Eupneic respiratory pattern and ventilatory responses to environmental CO₂ in a small lizard, Anolis carolinensis

Lizards and other reptiles are generally described as breathing intermittently, either with single breaths separated by variable periods of apnea or with clusters of breaths separated by prolonged apneas (i.e., episodic breathing). However, relatively little is known about the breathing of small lizards (≤ 10 g) even though the average body mass of extant species is only 8 g. Accordingly, head-body plethysmography was used to assess breathing in the green anole (Anolis carolinensis). Ventilation was measured in adult anoles (2 - 6 g) while exposed to room air (0 % CO₂) and hypercarbic gas mixtures (1 %, 3 %, and 5 % CO₂), and during recovery from hypercarbia. A. carolinensis exhibited a continuous, rhythmic breathing pattern in room air with most (90 %) breaths lacking any discernible end-inspiratory pause; apneas were infrequent (1.3 ± 0.2 apneas per 5 min (mean±SEM; n = 16)). Minute ventilation increased during exposures to 1 % and 5 % CO₂ largely through increases in tidal volume, but the response to 3 % CO₂ was variable due to the emergence of end-inspiratory pauses each respiratory cycle (and concomitant slowing of respiratory frequency) during the 3 % and 5 % CO₂ exposures. A marked post-hypercarbic hyperpnea was observed during the return to 0 % CO₂, the magnitude of which increased with the severity of the hypercarbic challenge. Although the ventilatory responses to CO₂ and post-hypercarbic hyperpnea are consistent with reports for other reptile species, these data highlight that the classic description of intermittent breathing in reptiles may not apply to all small lizards, perhaps due to their higher mass-specific metabolic demands.