Warm-blooded is what we call mammals and birds, but really what they have is the ability to keep their temperature constant, regardless of the outside temperature. This is a good idea, because the chemical reactions in a cell can then all be optimized to work best at a particular temperature. 'Cold-blooded' animals are not necessarily cold. A lizard has warmer blood than a mammal if both happen to be out in the midday sun in the Sahara desert. A lizard has colder blood than a mammal if they are out in the snow. The mammal has the same temperature all the time, and it has to work hard to keep it constant, using internal mechanisms. Lizards use external means to regulate their temperature, moving into the sun when they need to warm themselves up, and into the shade when they need to cool down. Mammals regulate their body temperature more accurately, and dolphins are no exception. Once again, their mammal history is written all over them, even though they have reverted to life in the sea, where most creatures don't maintain a constant temperature.
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