Animals at the extremes: Polar biology
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# 3.5.1 Summary of Section 3

Penguins and many other large polar animals fast for long periods while remaining active and at near-normal body temperature. Emperor penguins fatten before the breeding season and fast for weeks during courtship and reproduction. Very little protein is broken down until lipid stores are nearly exhausted. Energy reserves determine an individual's behaviour such as feeding or abandoning the chick. Omnivorous brown and black bears feed in summer and become dormant in winter: they stop feeding and enter dens, where their metabolism slows and is supported almost entirely by lipids released from adipose tissue. Urea is recycled and very little nitrogenous waste is excreted, so the protein in muscle, liver and other lean tissues is not depleted, as normally happens in prolonged fasting. Bone may be withdrawn from the skeleton during long periods of inactivity, but the tissue is restored to normal strength by rapid deposition of new bone in spring. Similar physiological processes occur in carnivorous polar bears when food is scarce but, except for breeding females, there is no regular, prolonged period of dormancy in a den.

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