Some species feed almost exclusively on other snakes, and often manage to swallow individuals as large as, or even a little larger than, themselves. Examples are known of harmless snakes showing a predilection for dangerous species, to whose poison they are immune (see p. [71]).
As a rule snakes that eat fish will also eat batrachians, but nothing higher in the scale, although exceptions have been reported, such as the Anaconda feeding on mammals, birds, reptiles, and fish, and our Grass-snake having taken mice and birds. Some that feed chiefly on lizards and snakes will occasionally eat also mammals, and vice versa, but rarely frogs. On the other hand, European Vipers accommodate themselves to a more varied bill of fare, being known to feed on mammals, birds, reptiles, batrachians, insects, and slugs, and they have even been observed to eat voles showing signs of putrefaction.
The enormous prey which some snakes are able to swallow is quite astounding. Anacondas and Pythons, the largest snakes, have been known to swallow calves and good-sized antelopes with their horns, animals which, even after being somewhat crushed by constriction, very much exceed the calibre of the snake. A Python molurus 17 feet long is reported on good evidence to have swallowed a gravid Axis deer. A Grass-snake half an inch in diameter can manage a frog or toad three times that width, and a Dasypeltis of the same size a hen’s egg. Such feats are rendered possible by the mobility of the jaws and palato-pterygoid arch on the cranium, and the elasticity of the ligaments by which they are attached (see above, p. [42]), as well as by the mobility of the ribs and the absence of sternal apparatus, together with the great distensibility of the skin. When a snake proceeds to dispose of a large prey, which, if it be a mammal or bird, is usually seized head-first, it pulls itself forward by alternate movements of the jaws, the maxillary and the mandibular ramus of the one side, and then of the other, being extended anteriorly and laterally, the snake at the same time producing an abundant salivation which renders the prey very slimy. Several repeated alternate movements of the jaws bring the head of the prey to the gullet, where the muscles and ribs come into play, and the two sides of the jaws work no longer alternately, but together. When once in the œsophagus, the prey progresses with much greater facility, and usually reaches the stomach in a few minutes, whilst the previous process of deglutition may have lasted half an hour. While this laborious operation is going on, the breathing of the snake is not impaired owing to a remarkable contrivance: the trachea can be protruded in such a manner as to bring its opening outside the mouth.
In cases where the victim is eaten alive, the snake has to contend with its struggles, but retrogression is rendered impossible by the backwardly-directed sharp teeth with which the jaws and palate are beset. A frog is usually caught by one of the hind limbs and swallowed back-first, the long hind limbs stretching forwards as they fold against the body; its struggles are often still apparent when it has reached the œsophagus. Snakes when caught immediately after a meal are in the habit of disgorging their food, and it sometimes happens that a frog or toad is thus vomited alive. An instance is known of a naturalist having captured a Grass-snake and put it in a linen bag. On opening it a short time after, great was his surprise to find the snake had escaped through a small hole in the bag, leaving instead a living toad too big to pass through the hole.
If not of too large a size, several animals will often be swallowed in rapid succession, after which the gorged snake will allow its digestive organs several days, or even weeks, of repose. A large Anaconda in the Paris Jardin des Plantes fed only thirty-six times in the course of seven years. Digestion is usually rapid in the small snakes, defecation taking place twenty-four to forty-eight hours after the feeding; it lasts much longer in the large Boas and Pythons. Thus, in the above-mentioned Anaconda it has been observed to take from nine to thirty-eight days. Even the hardest bones of birds are decomposed by the gastric juices, but hairs, feathers, and horny productions, are passed with the excrements, sometimes forming regular balls. It is in most cases possible to tell, from an inspection of the dried fæces, what a snake has been feeding on, hairs, feathers, beaks, claws, epidermal horny shields, bits of tooth-enamel, being found mixed with the chalky matter which represents the decomposed bones. As a rule there is but one defecation after each meal, but there are in addition more frequent renal dejections, consisting chiefly of uric acid.
