An elephant uses its ears to help maintain body temperature. When it gets too hot, blood flow to the ears increase and the elephant sticks them out away from its body, exposing more surface area to the air. The blood coursing through the thin membranes of the ears effectively transfers heat to the air returning a bit cooler into the rest of the body. The elephant can also flap its ears to increase this heat exchange or pull its ears tight to decrease it. Lots of animals have similar regulatory mechanisms – birds spread their wings and flap them slightly to increase heat dissipation, lizards raise spines on their backs exposing a membrane to the air, etc. Stegosaurus plates may have served a similar function.
While that was the leading theory at one time, it has been seriously questioned. Nowadays the leading theory is that they were for display, either to attract mates, intimidate rivals, or deter predators.
Regardless, the thermoregulation theory goes that the plates would have been full of blood vessels, and the greatly increased surface area would have acted kind of like a heat sink, cooling the animal. Alternatively, the animal could have used the plates to catch more sunlight and thus warm its blood quicker.
One knock against this theory is that most related stegosaurs like Kentrosaurus had long pointy spikes, not big flat plates, which would not have been nearly as effective for thermoregulation.
There are animals today that do something similar: elephants. Their ears are very thin and full of blood vessels which, combined with flapping of the ears, gives a large surface area to dissipate excess heat.
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