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Warm-blooded are those animal species which can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The only known living homeotherms are birds and mammals, though ichthyosaurs, plesiosaurs and non-avian dinosaurs are believed to have been homeotherms. Other species have various degrees of thermoregulation. Animal body temperature control varies by species, so the terms "warm-blooded" and "cold-blooded" (though still in everyday use) suggest a false idea of there being only two categories of body temperature control, and are no longer used scientifically. In general, warm-bloodedness refers to three separate categories of thermoregulation. Endothermy is the ability of some creatures to control their body temperatures through internal means such as muscle shivering or increasing their metabolism (Greek: ἔνδον endon "within" θέρμη thermē "heat"). Some writers restrict the meaning of endothermy to mechanisms that directly raise the animal's metabolic rate to produce heat. The opposite of endothermy is ectothermy. Homeothermy maintains a stable internal body temperature regardless of external influence and temperatures.