An ear tag (1) suitable for monitoring an animal comprises an ear plug (10). The ear plug (10) comprises a base (11) and a shaft (12) extending upwards from the base (11). A piercing pin (31) is mounted at the end of the shaft (12) and dimensioned to pierce the ear of the animal such that the shaft (12) becomes surrounded by ear tissue (460). The ear plug (10) further comprises a first light pipe (17； 413) designed to guide light from a light source (421) near a first base opening (13) towards ear tissue (460) of the animal near a first shaft opening (15). The ear plug (10) also comprises a second light pipe (18； 414) designed to guide reflected light from ear tissue (460) of the animal near a second shaft opening (16) to a light detector (422) near a second base opening (14).
