Ears

Rabbits are unable to sweat or pant effectively to dissipate body heat, and their main thermoregulatory mechanism is heat exchange by the ears, which have a large set of blood vessels

Can I determine the body temperature of my rabbit by their ears?
Due to rabbits using their ears as their main thermoregulatory device, ears are not a good or accurate method to determine a rabbit's internal body temperature. The blood vessels in the ears can contract or expand minute by minute. Their ears may be cool when they are trying to conserve heat, and their ears be warm when they are trying to dissipate heat. Normal rabbit body temperature is about 101-103&deg;F, which can feel warm to a human's touch even if a rabbit is relaxed and just passing normal blood flow through their ears.

Using a rectal thermometer is the proper method of reading a rabbit's body temperature.