Ventilatory and metabolic responses to hypoxia in the echidna, Tachyglossus aculeatus

Oxygen consumption (VO2), CO2 production (VCO2), and minute ventilation (VE) together with breathing pattern were measured in echidnas during normoxia and hypoxia. In normoxia, VO2, VCO2, and VE were all found to be approximately 30% of the allometric prediction for a eutherian. As a consequence VE/VO2 and VE/VCO2 are as predicted for a mammal. This is in contrast to previous reports on the echidna in which the VE was shown to be low and the echidna, subsequently, to be in a state of hypoventilation. It is possible that the difference between this and previous studies is related to the resting state of the echidna; echidnas in this study adopted a curled-up "sleeping" posture, and measurements were made without tactile disturbance. Breathing pattern was typical of a semifossorial species in that inspiration time to total breath time was short when compared with the normal eutherian value. In graded hypoxia VE increased [threshold fractional concentration of inspired O2 (FIO2) = 0.125], predominantly the result of changes in frequency achieved through a shortening in expiration time. In acute hypoxia (FIO2 = 0.10) VE/metabolic rate showed a tendency to increase, mainly because of the increase in VE. Approximately 50% of the increase in VE could be attributed to the 25% increase in VO2 and VCO2 that occurred in acute hypoxia. Given that the general mammalian response to hypoxia is a drop in metabolic rate, possible reasons as to why the echidna does not decrease metabolic rate in hypoxia are discussed.