3.4.1 Alarm arousal

A potentially life-threatening event, such as a fall in T _a to below zero, elicits a transient metabolic response in a hibernator. If the lowered temperature is maintained, the animal responds not just with transient increases in metabolism, but with a sustained rise in T _b and complete arousal.

Mechanical stimuli as well as temperature changes can evoke arousal. In animals fitted with electrodes just under the skin to monitor muscle action potentials, an externally applied stimulus results in a long-lasting burst of action potentials. The response in the fat dormouse (Glis glis) is very striking. This species hibernates with its bushy tail curled over its back. If the erect hairs are gently displaced, a burst of muscle action potentials occurs with a concurrent rise in respiratory and heart rates. Vibration, pressure, locally applied heat or cold, and the infusion of a variety of substances all produce the muscle response. In fact, the responsiveness of receptors in hibernators appears to increase with decreasing temperature, in marked contrast to the situation found when non-hibernators are made hypothermic.

The adaptive value of such a response is obvious. An animal torpid in a burrow seems quite defenceless. By retaining a high degree of surveillance, the animal can still perceive disturbances in air-flow or collapse of the burrow and make the appropriate response.