Control of breathing in the echidna (Tachyglossus aculeatus) during hibernation

Resting non-hibernating echidnas are characterised by low metabolic rates, but also have a very low respiratory frequency and a variable respiratory minute volume, often resulting in low levels of arterial O2 and high CO2. As the echidna lies at one physiological extreme among the hibernators, in terms of its large size and low metabolism and ventilatory requirement when not hibernating, a study of control of breathing during hibernation in echidnas should provide a useful test of the generality of various models. We used non-invasive techniques to study breathing patterns and the control of ventilation in 6 echidnas. Hibernating echidnas (Tb range 7-10 °C) showed episodic breathing with bursts of breaths (average 36±16 breaths in 24±5 min) followed by a period of apnea (76±17 min) then a series (8±4) of slow breaths at 14±1 min intervals leading up to the next burst. Increasing CO2 levels in the inspired air increased the number of breaths in a burst, eventually leading to continuous breathing. Inter burst breaths were controlled by O2: hypoxia increased inter burst breaths, and decreased burst length, while hyperoxia abolished inter burst breaths and increased the apneic period. Overall, while CO2 was a strong respiratory stimulus in hibernating echidnas, O2 had little effect on total ventilation, but did have a strong effect on the breathing pattern. © 2003 Elsevier Inc. All rights reserved.