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Introducing mammals

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This is a simplified graph of metabolic rate (vertical axis) against ambient temperature (horizontal axis) in contrasting types of mammal: a temperate mammal (a weasel), an arctic mammal (a polar bear cub), and a tropical mammal (a sloth). Each plot shows the range of body temperature over which the metabolic rate does not change and the point at which metabolic rate starts to increase with decreasing temperature. For the sloth, the metabolic rate is constant from 38 degrees to 30 degrees and then starts to rise. For the weasel, the figures are 38 to 18 degrees and for the polar bear, 38 to 10 degrees.

 6.3.2 Endotherms compared with an ectotherm under natural conditions