41.13 The Mouse-to-Elephant Curve On a weight-specific basis, the metabolic rate of small endotherms is much greater than that of larger endotherms. Using the data in the table, the graph plots O2 consumption per kg of body weight (a measure of the metabolic rate) against a logarithmic plot of body weight.
The thermoneutral zone is bounded by a lower critical temperature and an upper critical temperature (Figure 41.14). When the environmental temperature falls below the lower critical temperature, endotherms must produce heat to compensate for the heat they lose to the environment. Mammals can accomplish this thermogenesis in two ways: through shivering and nonshivering heat production. Birds use only shivering heat production.
Shivering uses the contractile machinery of skeletal muscles to consume ATP without causing observable behavior. The muscles pull against each other so that little movement other than a tremor results. The energy from the conversion of ATP to ADP in this process is released as heat. Shivering heat production is perhaps too narrow a term, however; increased muscle tone and increased body movements also contribute to increased heat production in cold environments.
Most nonshivering heat production occurs in specialized adipose tissue called brown fat (Figure 41.15). This tissue looks brown because of its abundant mitochondria and rich blood supply. In brown fat cells, a protein called thermogenin uncouples proton movement from ATP production, allowing protons to leak across the inner mitochondrial membrane rather than having to pass through the ATP synthase protein
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