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Why is daily hibernation more practical for small animals than for bears?

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The image shows a group of Virginia big-eared bats sleeping upside down in a dark cavern. — Dormancy Virginia big-eared bats (*Corynorhinus townsendii virginianushibernating*) are true hibernators, entering a state close to death for much of the winter.

Daily hibernation, or circadian torpor, is more practical for small animals than for large ones primarily due to the metabolic costs associated with warming the body.12

For a small animal like a hummingbird, the energy required to raise its body temperature from a torpid state to an active one is relatively small—only a fraction of its total daily energy expenditure. In contrast, a large animal like a bear would require a massive amount of energy to warm its large body mass. The metabolic cost of such a daily transition for a bear would be equivalent to its entire 24-hour energy budget, making the process metabolically extravagant and counterproductive to the goal of energy conservation.12

Furthermore, larger animals take significantly longer to enter and emerge from hibernation because heat must radiate from the body to lower its temperature. Even if a bear had enough time in a 24-hour period to complete these cycles, the energy expenditure would defeat the purpose of dormancy. Consequently, while small animals can efficiently use daily torpor to save energy, larger animals like bears rely on different strategies, such as winter lethargy, which does not involve the deep, energy-intensive temperature fluctuations of true hibernation.13