Physiological responses associated with feigned death in the American opossum.

Heart rate, respiratory rate and body temperature of four free-ranging American opossums, Didelphis marsupialis of both sexes (2.47 +/- 0.27 kg; mean +/- SE), were monitored during mild provocation and during death feigning ('play possum'). Mild provocations, caused by the approach of either man or dog, elicited a freezing response. The behavioural response was accompanied by a 12% decrease in heart rate (218 +/- 11 to 192 +/- 10 beats X min-1; mean +/- SE) and a 31% reduction in respiratory rate (26.7 +/- 1.7 to 18.4 +/- 1.4 breaths X min-1) from pre-stimulation values. Death feigning was induced by vigorous tactile stimulation by either investigator or dog. The response was marked by immobility, prone position and stiffness of the body. Mouth was open and the animal showed no response to touch or pinching. Death feigning was always accompanied by salivation, urination, defaecation and erection of the penis (in males). Heart rate decreased 46% (222 +/- 10.6 to 120 +/- 17 beats X min-1) and respiratory rate was reduced 30% (27.4 +/- 1.5 to 19.2 +/- 2.3 breaths X min-1) from pre-stimulation values. Body temperature dropped from 34.8 +/- 0.2 to 34.2 +/- 0.3 degrees C. During death feigning the animal was fully conscious as was evident by heart rate reduction during the re-approach of the dog. Atropine treatment had no obvious effect on behaviour but abolished the bradycardia when compared to pre-stimulus condition.