Scaling of hypercapnic ventilatory responsiveness in birds and mammals

The possible relationship between CO 2 responsiveness and body mass in birds was explored using newly acquired ventilatory data from the barn swallow, Hirundo rustica, and the pigeon, Columbia livia, and that from the literature on four other species. Ventilatory responsiveness (%ΔV̇) of birds to 5% inspired CO 2 is scaled to body mass to the 0.145 power (%ΔV̇ ∝ Mb 0.145). A similar allometric relationship exists for data on 7 species of eutherian mammals taken from the literature (%ΔV̇ ∝ Mb 0.130). The reduced responsiveness to CO 2 in small birds and mammals may be related to an elevated hypoxic ventilatory sensitivity, as demonstrated in mammals (Boggs and Tenney, Respir. Physiol. 58: 245–251, 1984). These scaling relationships may reflect a mechanism for minimizing the inhibition of ventilation resulting from excessive loss of CO 2 which thereby permits a higher hypoxic ventilatory response in small species. Other mechanisms, however, could include size related differences in mechanics or alveolar ventilation.