HABITS
Snakes may be grouped, according to their mode of life, in five
principal categories, gradually merging into each other, or two of them
not infrequently found combined in one and the same species. These
categories are:--Ground-snakes, Sand-snakes, Burrowing-snakes,
Tree-snakes, and Water-snakes.
Ground-snakes may be defined as living above ground, and only
occasionally climbing bushes or entering the water. Among European
genera, _Coronella_ and _Vipera_ are perfect examples of this type,
whilst _Coluber_ and _Zamenis_ approach the Tree-snakes in often
ascending bushes, or even trees.
Sand-snakes are adapted for living on loose sand, in which they seek
concealment. Such are _Lytorhynchus_ and some _Psammophis_ among the
Colubridæ, _Cerastes_ among the Viperidæ. _Eryx_ connects this category
with the next.
Burrowing-snakes live chiefly underground, and often have the visual
organ atrophied in consequence, as in _Typhlops_; all the Typhlopidæ,
Glauconiidæ, and Uropeltidæ, belong to this category; the Viperid
_Atractaspis_ is also a burrowing type.
Tree-snakes spend the greater part of their life on bushes or trees.
_Corallus_ among the Boidæ, _Dendrophis_ and _Dendraspis_ among the
Colubridæ, _Atheris_ and various species of _Lachesis_ among the
Viperidæ, may be quoted as examples.
Of Water-snakes, some are exclusively aquatic, like the marine
Hydrophiinæ and the typical Acrochordinæ (_Acrochordus_, _Chersydrus_)
and Homalopsinæ (_Hipistes_, _Herpeton_). _Chersydrus_ and _Hipistes_
occur in the sea as well as in fresh water. Many species of
_Tropidonotus_ (_T. tessellatus_ and _T. viperinus_ in Europe), as well
as the genera _Helicops_, _Grayia_, _Boulengerina_, etc., among the
Colubridæ, _Eunectes_ among the Boidæ, _Ancistrodon piscivorus_ among
the Viperidæ, are chiefly but not exclusively aquatic.
Our _Tropidonotus natrix_ stands between the Ground-snakes and the
Water-snakes; Boas and Pythons are as much Water-snakes as Tree-snakes.
As shown by these and many other examples which might be given, a
division into categories cannot always be applied with precision, nor
does it convey an expression of the natural relationships of the
species, as was believed by many systematists of the last century, who
appealed to such adaptations for the definition of families.
A vertical pupil denotes more or less nocturnal habits. Nevertheless our
European Vipers, which are provided with such a contractile pupil, are
far from exclusively nocturnal, delighting to bask in the sun, and
pairing and feeding in the day-time. The Boidæ appear to be more
nocturnal, but no snake is known to be absolutely so, and the two
species of _Coluber_ which have been found living in perfect darkness in
limestone caves in the Malay Peninsula and China, where they feed
chiefly on bats, occur also outside the caves, and probably never breed
in them.
It is often stated in books that the organs of locomotion for the
exceedingly elongate body of snakes are the ribs, and these creatures
have even been compared to Centipedes. This statement is no doubt true
to a certain extent for slow locomotion on uneven ground, when the ribs
and the corresponding ventral shields afford a point of support; but it
does not account for the rapid movements, as when a snake darts like an
arrow in pursuit of its prey or to escape from an enemy. Besides, the
winding motions are not different from those of a Slow-worm or
Glass-snake, in which, encased as they are in a bony armour, the ribs
cannot come into play at all. The action of the muscles alone is quite
sufficient to account for the reptation of snakes, without the ribs
having to play an essential part.
Not only the Cobras, but several harmless snakes, are able to raise the
anterior third of the body vertically, when taking up a threatening
attitude in the presence of an enemy, at the same time widening or
inflating the region behind the head.
Most snakes can climb, and in this case the ribs and ventral shields are
of great assistance. The Tree-snakes, usually characterized by a very
slender, sometimes compressed, body, or by a prehensile tail, are
specially adapted for twining themselves round branches, and in several
of them the presence of a keel on each side of the ventral and subcaudal
shields, accompanied by a notch corresponding to the keel, affords an
additional help for climbing on vertical uneven surfaces, such as the
trunks of trees. This condition of the ventral shields has a bearing on
the extraordinary mode of locomotion with which some Tree-snakes
(_Chrysopelea_, and probably also _Dendrophis_) have long been credited
by the Malays. We allude to the so-called Flying-snakes, remarkable for
their habit of shooting down from trees and descending to the ground at
an oblique angle, the body being kept rigid the whole time of the
"flight." It has been observed in _Chrysopelea_ that the ventral surface
between the lateral keels, which may be compared to hinges, can be drawn
in and become deeply concave, whilst at the same time a slight
dorso-ventral flattening of the body takes place. During this muscular
contraction the snake is like a piece of bamboo bisected longitudinally,
and is buoyed up in such a way as to explain its parachute-like descent.
All snakes are able to swim, and the more aquatic kinds may spend a few
hours under the water. A _Python molurus_ is known to have remained
alive in a basket sunk for thirty-six hours in a river. The best adapted
for aquatic life are the Hydrophiinæ, or Sea-snakes, most of which never
leave the water, and are quite helpless and soon die when brought on
shore; their body is more or less compressed posteriorly, and the tail
oar-shaped. Sea-weeds and barnacles sometimes settle on them. Algæ have
also been observed growing on the fresh-water snake _Herpeton
tentaculatum_.
