Popular Science Monthly/Volume 72/February 1908/The Instinct of Feigning Death
|THE INSTINCT OF FEIGNING DEATH|
UNIVERSITY OF WISCONSIN
THE so-called instinct of feigning death is one which is very widely distributed in the animal kingdom. It crops out sporadically, as it were, in forms which are but very distantly related, and hence it must have been independently evolved a great many times. The expression feigning death is a misleading one to the extent that it is apt to give rise to the idea that the animal consciously adopts this device with the intent to deceive. While it is probable, however, that among the higher animals which sometimes feign death there may be an attempt to mislead their enemies, it is quite certain that among the insects, spiders and other low forms there is no such aim in the creature's mind if we grant (what some naturalists are disposed to deny) that these animals have minds. The veteran French naturalist, Fabre, who has devoted the leisure periods of a long life to the enthusiastic study of the ways of insects, performed several experiments on beetles in order to ascertain if the duration of their feint was in any way affected by his own presence or movements. Most of Fabre's observations were made on a large scarab beetle. When handled the beetle would throw itself into an immobile state with its head bent down and its legs drawn in close to the body. It would remain in this attitude perfectly quiet for several minutes—sometimes for over an hour. Its awakening would be first manifested by a slight trembling of the feet and a slow oscillation of the antennæ and palps; then its legs would move about more vigorously, and finally the insect would arise and scamper off. Seized again, it would repeat the performance several times in succession, the duration of the feint often increasing with successive trials. Finally, as if wearied, or convinced that the ruse were vain, the beetle would refuse to feign longer.
Were the feints attempts to deceive its captor by simulating death? Fabre placed the insect on its back, went to a distant part of the room and remained perfectly quiet. The beetle still lay as usual. He then went out of the room, carefully looking in at intervals to watch the course of events. Still the same immobility. In other cases he covered the insect so that it could not see out and then quietly went away. This was also found to make no difference. In fact, whether the insects were surrounded by sounds and sights of moving objects or entirely excluded from these influences made no difference in the average length of time they would remain in a motionless condition. Similar experiments have been made on other insects by different observers, who have all arrived at the conclusion that conscious deception plays no part in the process.
The attitudes assumed by insects and other forms when feigning death are usually quite different from those of dead specimens. This general fact was pointed out by Darwin, who says that "I carefully noted the simulated positions of seventeen different kinds of insects (including an Iulus, spider and Oniscus) belonging to distinct genera, both poor Fig. 1. Larva of a Geometrid Moth attached to a Twig. and first-rate shammers; afterward I procured naturally dead specimens of some of these insects, others I killed with camphor by an easy slow death; the result was that in no instance was the attitude exactly the same, and in several instances the attitudes of the feigners and of the really dead were as unlike as they possibly could be."
The attitudes of animals in the death feint are frequently very characteristic. Many beetles as well as other forms feign with the legs drawn up to the body and the antennæ closely appressed, so that the whole insect assumes as compact a form as possible. The woodlouse, Armadillo, rolls itself up into a ball with its legs drawn into the center, a habit which lias doubtless caused the name pill-bug to be given to this crustacean. A beetle, Geotrupes, according to Kirby and Spence, "when touched or in fear sets out its legs as stiff as if they were made of iron wire—which is their posture when dead—and remaining motionless thus deceives the rooks which prey upon them. A different attitude is assumed by one of the tree-chafers probably with the same end in view. It sometimes elevates its posterior legs into the air, so as to form a straight vertical line, at right angles with the upper surface of its body." Spiders usually feign by folding up their legs, dropping down and remaining motionless. The caterpillars of some of the geometrid moths have the curious habit of attaching themselves to a branch by their posterior legs and holding the body straight and stiff at an angle to the stem, thus forming a remarkably close resemblance to a short twig. Frequently the deceptiveness is increased by a marked similarity in color to that of the branch to which they are attached.
