Humans and natural predators induce different fear/anxiety reactions and response pattern to diazepam in marmoset monkeys

The behavioral response of marmoset monkeys in the Human Threat (HT) test of anxiety, and the effects of diazepam (DZP), were compared to those in the Predator Confrontation (PC) procedure. Subjects (n = 13) were initially submitted to four habituation trials, followed by four random confrontation sessions for each test (DZP 0, 1, 2 and 3 mg/kg). Each trial was divided into three consecutive 5-min intervals: pre-exposure, exposure (human observer, taxidermized oncilla cat) and post-exposure. As DZP induced sedation, marmosets (n = 10) were re-tested in a second experiment, consisting of two habituation trials and four confrontation sessions per stimulus, with lower DZP doses (0, 0.10, 0.25 and 0.50 mg/kg). Exposure to both stimuli significantly increased direct gazes and alarm calls, being dose-dependently reduced by DZP only in the PC test. In the HT protocol, the significant decrease in aerial scans was not detected with 0.10 mg/kg DZP. Locomotion, proximity, displacement activities and vigilance were not consistently influenced by the stimuli and/or DZP. The results thus suggest that the HT test had a greater impact on the marmosets' behavior, while DZP was more effective on the reactions observed in the PC test, possibly due to the inherent nature of each stimulus, distinct threat levels and/or presentation order.