A tool for applying an identification tag body and a tag retainer to an ear of an animal. The tool includes a main handle member on which a tag body is mounted, a swinging bracket pivotally mounted on the main handle member, a second handle member pivotally connected to the swinging bracket, a first sliding link member pivotally connected to the second handle member, and a second sliding link member pivotally connected to the second handle member. The tag retainer is mounted on the swinging bracket. As the swinging bracket swings, the tag retainer can pass through the animal's ear and enter a boss of the tag body for locking therein. Support structure on each of the sliding link members can engage support pins mounted on the main handle member so that linkage of the tool can advance in two stages as the handles are advanced toward each other. The support structure of the sliding link members can come free of the associated support pins, one at a time. Both support pins can engage support structure of the sliding link members when the handles are in an intermediate or transistion position. The swinging bracket moves in ear puncturing direction with increasing mechanical advantage as the handles are urged toward each other in the first stage with a maximum advantage as the transition position is reached and the tag retainer penetrates the animal ear. Further advance during the second stage following passing of the transition position causes advance with mechanical advantage increasing from a lower value than that reached in the first stage.