While in most cases a species has a particular attitude which it maintains when simulating death, there are some forms which feign in whatever posture they may be in when disturbed. A good example of this is afforded by the water-scorpion, Ranatra. This insect has the two hinder pairs of legs, which are employed in walking and swimming, very long and slender; the first pair are fitted for grasping the small aquatic animals on which it feeds and are carried straight out in front of the body. It is only necessary to pick one of these insects out of the water to throw it into a stiff, immobile condition which usually lasts several
minutes and sometimes for over an hour. The legs may be closely pressed to the body so that the creature resembles a stick, or they may stand out at right angles to it, or be bent in any position, some in one way and some in another, depending upon how they happen to lie when the feint began. And no matter how awkward the position, it is rigidly maintained until the feint wears off. I have found that young Ranatras, the first day they emerged from the egg and while their appendages were still soft and easily bent, showed the same death-feigning instinct as the adults, although they did not persist in it for so long a time. It is a curious fact that the mature insects can not by any sort of manipulation be caused to feign death while underwater; but as soon as it is in the air it can be caused to feign repeatedly; sometimes a slight touch is all that is necessary to throw it into a rigid state of an hour's duration.
Death feigning does not seem to occur among the lower invertebrate animals such as the Protozoa, Cœlenterates, Molluscs and worms, although some of them may exhibit reactions which are prophetic of this instinct. Among Crustaceans the instinct in its fully developed form is quite uncommon. Some years ago I described the death-feigning of certain species of terrestrial amphipod crustaceans which are frequently found on sandy beaches near the seashore. On account of their peculiar hopping movements these crustaceans are commonly known as sand-hoppers or sand-fleas, although they have of course no relation to the fleas of human experience. One of the largest species of sand-hopper, Talorchestia, is common along our Atlantic coast, where it lives during the day in burrows made in the sand, coming out only at night to feed upon the seaweed and other material washed ashore by the waves. When the Talorchestias are dug out of their burrows,
they usually lie curled up with their long antennae bent under the body and their legs drawn up so as to assume a compact form. They will lie in this way for several minutes, when they may be seen slowly to relax; the legs then move about, and soon the creature hops away by a sudden extension of its abdomen. When caught in the hand they will feign death again, and repeat the performance many times in succession. Other species of sand-hoppers exhibit the same instinct, though less perfectly, and there are traces of it in many of the reactions of their aquatic relatives.
The various species of wood lice exhibit the instinct of feigning death in various degrees. Some species are able to roll up into an almost perfect ball and will remain in that state for a considerable time. Other species curl up, but make only a very imperfect approximation to a sphere, and they may not maintain this attitude but for a short period. Some myriapods when disturbed curl up in much the same way. Among spiders death-feigning is not uncommon, especially among the orb weavers.
It is among the insects that the death-feigning instinct reaches its fullest development, occurring to a greater or less extent in most of the orders. It is especially common in beetles and not unusual among the bugs, but it is quite rare in the highest orders such as the Diptera or flies, and the Hymenoptera, or the ants, bees and their allies. It occurs in a few cases among butterflies and moths, both in the imago as well as the larval state. The instinct is exhibited in different species in all stages of development from a momentary feint to a condition of intense rigor lasting for over an hour. Some insects may be severely mutilated, or, according to De Geer, even roasted over a fire before they will cease feigning.
Among the vertebrate animals death-feigning has been observed only rarely in the fishes. In the Amphibia it is not exhibited in the striking way it occurs in insects and spiders, although frogs and toads may be thrown by the proper manipulation into an immobile condition more or less resembling it. A phenomenon apparently related to the death feigning of insects has long been known in certain reptiles. Darwin in his "Journal of Researches" describes a South American lizard which when frightened "attempts to avoid discovery by feigning death with outstretched legs, depressed body, and closed eyes; if further molested it buries itself with great quickness in the loose sand." The Egyptian snake charmers by a slight pressure in the neck region are able to make the asp suddenly motionless so that it remains entirely passive in the hands of the operator. And similar phenomena have been found in other species.
In birds the instinct crops out only here and there. A few summers ago when on the island of Penikese I was somewhat surprised to find the instinct well developed in the young terns which were hatched out in abundance on the hillsides. For a short time after being hatched the little downy fellows betray no fear of man and will cuddle under one's hand in perfect confidence. When the birds become larger and acquire their second coat of feathers the instinct of fear takes possession of them and they run and hide in the grass when you approach. Here they lie perfectly quiet; you may pull them about, stretch out their legs, necks, or wings and place them in the most awkward positions, and they will remain as limp and motionless as if really dead. They will even suffer their wing or tail feathers to be plucked out one by one without a wince. But all of a sudden the bird becomes a very different creature. It screams, pecks and struggles to escape. I have made several attempts to make a bird feign death a second time, but never met with success. According to Couch the land rail and skylark feign death, and Wrangle states that the wild geese of Siberia have the same habit during their molting season, when they are unable to fly. Hudson states in his most interesting "Naturalist on the La Plata" that the common partridge of the pampas, when captured, "after a few violent struggles to escape drops its head, gasps two or three times, and to all appearances dies. If, when you have seen this, you release your hold, the eyes open instantly, and with startling suddenness and noise of wings, it is up and away and beyond your reach forever."
