Printed in Great Britain VENTILATION IN AN AQUATIC AND A TERRESTRIAL CHELONIAN REPTILE

1. Periods of breath-holding are interrupted by episodes of continuous breathing in the aquatic turtle Pelomedusa subrufa, whereas single breaths and short periods of breath-holding alternate in the terrestrial tortoise Testudo pardalis. This implies that partial pressures of O2 and COa in expired air are stable in Testudo in contrast to cyclic fluctuations in Pelo-medusa. 2. In spite of this, air convection requirements (J^/tfb,, ml BTPS/ml STPD) are not significantly different for the two species (25.4 in Testudo, 27-3 in Pelomedusa), and differences in weight-specific ventilation between the species can only be ascribed to a difference in mean body weight. 3. The end-tidal POOl in Pelomedusa (mean 152 mmHg) is lower than in Testudo (mean 24-7 mmHg), which reflects aquatic COa elimination in Pelomedusa. 4. In Testudo, the time course of ventilation correlates with the time course of increase of end-tidal PCOt during CO8 breathing, but no simple relationship is evident between ventilation and blood PCOi in Pelomedusa. 5. In both species tidal volume as well as respiratory frequency increase approximately in proportion to the end-tidal POo. ' although the response to 6 % CO2 breathing could be less than expected from extrapolation of the responses to 2 and 4 % CO2. Both species also increase ventilation during hypoxia, but hyperoxia depresses ventilation