Bradycardia during Death-Feigning of Heterodon platyrhinos Latreille (Serpentes)

Bradycardia occurs during death-feigning in Heterodon platyrhinos. Its onset is abrupt and is closely associated with body inversion. Recovery is gradual, and heart rate has usually risen to between 40 and 60 beats/minute before righting occurs. At room temperature (23-26 C) all but 3 of 26 death-feigning specimens had minimum instantaneous rates between 3 and 15 beats/minute. During the immediate prelude to death-feigning, bradycardia and other physiological events constitute a virtual "parasympathetic syndrome". An apparent correlation between death-feigning and toad-eating in a number of snakes suggests that resistance to toad toxins may somehow be associated with death-feigning. It is speculated that Heterodon (and perhaps other toad-eating death-feigners) may compensate for ingested sympathomimetic toxins by reducing the corresponding endogenous component of a system comprising relatively high but balanced levels of sympathetic and parasympathetic activity. Non-adaptive triggering of the compensatory mechanism by transient extra endogenous catecholamines during fright or excitement might lead to wide-spread parasympathetic effects. If suppression of catecholamine-secreting CNS centers were included in the compensatory mechanism, the neurohumoral imbalance resulting from false triggering might account for body inversion and death-feigning per se.