An animal that cannot generate its own body heat and must rely on external sources like the sun to regulate its temperature, such as reptiles, fish, and insects.
From Greek 'ektos' (outside) and 'therme' (heat), literally 'outside heat.' Coined in the 20th century to replace the older term 'cold-blooded.'
Snakes, lizards, and frogs are ectotherms, which means they're not lazy or sluggish—they're actually brilliantly adapted to their environments and can be incredibly active; they just depend on the sun like solar-powered robots!
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