Rabbits use their ears as air conditioners in warm weather. Since they cannot sweat like people do, they have to lose heat somehow and they do it a bit through panting, but mostly through their ears. There are large blood vessels in the ears which dilate when the rabbit is warm, either because the outside temperature is warm (resulting in heavy breathing and a rise in body temperature) or if the animal is overheated because of exercise or disease. The air moving across the ear cools the blood as it passes through the blood vessels, thus returning cooler blood to the body to cool the rabbit. Air temperature, even when it is in the 80s, is cooler than the rabbit's normal body temperature of 102 degrees, which may rise a degree or two when the pet is overheated.

This is why rabbits from hot climates, such as the Jack rabbit, have such large ears, so they can lose a lot of heat; and rabbits from cooler climates, such as the Arctic hare, have tiny ears because they don't want to lose precious body heat.

To cool down a rabbit who may be overheated, wet the ears or hold an ice pack wrapped in a wet washcloth over the ears. This cools the blood as it circulates through the ears. Unless your rabbit is having any difficulties such as lethargy, panting or difficulty in breathing, just hot ears don't need to be treated in any special way.