Cardiac and Respiratory Function During Sudden Prolonged Immobility in Wild Rodents

&NA; Recently captured wild rodents of six different species were tested in a standard “open field” arena where they were subjected to sudden visual and auditory stimulation as well as to a natural predator. Periods of prolonged immobility ranging from 2 to more than 60 minutes' duration were elicited in 21 members of four species. This behavioral state was accompanied by very low heart rates and a high incidence of cardiac arrhythmias. Nine members of two other species which did not show prolonged immobility, had increased heart rates during testing and a significantly lower incidence of arrhythmias. Respiratory rate was elevated during testing in all species. Physiologic recordings were also obtained during “feigned death” responses and transient “torpor.” The adaptive value of these responses is discussed, and the physiologic changes compared to those known to occur during syncope, sleep, expectation of attack and sudden death.