What structure forms the warning rattle at the end of a rattlesnake’s tail?

A rattlesnake’s warning rattle is formed by interlocking keratin segments.

Keratin is the tough structural protein also found in human fingernails and hair. Each time a rattlesnake sheds its skin, a new segment is added at the base of the rattle. The segments remain loosely connected, so rapid tail movement makes them vibrate against one another and produce the familiar buzzing sound.

The rattle is not made of loose stones, and it is not an extension of the snake’s bones. Segments can break off, so the number of segments cannot reliably reveal a snake’s age. Young rattlesnakes already possess a small button at the tail tip and can produce sound after their first shed.

Rattlesnakes use the sound mainly as a warning, helping them avoid an unnecessary confrontation.