In mammals the instinct is so well shown in one of the lower members of the group, the opossum, that the expression "playing possum" is familiar to every one. Foxes when trapped or hard pressed often drop down limp and apparently lifeless and will even endure a good deal of maltreatment without making any response. Hudson records that he was "once riding with a gaucho when we saw, on the open level ground before us, a fox not yet fully grown standing still and watching our approach. All at once it dropped, and when we came up to the spot it was lying stretched out, with eyes closed, and apparently dead. Before passing on my companion, who said it was not the first time he had seen such a thing, lashed it vigorously with his whip for some moments, but without producing the slightest effect."
Mr. Morgan in his book on the beaver gives the following instance on what he assures us is excellent authority: "A fox one night entered the hen-house of a farmer, and after destroying a large number of fowls, gorged himself to such repletion that he could not pass out through the small aperture by which he had entered. The proprietor found him in the morning sprawled out upon the floor apparently dead from surfeit; and taking him up by the legs carried him out unsuspectingly, and for some distance to the side of his house, where he dropped him upon the grass. No sooner did Reynard find himself free than he sprang to his feet and made his escape." Dogs are frequently deceived by this ruse of the fox and doubtless foxes have many times owed their lives to its aid. It has been often noticed that if one withdraws from a fox when it is feigning it may be seen to slowly open its eyes, then raise its head and carefully look around to see if its foes are at a safe distance, and finally scamper off.
While in insects the instinct of feigning death is probably a simple reflex reaction to outer stimuli, it is doubtless associated in birds and especially mammals with a tolerably acute consciousness of the situation. It involves a more or less deliberate intention to profit by the deception, yet at the same time it is probably not a result of conscious reflection. The instinct is there, or else such a course of action would not occur to the animal's mind. Were it otherwise it would be difficult to understand why the ruse is adopted only by certain species while many others, equally intelligent and for whom it would be an equally advantageous stratagem never manifest it. There can be little doubt that a fox which slowly opens its eye and warily looks around is acting with an intelligent appreciation of his predicament, but it is not to be inferred that he could have reasoned out his course of action did not an innate proclivity in that direction form a part of his instinctive make-up.
The physiological condition in what is called death-feigning is quite different in different forms. In most of the lower animals it is characterized by a tetanic contraction of the muscles. The attitudes assumed by many forms, such as rolling into a ball, keeping the legs and other appendages drawn close to the body, or in some cases holding them straight and rigid, are such as can be maintained only at the cost of considerable muscular effort. If a Ranatra is picked up by one of its slender legs it may be held out horizontally for a considerable time without causing the leg to bend. It is as if a man were seized below the knee and held out straight, face upward, without causing the knee to bend; only the legs of a Ranatra are several times more slender than those of the most attenuated of the human species, and the muscular tension which the insect maintains must therefore be intense.
The death feint of insects and other low forms is not entirely dependent on the brain. It is due rather to a general physiological state of the animal. I have found that the posterior part of the body of a Ranatra can still be induced to feign death, though less perfectly, when entirely removed from the head and prothorax. When it would come out of the feint a few light strokes would cause it to feign again. It has been found that spiders also may still feign after entire destruction of the brain.
The instinct of feigning death is doubtless closely connected with much of what has been called hypnotism in the lower animals. Crayfishes, frogs, lizards, certain snakes and many birds and mammals, may by a very simple process be thrown into an inactive condition from which they are not readily aroused by external stimuli. In ordinary death feigning the animal falls into its immobile state upon slight provocation; a touch, or even a jar is sometimes all that is necessary. In the so-called cases of hypnosis more or less manipulation is necessary. The exciting cause in both cases is generally some form of contact stimulus. In the hypnotism of animals, as Verworn and others have shown, there is diminished reflex irritability, and usually tonic contraction of many at least of the muscles. Similar phenomena are observed in the death feigning of many forms, some of the insects showing a lack of responsiveness that is truly remarkable. In a water-scorpion that is feigning death the legs may be cut off one by one, or the body cut in two without eliciting the least reaction from the unfortunate victim. We can only speculate at present on the condition of the nervous system which makes such a result possible.