In captivity snakes show themselves capricious in the choice of food, one individual preferring mammals, whilst another, of the same species, will only take birds; and many, although to all appearances perfectly healthy, will persist in refusing all food, and allow themselves to die of starvation—a suicide which may require months, or even years, to accomplish. A Rattle-snake in the menagerie of the Jardin des Plantes in Paris has lived two years and two months without taking any food, a Python sebæ nearly two years and a half, a Boa madagascariensis four years and a month. A Viper aspis was kept for three years without food and without losing its vicious temper. Specimens thus fasting do not, as a rule, renew their epidermis, or do so but very rarely. Our Common Adder can very seldom be induced to feed in captivity. Other snakes may rid themselves of all shyness to the extent of taking food from the hand, or show such appetite as to seize a prey immediately on being released from the small box or bag in which they have travelled for a considerable time.
Most snakes drink, and pretty often—not by lapping with the tongue, but by drawing in water from the mouth and immersing the anterior part of the head. Some are said to be fond of milk, but there is no foundation for the belief held by peasants, that they enter sheds with the object of sucking milk from the cows, which would be a material impossibility; their real purpose in visiting such places being a search for suitable dung-heaps in which to deposit their eggs.
Snakes cannot be credited with much intelligence or educability, nor do they display any very marked instincts. The least stupid and most easily tamed are the species of the genera Coluber and Coronella. There is, however, considerable difference in this respect between individuals of the same species. Most snakes, when freshly caught, defend themselves by biting, and some individuals retain their savage temper after months of captivity; others hardly ever bite, even if molested. The Common Grass-snake, for instance, hisses loudly and takes up a very threatening attitude, or even pretends to snap with open mouth, but very seldom bites; its principal defensive action when caught consists in voiding a most repulsive secretion from its anal glands, which it evidently controls, as it ceases doing so when accustomed to being handled. The same snake also produces, during the spring, an oily exudation from the skin which has the same repulsive smell. Mr. H. N. Ridley has observed a Malay snake allied to Tropidonotus, Macropisthodon rhodomelas, to exude drops of a white viscid liquid from the skin of its neck, which is flattened out like that of a Cobra when in an attitude of defence, and he noticed that his dog, seizing the snake to worry it, foamed at the mouth as if he had been biting a toad.
The hissing is produced by the rapid expulsion of air from the lungs through the trachea and the notch at the end of the mouth, which is kept shut at the time. Snakes provided with an epiglottis (see p. [79]) produce a much louder hissing. Other sounds are produced by some snakes. Thus, the Indian and African Vipers of the genera Echis and Cerastes make a curious, prolonged, rustling noise, by rubbing the folds of the sides of the body against one another. This sound is produced by friction between the serrated keels of the lateral scales, which are disposed obliquely with their tips directed downwards and backwards; the noise can even be repeated after the death of the animal, by twisting the body and thus rubbing or rasping these little saws against one another. The same thing probably takes place in the African genus Dasypeltis, in which we find a similar arrangement of the scales, though to a less degree.
The best known sounding apparatus is that of the Rattlesnakes, described on p. [20]. When alarmed, these snakes gather the body in a few coils or roll themselves up in a spiral, with the tail erect in the centre, and vibrating with great rapidity, whilst the head is ready for attack. Other snakes, such as the Ancistrodon and some species of Coluber and Zamenis, when excited, vibrate the tail in the same manner; but, being deprived of the sound-producing apparatus, this expression of their anger does not attract the same attention. It is from such a habit, however, that the rattle must have been evolved and perfected, not necessarily in a Lamarckian sense, but through the different steps by which evolution or creation has proceeded; Natura non fecit saltus, as Linnæus well said. Many suggestions have been made as to the use of the rattle. One of them is that the rattling resembles the sound made by locusts, and serves to decoy insect-eating birds; another, that it serves to call the sexes together. Probably it is useful to the snake as a warning to keep off disturbers which cannot serve as food, and thus prevents useless expenditure of venom, or even the breaking of the fangs. At any rate, it gives expression to the snake’s excitement, as does the voice in the case of many other animals, and it seems reasonable to suppose that it may be applied to different purposes. With the advent of man, this means of attracting attention must tend to the more rapid extermination of the snakes which possess it.