Daily and seasonal rhythms in the respiratory sensitivity of red-eared sliders (Trachemys scripta elegans).

The purpose of the present study was to determine whether the daily and seasonal changes in ventilation and breathing pattern previously documented in red-eared sliders resulted solely from daily and seasonal oscillations in metabolism or also from changes in chemoreflex sensitivity. Turtles were exposed to natural environmental conditions over a one year period. In each season, oxygen consumption, ventilation and breathing pattern were measured continuously for 24 h while turtles were breathing air and for 24 h while they were breathing a hypoxic-hypercapnic gas mixture (H-H). We found that oxygen consumption was reduced equally during the day and night under H-H in all seasons except spring. Ventilation was stimulated by H-H but the magnitude of the response was always less at night. On average, it was also less in the winter and greater in the reproductive season. The data indicate that the day-night differences in ventilation and breathing pattern seen previously resulted from daily changes in chemoreflex sensitivity whereas the seasonal changes were strictly due to changes in metabolism. Regardless of mechanism, the changes resulted in longer apneas at night and in the winter at any given level of total ventilation, facilitating longer submergence at times of the day and year when turtles are most vulnerable.