According to Rubio, "The most widely accepted hypothesis for the evolution of the rattle is that the rattle is a warning device for predatory animals that might be a threat to the rattlesnake. It produces a signal to drive them away."
The rattle is composed of a series of hollow, interlocked segments made of keratin, which are created by modifying the scales that cover the tip of the tail. The contraction of special "shaker" muscles in the tail causes these segments to vibrate against one another, making the rattling noise (which is amplified because the segments are hollow). The muscles that cause the rattle to shake are some of the fastest known, firing 50 times per second on average, sustained for up to three hours.
At birth, a “prebutton” is present at the tip of the snake's tail; it is replaced by the “button” several days later when the first skin is shed. However, no sound can be made by the rattle until a second segment is added when the skin is shed again. A new rattle segment is added each time the snake sheds its skin, and the snake may shed its skin several times a year, depending on food supply and growth rate.
Rattlesnakes travel with their rattles held up to protect them from damage, but in spite of this precaution, their day-to-day activities in the wild still cause them to regularly break off end segments. Because of this, the age of a rattlesnake is not related to the number of rattles on its tail.
One of the differentiating features of males and females is the males have thicker and longer tails (because they contain the inverted hemipenes). Also, the tails of males taper gradually from the body, whereas the tails of females narrow abruptly at the vent.
Just a thought here, but, no rattlesnake is a perfect, they don't all have rattles that we love to hear as a warning shot to jump, flight or fight. Predominately, the rattlesnakes that end up being hunted down, do have rattles. Evolutionary ????? elimination of the 'fittest' rattlesnakes will be replaced by the rattlesnake without a rattle.
No warning, more deaths, no flight, no fight.
Of course not all of the rattlesnakes with rattles will ever be hunted down to extinction, but evolution is a funny thing.....