Heart rate dynamics in a marsupial hibernator

The eastern pygmy possum (Cercartetus nanus) 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 (f_H) control at various stages of torpor in a marsupial hibernator, and to see whether f_H variability differs from that of deep placental hibernators, we used radiotelemetry to measure ECG and body temperature (T_b) while measuring the rate of O₂ consumption and ventilation. f_H and O₂ consumption rate during euthermia were at a minimum (321±34 beats min^-1, 0.705±0.048 ml O₂ g^-1 h^-1) at an ambient temperature (T_a) of 31°C. f_H had an inverse linear relationship with T_a to a maximum of 630±19 beats min^-1 at a T_a of 20°C. During entry into torpor at a T_a of 20°C, f_H 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 T_b was stable at 24°C in these torpor bouts, the episodic nature of f_H had disappeared (i.e. no asystoles) with a rate of 34±3 beats min^-1 For multi-day bouts of deep torpor, T_a was lowered to 6.6±0.8°C. During these deep bouts of torpor, T_b reached a minimum of 8.0±1.0°C, with a minimum f_H of 8 beats min^-1 and a minimum O₂ consumption rate of 0.029±0.07 ml O₂ g^-1 h^-1 Shivering bouts occurred in deep torpor about every 8 min, during which ventilation occurred, and f_H was elevated to 40 beats min^-1 The duration of the QRS complex increased from 12 ms during euthermia to 69 ms at a T_b of 8°C. These findings demonstrate the dynamic functioning range of f_H to be about 600 beats min^-1 (∼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.