Because it has a series of cups appended to its tail; which cups, when the serpent moves its body, likewise move one upon another, and make a rattling noise, not unlike the folding of dried parchment. This noise is said to be audible at the distance of twenty yards, and is thus useful in giving warning of the approach of the destructive reptile, to which it is attached. Its bite is attended with frightful consequences, as in the following instance : - " An emigrant family inadvertently fixed their cabin on the shelving declivity of a ledge, that proved a den of rattle-snakes. Warmed by the fire on the hearth of the cabin, the terrible reptiles entered in numbers, and, of course, in rage, by night, into the room where the whole family slept. As happens in those cases, some slept on the floor, and some in beds. The reptiles spread in every part of the room, and mounted on every bed. Children were stung in the arms of their parents, and in each other's arms. Most of the family were bitten to death; and those who escaped, finding the whole cabin occupied by these horrid tenants, hissing and shaking their rattles, fled from the house by beating off the covering of the roof, and escaping in that direction." - Flint's Geography and History of the United States.
Dr. Mead supposes this rattle may serve to bring birds, etc, within the reach of the snakes, from the effect its sound produces. Major Gardner, who had lived long in East Florida, affirms, that the young Indians of that country, were accustomed to imitate the noise of the rattle-snake, for the purpose of taking squirrels, etc.
Blumenbach says, " we are assured by credible eyewitnesses, that squirrels, small birds, etc. fall from the trees on which they stand, into the throat of the rattlesnake below; the circumstance is not, however, by any means confined to this genus, as it has been remarked in many other serpents of both the Old and the New-World. Rattle-snakes are eaten by hogs and birds of prey. They may also be tamed, and rendered docile."
Because, as the cups of the rattle consist merely of dried matter, which, in the dry season, is brought into a condition to make a noise when the animal moves, so, in like manner, the rattle, in the wet season, is soft and mute.