Hyperthermia means an increase in body temperature above normal range, and an animal is hyperthermic once it's temperature reaches a critical point. The "normal" body temperature range may vary depending upon the method of measurement used. The most widely used method is to take the temperature using a mercury thermometer inserted into the animal's rectum although other orifices can be used (e.g. the vagina). Recently modern technological developments have lead to the use of monitors which are inserted into the external ear canal.