Changes in Ventilation and Respiratory Sensitivity Associated with Hibernation in Columbian (Spermophilus columbianus) and Golden-Mantled (Spermophilus lateralis) Ground Squirrels

During entrance into hibernation, the decrease in minute ventilation associated with the fall in body temperature and metabolic rate is accompanied by the conversion of continuous breathing to intermittent breathing in both the goldenmantled ground squirrel (Spermophilus lateralis) and the Columbian ground squirrel (S. columbianus). In hibernation, the former species exhibits an episodic breathing pattern, while the latter species exhibits a pattern of evenly spaced single breaths. The goal of this study was to compare the changes in the ventilatory responses of these two species of ground squirrel to changes in levels of inspired O₂ and CO₂ during hibernation, to test the hypothesis that species differences in intermittent breathing patterns may be explained on the basis of differences in sensitivity to hypoxia and hypercapnia. During entrance into hibernation, changes in ventilation and breathing pattern were accompanied by a change in the relative contribution of changes in arterial Po₂ (Pao₂), Pco₂ (Paco₂), and pH (pHa) as stimuli to ventilation. When awake at an ambient temperature of 22° C, both species showed strong hypoxic ventilatory responses and comparatively blunted hypercapnic ventilatory responses. During hibernation, both showed greatly reduced hypoxic responses but comparatively elevated hypercapnic ventilatory responses. This suggests that changes in Pao₂ play a much greater role in the control of ventilation in euthermic animals, while changes in Paco₂/pHa play the predominant role during hibernation. Furthermore, during hibernation, the nonventilatory period-rather than the breath-became the primary controlled variable in the breathing pattern. This was true for both species, despite the differences in the intermittent breathing patterns each exhibited, and thus our results do not support the hypothesis that differences in intermittent breathing patterns are due to species-specific changes in sensitivity to respiratory stimuli as animals enter hibernation.