Self-injurious behavior in male rhesus macaques does not reflect externally directed aggression

Self-injurious behaviors (SIB), such as self-biting and self-wounding, have been observed in a small percentage of captive nonhuman primates. Because rhesus monkeys that exhibit SIB also tend to be more aggressive, it was hypothesized that SIB is related to externally directed aggression and is associated with contexts in which physical contact between participants is prevented. The purpose of this study was to test the hypothesized relationship between SIB and outward aggression. Subjects were first presented with videotapes of conspecifics, scenery and a blank screen, and their behavior was recorded. Levels of salivary cortisol, an indicator of stress, were also measured before and after presentation of the videos. Although aggression increased when subjects viewed tapes containing conspecifics, neither cortisol levels nor self-biting behavior varied as a function of tape content. The subjects were then placed in two additional test situations: an empty room and the same room containing an unfamiliar conspecific. Aggression was significantly higher in the stranger condition compared to the empty room condition. The two situations yielded parallel increases in cortisol, suggesting that being alone was just as stressful as being paired with an unfamiliar conspecific. Self-biting rates were also similar in these two conditions. Thus, contrary to our prediction, increases in aggression did not correlate with increases in SIB. These results suggest that under similarly stressful conditions, SIB and externally directed aggression are unrelated.