How stiff is the armadillo? A comparison with the allometrics of mammalian respiratory mechanics.

Static respiratory mechanics were examined in the armadillo (Dasypus novemcinctus) and compared with allometric relationships newly derived for adult mammals from values in the literature. Normalised by body weight, chest wall compliance (Cw) in the armadillo is lower than predicted. Lung compliance (Cl) is also low in the armadillo, however it is appropriately matched to the resting lung volume (Vr) (ie. Cl/Vr infinity Mass0.0) and the ratio of Cw/Cl is appropriate for the size of the animal. Respiratory system resistance is high in the armadillo, presumably because of smaller airways associated with the small lung. The power of breathing in the armadillo is comparatively high, mainly due to the high resistive forces. Indeed, the oxidative cost of breathing is approximately double that of a mammal with similar Vr. Hypoxia or hypercapnia are known to invoke an attenuated ventilatory response in the armadillo and one that relies more on changes in frequency rather than volume. While such a breathing pattern helps to reduce the power of breathing it also compromises the degree of hyperventilation achieved.