Abstract: An animal number ear tag is provided that consists of an improved number tag element that is fastened to a lock pin and is suspended from the ear of an animal. The entire surface area of the lock pin is plated or coated with a layer of temperature-sensitive ink and when the set temperature of the aforesaid temperature-sensitive ink is exceeded, an instantaneous color difference is manifested. The lock pin is in contact with the skin of the animal's ear, therefore, the lock pin not only provides for the ensured positioning of the number tag element, but also indicates whether the animal develops a fever during the growth process. Since such a phenomenon is immediately observed, treatment can be applied earlier, thereby effectively providing for the reduction of unnecessary losses and degree of danger to the animal.