The Expression of Emotion in Man and Animals - Charles Darwin (an ebook reader TXT) 📗

to their other enemies, if not of overwhelming power, will on an average have left more offspring to inherit their characteristic qualities, whatever these may be and however first acquired, than have other males.

 

[20] `Lehrbuch der Histologie,’ 1857, s. 82.

 

The inflation of the body, and other means of exciting fear in an enemy.—Certain Amphibians and Reptiles, which either have no spines to erect, or no muscles by which they can be erected, enlarge themselves when alarmed or angry by inhaling air.

This is well known to be the case with toads and frogs.

The latter animal is made, in AEsop’s fable of the `Ox and the Frog,’

to blow itself up from vanity and envy until it burst.

This action must have been observed during the most ancient times, as, according to Mr. Hensleigh Wedgwood,[21] the word toad expresses in all the languages of Europe the habit of swelling. It has been observed with some of the exotic species in the Zoological Gardens; and Dr. Gunther believes that it is general throughout the group.

Judging from analogy, the primary purpose probably was to make the body appear as large and frightful as possible to an enemy; but another, and perhaps more important secondary advantage is thus gained.

When frogs are seized by snakes, which are their chief enemies, they enlarge themselves wonderfully; so that if the snake be of small size, as Dr. Gunther informs me, it cannot swallow the frog, which thus escapes being devoured.

 

[21] `Dictionary of English Etymology,’ p. 403.

 

Chameleons and some other lizards inflate themselves when angry.

Thus a species inhabiting Oregon, the Tapaya Douglasii, is slow in its movements and does not bite, but has a ferocious aspect; “when irritated it springs in a most threatening manner at anything pointed at it, at the same time opening its mouth wide and hissing audibly, after which it inflates its body, and shows other marks of anger.”[22]

 

Several kinds of snakes likewise inflate themselves when irritated.

The puff-adder (_Clotho arietans_) is remarkable in this respect; but I believe, after carefully watching these animals, that they do not act thus for the sake of increasing their apparent bulk, but simply for inhaling a large supply of air, so as to produce their surprisingly loud, harsh, and prolonged hissing sound.

The Cobras-de-capello, when irritated, enlarge themselves a little, and hiss moderately; but, at the same time they lift their heads aloft, and dilate by means of their elongated anterior ribs, the skin on each side of the neck into a large flat disk,—the so-called hood.

With their widely opened mouths, they then assume a terrific aspect.

The benefit thus derived ought to be considerable, in order to compensate for the somewhat lessened rapidity (though this is still great) with which, when dilated, they can strike at their enemies or prey; on the same principle that a broad, thin piece of wood cannot be moved through the air so quickly as a small round stick.

An innocuous snake, the Trovidonotus macrophthalmus, an inhabitant of India, likewise dilates its neck when irritated; and consequently is often mistaken for its compatriot, the deadly Cobra.[23] This resemblance perhaps serves as some protection to the Tropidonotus.

 

[21] See the account of the habits of this animal by Dr, Cooper, as quoted in `Nature,’ April 27, 1871, p. 512.

 

[22] Dr. Gunther, `Reptiles of British India,’ p. 262.

 

Another innocuous species, the Dasypeltis of South Africa, blows itself out, distends its neck, hisses and darts at an intruder.[24] Many other snakes hiss under similar circumstances.

They also rapidly vibrate their protruded tongues; and this may aid in increasing their terrific appearance.

 

Snakes possess other means of producing sounds besides hissing.

Many years ago I observed in South America that a venomous Trigonocephalus, when disturbed, rapidly vibrated the end of its tail, which striking against the dry grass and twigs produced a rattling noise that could be distinctly heard at the distance of six feet.[25]

The deadly and fierce Echis carinata of India produces “a curious prolonged, almost hissing sound in a very different manner, namely by rubbing “the sides of the folds of its body against each other,” whilst the head remains in almost the same position.

The scales on the sides, and not on other parts of the body, are strongly keeled, with the keels toothed like a saw; and as the coiled-up animal rubs its sides together, these grate against each other.[26] Lastly, we have the well-known case of the Rattle-snake. He who has merely shaken the rattle of a dead snake, can form no just idea of the sound produced by the living animal.

Professor Shaler states that it is indistinguishable from that made by the male of a large Cicada (an Homopterous insect), which inhabits the same district.[27] In the Zoological Gardens, when the rattle-snakes and puff-adders were greatly excited at the same time, I was much struck at the similarity of the sound produced by them; and although that made by the rattle-snake is louder and shriller than the hissing of the puff-adder, yet when standing at some yards distance I could scarcely distinguish the two.

For whatever purpose the sound is produced by the one species, I can hardly doubt that it serves for the same purpose in the other species; and I conclude from the threatening gestures made at the same time by many snakes, that their hissing,—the rattling of the rattle-snake and of the tail of the Trigonocephalus,—the grating of the scales of the Echis,—and the dilatation of the hood of the Cobra,—

all subserve the same end, namely, to make them appear terrible to their enemies.[28]

 

[24] Mr. J. Mansel Weale, `Nature,’ April 27, 1871, p. 508.

