Thermoregulatory, brain and behavioral mechanisms during entrance into hibernation in the squirrel, Citellus beecheyi

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Abstract
In the continuous record of brain temperature during single entrances into hibernation there are gently falling temperature steps and longer maintained plateaus. Without these plateaus, brain temperature would decline roughly 18°C/hr. The normal rate of decline is 2–4°C/hr. with considerable slowing as the critical point is approached. The plateaus are initiated and maintained by periodic shivering. The ‘steps’ are ascribed to a vasodilation in the dorsal skin and a relative reduction in muscle tone. Each of the consecutive temperature curves of entering hibernation shows a faster rate of decline of temperature. During entrance, the heart rate declines 50%, with as small a change in temperature as 0.6°C occurring entirely around 33–34°C. Evidence is presented for a shift of balance toward the parasympathetic system during entrance into hibernation. As temperatures decline, the squirrel is capable of ‘spontaneous’ complex behavior. Neuronal activity changes correlatable with events during entrance into hibernation consisted of a 10 cps motor cortex discharge preceding and continuing during shivering and an amygdaloid-hippocampal 5 cps rhythm occurring after shivering at a time when the motor cortex discharge started to disintegrate. The correlates mentioned are absent during arousal shivering, indicating that a different controlling system was active here.