Unlike fish, reptiles and other cold-blooded animals, mammals can maintain their body temperature over a wide range of conditions. This enables us to live in a great variety of habitats - fromthe frigid arctic to the scorching Sahara.
Compare what happens to the body temperatures of the desert lizard and the fennec fox as the sun rises and sets.
The lizard's body temperature rises and falls with that of the outside air. After a cold night, it must bask in the sun until its body is warm enough to function.
The fox maintains it's body temperature within a few degrees even when air temperature changes. So it can be active both during the day and night.
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