Heart rate dynamics in a marsupial hibernator.

The eastern pygmy possum () is a small marsupial that can express spontaneous short bouts of torpor, as well as multi-day bouts of deep hibernation. To examine heart rate () control at various stages of torpor in a marsupial hibernator, and to see whether variability differs from that of deep placental hibernators, we used radiotelemetry to measure ECG and body temperature () while measuring the rate of O consumption and ventilation. and O consumption rate during euthermia were at a minimum (321±34 beats min, 0.705±0.048 ml O g h) at an ambient temperature () of 31°C. had an inverse linear relationship with to a maximum of 630±19 beats min at a of 20°C. During entry into torpor at a of 20°C, slowed primarily as a result of episodic periods of cardiac activity where electrical activity of the heart occurred in groups of 3 or 4 heart beats. When was stable at 24°C in these torpor bouts, the episodic nature of had disappeared (i.e. no asystoles) with a rate of 34±3 beats min For multi-day bouts of deep torpor, was lowered to 6.6±0.8°C. During these deep bouts of torpor, reached a minimum of 8.0±1.0°C, with a minimum of 8 beats min and a minimum O consumption rate of 0.029±0.07 ml O g h Shivering bouts occurred in deep torpor about every 8 min, during which ventilation occurred, and was elevated to 40 beats min The duration of the QRS complex increased from 12 ms during euthermia to 69 ms at a of 8°C. These findings demonstrate the dynamic functioning range of to be about 600 beats min (∼80-fold), one of the largest known ranges in mammals. Our study shows that despite a separation of ∼160 million years, the control and function of the cardiac system seems indistinguishable in marsupial and placental hibernating mammals.