Tonic immobility in the squirrel monkey (Saimiri sciureus)

Tonic immobility and several accompanying behavioral changes were examined in the squirrel monkey (Saimiri sciureus). It was found that either the occurrence of a loud noise immediately before induction or the presence of a stuffed Cooper's hawk increased the duration of immobility in some subjects. Subspecies differences in durations of immobility were also found. The number of inductions required to produce immobility in squirrel monkeys were unusually large, and were negatively correlated with duration of the response. Behavioral variables accompanying tonic immobility showed qualitative differences when compared to what is known concerning their relation to immobility in other species. The findings support the belief that tonic immobility evolved as a predator defense in squirrel monkeys or one of their ancestors, but suggest that it no longer plays an active role in their survival.