The distinctive clicking sound made by reindeer as they move is commonly called a knee click.
The sound is produced by anatomical structures in the feet and lower legs, including tendons and specialized areas near the foot joints. It can be audible when reindeer walk or run, especially in a herd. Researchers have suggested that the clicks may help animals maintain contact in crowded or low-visibility conditions.
The name is informal and does not mean that the kneecap itself is striking like a drum. Reindeer anatomy and the exact source of the sound have been discussed in studies of cervid locomotion and communication.
Reindeer are also called caribou in North America, but “reindeer” is the usual name for domesticated or Eurasian populations. Their sounds include grunts, bellows, and clicks, so the knee click is one part of a broader communication system.