Periodically people ask why we do not use ear thermometers, and here is the answer. They do exist for cats and dogs. The first model that I bought was good and worked pretty well. Then the company came out with a new model that worked very poorly. I did not believe the low temperatures on the new model and was constantly doing the ear and a rectal to double check. Rarely was the ear model comparable to the rectal digital or glass; so we could not count on accuracy with the ear thermometer.
Cats and dogs have very long ear canals compared to people. The first part of the ear canal is vertical, then it become horizontal to the ear drum (tympanic membrane). The ear thermometer must hit the ear drum to read. It is more difficult to manipulate an ear thermometer down a vertical and horizontal canal; so error can be greater than people. An inaccurate body temperature is of no value in veterinary medicine as well as human medicine.
We use a digital rectal thermometer now. It is faster than a glass one and accurate.