Factors in the pattern, timing and predictability of hibernation in the squirrel, Citellus beecheyi

Ground squirrels, Citellus beecheyi, bearing permanently implanted brain thermocouples never entered deep hibernation in one simple temperature decline. The pattern consisted of entering into and arousing from hibernation several times, each succeeding hibernation terminating at a lower brain temperature until a final plateau was reached. The entrances before the final plateau are considered test drops and evidence is presented that each of the low brain temperatures reached is a critical point. This pattern of hibernation was independent of season. The critical brain temperature/time curve is suggestive of the integrated rate of metabolic preparations necessary for entering hibernation. From the thermoregulatory behavior of the critical points, it appears that the temperature regulating mechanism is being informed of the state of biochemical preparations for hypothermia and controls the depth of each temperature drop accordingly. Arousals from hibernation, particularly during the test drop period, occur at preferred times, at statistically significant 1-hour intervals, but the time of entrance and the first plateau are randomly distributed within two selected portions of the day. Evidence is presented that this species' time sense is due to a temperature-independent internal clock. A ‘three-factor theory’ explaining the triggering of hibernation is proposed and developed.