 

[25] `Journal of Researches during the Voyage of the “Beagle,”

‘ 1845, p. 96. I have compared the rattling thus produced with that of the Rattle-snake.

 

[26] See the account by Dr. Anderson, Proc. Zool. Soc. 1871, p. 196.

 

[27] The `American Naturalist,’ Jan. 1872, p. 32. I regret that I cannot follow Prof. Shaler in believing that the rattle has been developed, by the aid of natural selection, for the sake of producing sounds which deceive and attract birds, so that they may serve as prey to the snake.

 

It seems at first a probable conclusion that venomous snakes, such as the foregoing, from being already so well defended by their poison-fangs, would never be attacked by any enemy; and consequently would have

 

{note [27] continued} I do not, however, wish to doubt that the sounds may occasionally subserve this end.

But the conclusion at which I have arrived, viz. that the rattling serves as a warning to would-be devourers, appears to me much more probable, as it connects together various classes of facts.

If this snake had acquired its rattle and the habit of rattling, for the sake of attracting prey, it does not seem probable that it would have invariably used its instrument when angered or disturbed.

Prof. Shaler takes nearly the same view as I do of the manner of development of the rattle; and I have always held this opinion since observing the Trigonocephalus in South America.

 

[28] From the accounts lately collected, and given in the `Journal of the Linnean Society,’ by Airs. Barber, on the habits of the snakes of South Africa; and from the accounts published by several writers, for instance by Lawson, of the rattle-snake in North America,—it does not seem improbable that the terrific appearance of snakes and the sounds produced by them, may likewise serve in procuring prey, by paralysing, or as it is sometimes called fascinating, the smaller animals.

no need to excite additional terror. But this is far from being the case, for they are largely preyed on in all quarters of the world by many animals.

It is well known that pigs are employed in the United States to clear districts infested with rattle-snakes, which they do most effectually.[29] In England the hedgehog attacks and devours the viper.

In India, as I hear from Dr. Jerdon, several kinds of hawks, and at least one mammal, the Herpestes, kill cobras and other venomous species;[30]

and so it is in South Africa. Therefore it is by no means improbable that any sounds or signs by which the venomous species could instantly make themselves recognized as dangerous, would be of more service to them than to the innocuous species which would not be able, if attacked, to inflict any real injury.

 

Having said thus much about snakes, I am tempted to add a few remarks on the means by which the rattle of the rattle-snake was probably developed.

Various animals, including some lizards, either curl or vibrate their tails when excited. This is the case with many kinds of snakes.[31]

In the Zoological Gardens, an innocuous species, the Coronella Sayi, vibrates its tail so rapidly that it becomes almost invisible.

The Trigonocephalus, before alluded to, has the same habit; and the extremity of its tail is a little enlarged, or ends in a bead.

In the Lachesis, which is so closely allied to the rattle-snake that it was placed by Linnaeus in the same genus, the tail ends in a single, large, lancet-shaped point or scale. With some snakes the skin, as Professor Shaler remarks, “is more imperfectly detached from the region about the tail than at other parts of the body.”

Now if we suppose that the end of the tail of some ancient American species was enlarged, and was covered by a single large scale, this could hardly have been cast off at the successive moults.

In this case it would have been permanently retained, and at each period of growth, as the snake grew larger, a new scale, larger than the last, would have been formed above it, and would likewise have been retained.

The foundation for the development of a rattle would thus have been laid; and it would have been habitually used, if the species, like so many others, vibrated its tail whenever it was irritated.

That the rattle has since been specially developed to serve as an efficient sound-producing instrument, there can hardly be a doubt; for even the vertebrae included within the extremity of the tail have been altered in shape and cohere. But there is no greater improbability in various structures, such as the rattle of the rattle-snake,—

the lateral scales of the Echis,—the neck with the included ribs of the Cobra,—and the whole body of the puff-adder,—having been modified for the sake of warning and frightening away their enemies, than in a bird, namely, the wonderful Secretary-hawk (_Gypogeranus_) having had its whole frame modified for the sake of killing snakes with impunity.

It is highly probable, judging from what we have before seen, that this bird would ruffle its feathers whenever it attacked a snake; and it is certain that the Herpestes, when it eagerly rushes to attack a snake, erects the hair all over its body, and especially that on its tail.[32] We have also seen that some porcupines, when angered or alarmed at the sight of a snake, rapidly vibrate their tails, thus producing a peculiar sound by the striking together of the hollow quills.

So that here both the attackers and the attacked endeavour to make themselves as dreadful as possible to each other; and both possess for this purpose specialised means, which, oddly enough, are nearly the same in some of these cases. Finally we can see that if, on the one hand, those individual snakes, which were best able to frighten away their enemies, escaped best from being devoured; and if, on the other hand, those individuals of the attacking enemy survived in larger numbers which were the best fitted for the dangerous task of killing and devouring venomous snakes;—

then in the one case as in