As regards food, Burrowing-snakes, as well as a few small Ground-snakes,
subsist mostly on worms, insects, and myriopods; Tree-snakes on lizards,
frogs, birds and their eggs; Water-snakes on fishes and batrachians.
Among the other types, some show a predilection for mammals, others for
lizards or snakes, whilst not a few feed indiscriminately upon mammals,
birds, reptiles and batrachians, even on slugs, insects, and worms, in
addition. However surprising, it is a fact that spiny mammals are
occasionally eaten, spines of the Madagascar Hedgehog (_Ericulus_)
having been found in the excrements of a _Boa madagascariensis_. Even
hard-shelled eggs and molluscs may constitute the principal or exclusive
food of certain snakes.
Thus, _Dasypeltis_ eats nothing but birds' eggs, the shells of which are
crushed in the gullet, by a special contrivance mentioned above (p. 80),
and are soon after rejected through the mouth as a pellet. Other snakes,
such as _Coluber_ and _Lioheterodon_ show themselves partial to eggs in
addition to live prey, but their alimentary canal does not depart from
the normal, the eggs being broken in the stomach and the remains of the
shells passed with the excrements.
The Amblycephalidæ subsist almost entirely on snails and slugs, the
shells of the former being crushed in the anterior part of the intestine
after their contents have been digested, and the débris are rejected
through the vent. A small land tortoise has been found in the stomach of
a Cobra (_Naia haie_) from Algeria.
Snakes which take large prey secure it according to three methods: By
catching it simply with the jaws, and immediately proceeding to swallow
it, as in _Tropidonotus_ and in some of the Constrictors when dealing
with small animals; by constriction, after having seized it with the
jaws, crushing it in the coils of their body and thus killing it
previous to feeding, as in the Boidæ and _Coluber_; or by poisoning, by
a mere stroke with the fangs, the result being awaited before the meal
is begun, as in most of the Viperidæ. Other poisonous snakes proceed
according to the first method, the use of the venom being to reduce the
struggles of the victim and to relax its muscles. Such snakes as are in
the habit of previously killing their prey show little reluctance to
accept dead food in confinement, a thing which others usually refuse to
do; they may, however, be deceived by the dead animal being agitated
before them, and the system now adopted in our Zoological Gardens, of
offering all snakes previously-killed animals, has been attended with
comparative success.
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 oesophagus, 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
oesophagus. 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.
Another curious behaviour is that of feigning death, as observed in a
harmless but vicious-looking snake, _Heterodon_, often called Puff-adder
in America. It looks more like a Viper than a harmless snake, and when
disturbed hisses loudly and flattens out the anterior part of the body,
much as does a Cobra, and pretends to strike, although it is one of the
few snakes that never bite man. If, however, this display proves of no
avail in frightening away the intruder, the snake rolls on its back and
opens its mouth, and then lies for a time, which may exceed a quarter of
an hour, absolutely motionless, as if dead. As soon as it thinks the
danger over, it awakens from its spasm and rapidly moves off. It is the
opinion of those who have most experience of this snake that this
extraordinary behaviour is not to be explained as a convulsion or faint
due to fright, but constitutes a deliberate trick to save its life.
Individuals of the South African Ringhals (_Sepedon hæmachates_) and of
the Common Grass-snake have also been observed to feign death.
The notion that snakes fascinate their prey, attracting it or reducing
it to immobility by a mysterious power in their glittering eyes, is pure
fable. Animals placed in a cage with a snake evince no particular
fright, and fly away when pursued, if not actually turning round to
defend themselves. It is even dangerous to offer a good-sized snake a
wild rat for food, as all keepers of menageries know.
In cold and temperate climates snakes hibernate, lying more or less
torpid in holes or hollow trees, sometimes assembled in numbers and
coiled together in a mass. The first thing they do in awakening in the
spring is to cast the outer coating of the epidermis, as described above
(p. 20). Several exuviations take place during the period of activity,
sometimes pretty regularly every month, sometimes at very irregular
intervals. A few days previous to this operation the snake is languid
and abstains from feeding; its skin is dull and the sight impaired by
the opaque condition of the lid; a day or two before moulting, the outer
stratum of the epidermis becomes again transparent and the eye clear,
through this stratum becoming detached from the subjacent tissue, until
it is pulled off in one piece, by the snake rubbing itself against
stones or bushes. The first exuviation takes place very shortly after
birth.
Snakes are long-lived, although the limit of duration of life is not
known in any of them. They grow slowly, and do not appear ever to reach
sexual maturity until the fourth year, when they continue increasing in
size for a long period. A _Python reticulatus_ and an _Ancistrodon
piscivorus_ are reported to have lived twenty-one years in captivity in
Paris. The young of many snakes are very secretive, and are not often
found in the open, those that are met with being as a rule either
new-born or approaching sexual maturity.
Snakes are tenacious of life, and remarkable for the reflex movements
which take place after they have been cut to pieces, the severed parts
of the body and tail wriggling for a considerable time, and the head
endeavouring to bite. Accounts of decapitated Rattlesnakes turning round
and striking with their bloody stumps are probably not snake stories.
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