What is the purpose of the rattle on a rattlesnake?

Answer 1

The rattle is used to warn predators not to come to close, it's a defense mechanism saying "I'm here and I'm aware of your presence, don't come any closer"

Surprisingly, scientists have noticed that the rattle is becoming smaller or disappearing completely in new generations of rattlesnakes in Arizona. (I don't know about elsewhere, I live in AZ) They theorize that the rattle is no longer working as a defense mechanism because people ignore it and approach the snake to kill it or remove it regardless of whether it is rattling or not. Perhaps it would be safer for the snake to not warn people it was there, thus surviving by going unnoticed. Well I didn't know that about Arizona but I can assure you they are not getting any smaller in Colorado. I found one the other day with 10 links on it. Anyways I would probably agree with the first comment. The rattle just lets you know "Your in my territory and I will kill you if I want."

Answer 2

It is primarily a warning device. It helped save many a rattler from being trampled by a large hoofed animal. It is used to advise a human or other animal to stop and avoid a confrontation that could turn out badly for both.