RATTLESNAKE. Rattlesnakes are a small group of the sub-family of pit-vipers (Crotalinae, see Snakes; Viperidae), characterised by a tail which terminates in a chain of horny, loosely connected rings, the so-called “rattle.” The “pit” by which the family is distinguished from the ordinary vipers is a deep depression in the integument of the sides of the snout, between the nostrils and the eye; its physiological function is unknown. The rattle is a complicated and highly specialized organ, developed from the simple conical scale or epidermal spine, which in the majority of snakes forms the termination of the general integument of the tail. The bone by which the root of the rattle is supported consists of the last caudal vertebrae, from three to eight in number, which are enlarged, dilated, compressed and coalesced (fig. 1, a). This bone is covered with thick and vascular cutis, transversely divided by two constrictions into three portions, of which the proximal is larger than the median, and the median much larger than the distal. This cuticular portion constitutes the matrix of a horny epidermoid covering which closely fits the shape of the underlying soft part and is the beginning of the rattle, as it appears in young rattlesnakes before they have shed their skin for the first time. When the period of a renewal of the skin approaches a new covering of the extremity of the tail is formed below the old one, but the latter, instead of being cast off with the remainder of the epidermis, is retained by the posterior swelling of the end of the tail, forming now the first loose joint of the rattle. This process is repeated on succeeding moultings — the new joints being always larger than the old ones as long as the snake grows. Perfect rattles therefore taper towards the point, but generally the oldest (terminal) joints wear away in time and are lost. As rattlesnakes shed their skins more than once every year, the number of joints of the rattle does not indicate the age of the animal but the number of exuviations which it has undergone. The largest rattle in the British Museum has twenty-one joints. The rattle consists thus of a variable number of dry, hard, horny cup-shaped joints, each of which loosely grasps a portion of the preceding, and all of which are capable of being shaken against each other. If the interspaces between the joints are filled with water, as often happens in wet weather, no noise can be produced. The motor power lies in the lateral muscles of the tail, by which a vibratory motion is communicated to the rattle, the noise produced being similar to that of a child's rattle and perceptible at a distance of from 10 to 20 yds.
Fig. 1. — Rattle of Rattlesnake (after Czermak).
1. Caudal vertebrae, the last coalesced in a single bone a. 2. End of tail (rattle removed); a. cuticular matrix covering terminal bone. 3. Side view of a rattle; c and d the oldest, a and b the youngest joints. 4. A rattle with joints disconnected; x fits into b and is covered by it; z into d in like manner.
The habit of agitating the tail is not peculiar to the rattlesnake, but has been observed in other venomous and innocuous snakes with the ordinary tail, under the influence of fear or anger. It is significant that the tip of such snakes is sometimes rather conspicuously coloured and covered with peculiarly modified scales, notably in Acanthophis. The use of such a tail probably consists in attracting or fixing the attention of small animals, by slightly raising and vibrating the tip. The rattle no doubt acts as a warning, every snake preferring being left alone to being forced to bite. Many a man has been warned in time by the shrill sound, and this principle applies undoubtedly to other mammals. Moreover, rattlesnakes are rather sluggish, and comparatively not vicious. First they try to slink away; when overtaken or cornered they use every means of frightening the foe by swelling up, puffing, rattling and threatening attitudes; it is as a rule not until they are touched, or provoked by a rapid movement, that they retaliate, but then they strike with fury. They are viviparous, and as destroyers of rats, mice and other small rodents they are useful. The surest way of clearing a ground of them and any other snakes is to drive in pigs, which are sure to find and to eat them, without harm to themselves. They inhabit localities to which the sun has free access, prairies, rough stony ground, &c. Specimens of 5 ft. in length are not rare. Formerly common in the eastern parts of the United States, and still so in thinly inhabited districts, rattlesnakes, like the vipers of Europe, have gradually succumbed to the persecution of man.
Fig. 2.—Rattlesnake (Crotalus adamanteus s. durissus).
Rattlesnakes are confined to the New World. North-American authors distinguish a great number of different kinds, S. W. Carman (“Reptiles and Batrachians of North America,” Harvard Mus. Zool. Mem., 1883, 410) enumerating twelve species and thirteen additional varieties. E. D. Cope has split them into twenty; but all these species or varieties fall into two groups. One, Sistrurus, has the upper side of the head covered with the ordinary nine shields; only three species, of comparatively small size, in North America (Sistrurus miliarius from Florida to Sonora; S. catenatus in many of the middle states of the Union, and elsewhere, as far north as Michigan; S. ravus in Mexico).
The second group forms the genus Crotalus, in which the shields between and behind the eyes are broken up and replaced by small scales. This genus ranges throughout the United States through Central and South America into Patagonia, but is not represented on any of the West Indian islands. C. horridus, with the tail uniformly black, from Maine to Kansas and Louisiana to Florida. C. adamanteus, tail light, with black crossbands, body with a handsome pattern of rhombs with lighter centres and yellowish edges; chiefly south-eastern states, to Arizona and Mexico; the largest of rattlers, giants of 8 ft. in length having been recorded. C. confluentus, tail with brown or indistinct bands; with a continuous series of large brown or reddish rhomboidal spots on the back; Texas to California. C. cerastes, with a pair of horns above the eyes; the “sidewinder” of Arizona and California to Nevada. C. terrificus, easily distinguished by the possession of three pairs of symmetrical shields on the top of the muzzle, ranging from Arizona into Argentina. It is the only kind of rattlesnake in Central and South America. C. triseriatus, a small species, with a feebly developed rattle, on Mexican mountains, on the pic of Orizaba up to 12,500 ft.