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Animals at the extremes: hibernation and torpor
Animals at the extremes: hibernation and torpor

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6.8 Summary

Hibernation shares physiological mechanisms with aestivation. These adaptations are fundamental properties of animal cells, but they come under central control and coordination mediated by the vascular, endocrine and central nervous systems.

Neuronal nuclei of the hypothalamus at the base of the forebrain combine the ability to integrate internal and external signals, set biological rhythms and control metabolism and body core temperature, and are indispensable to the process of hibernation. Hibernating animals are characterized by their ability to adjust their core temperature set-point. Experiments on ground squirrels suggest a progressive and smooth decline in the T b set-point (rheostasis), but this is not seen in all species that have been investigated. An alternative strategy is to practice thermogenesis at the point of reaching a T alarm resulting in an explosive increase in metabolism leading to arousal. Displays of apparently spontaneous metabolic thermogenesis, independent of T b, in marmots, is additional evidence that does not support the rheostasis theory for all mammals.

The ability of parts of the hypothalamus to engage in seasonal regulation of physiological functions is indicated by changes in genetic, molecular and structural changes in neurons. Rapid-response genes move from a cyclical to a sustained pattern of expression in parts of the hypothalamus implicated in circadian rhythm generation and locomotor activity. Changes in the numbers of cold-sensitive cells, the organization of neuronal dendrites and the properties of voltage-sensitive ion channels cause long-term changes in the ability of the hypothalamus to process sensory information into appropriate behavioural resonses at low T b. The hypothalamic neurotransmitters histamine and serotonin and the pineal gland hormone melatonin appear to have an important role in synaptic integration during the onset and maintenance of hibernation. A mixture of blood-borne factors have been shown to induce hibernation behaviour in a variety of birds and mammals in plasma transfusion experiments. As well as a large protein component, HIT contains at least one member of the opioid peptide family of neuromodulators.

Hibernation leads to the suppression of slow-wave sleep activity similar to that seen in sleep deprivation. In ground squirrels, the region of the hypothalamus, the SON, which controls the day-night sleep/waking cycle also regulates cyclical changes in T b. The modification of circadian cycles is therefore linked to the overriding need to govern the duration of ‘torpor bouts.