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

Abstract

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.

Reexamining Echidna Physiology: The Big Picture forTachyglossus aculeatus acanthion

“…In terms of the respiratory physiology of the short-beaked echidna, we confirmed the previous observations of Bech et al (1992) higher f R (7 -17.5 min -1 ; 1-sample t-test t 5 > 5.38, P < 0.003; Bentley et al 1967;Augee et al 1971;Parer and Hodson 1974;Frappell et al 1994;Nicol and Anderson 2003). Our V T measurement is similar to that of Frappell et al (1994;44.8 ml) but other studies report significantly lower values (19 -24 ml; t 5 > 3.21, P < 0.024; Bentley et al 1967;Parer and Hodson 1974;Bech et al 1992).…”

“…Our measured basal T b of 29.5°C for Western Australian echidnas was 0.6 -2.9°C lower than other basal T b estimates for this species (at T a between 23 and 30°C; Martin 1903;Warlow 1914;Schmidt-Nielsen et al 1966;Augee 1976;Dawson et al 1979;Frappell et al 1994). It is unclear if this reflects differences between sub-species (e.g.…”

“…Lovegrove (2012a) The low BMR that we measured here for T. a. acanthion (0.083 ml O 2 g -1 h -1 ) supports previous conclusions for short-beaked echidnas. Our value is even lower (1 sample t-tests; P ≤ 0.005) than that reported for short-beaked echidnas from previous studies (range 0.132 -0.217 ml O 2 g -1 h -1 , mean 0.163 ml O 2 g -1 h -1 ; Martin 1903;Schmidt-Nielsen et al 1966;Augee 1976;Dawson et al 1979;McNab 1984;Bech et al 1992;Frappell et al 1994), except that of Waugh et al (2006) of 0.108 ml O 2 g -1 h -1 (t 5 = 2.45, P = 0.058). Approximately 80% of the difference between our estimate of BMR and these varying estimates for the species can be accounted for by differences in T b (i.e.…”

“…The data of Bech et al (1992) suggest that an especially low V T results in a low ventilatory requirement. Bentley et al (1967), however, reported higher values for V T and V I , within the expected range for mammals, and both Bentley et al (1967) and Frappell et al (1994) measured an oxygen extraction (EO 2 ) and ventilatory requirement comparable to that of other mammals.…”

“…Previous studies of short-beaked echidnas have reported a lower respiratory frequency (f R ) than for marsupials, that did not vary greatly over a range of T a , except at high T a where f R increased (Martin 1903;Robinson 1954;Parer and Hodson 1974;Bech et al 1992;Frappell et al 1994). Tidal volume (V T ) and minute volume (V I ) values have been variable between studies.…”