biting remained constant. SIB is a problem in primate facilities in part due to rearing and housing conditions. Although aggression was not associated with SIB, social settings were, and stressful environments may still play a role.