Numerous stories have been related of cases in which an ant, having found food, returns to the nest with as much of it as she can carry, and when she comes out again brings with her a number of other ants. This has been interpreted to mean that in some mysterious way the ant communicates her discovery to her fellow-ants. A simpler explanation is probably more correct. The odor of the food, or of the trail, serves as a stimulus to other ants, that follow to the place where the first ant goes for a new supply of the food. The fact that the first individual returns to the supply of food seems to indicate that the ant has memory, and this is obviously of advantage to her and to the whole colony.
The peculiar habits of some of the solitary wasps, of stinging the caterpillar or other insect which they store up as food for their young, is often quoted as a wonderful case of adaptive instinct. The poison that is injected into the wound paralyzes the caterpillar, but as a rule does not kill it, so that it remains motionless, but in a fresh state to serve as food for the young that hatch from the egg of the wasp. A careful study of this instinct by Mr. and Mrs. Peckham has shown convincingly that the act is not carried out with the precision formerly supposed. It had been claimed that the sting is thrust into the caterpillar on the lower side, a ventral ganglion being pierced, the poison acting with almost instantaneous effect. But it may be questioned whether this is really necessary, and whether the same end might not be gained, although not quite so instantaneously, if the caterpillar were pierced in almost any other part of the body. Can we be seriously asked to believe that this instinct has been perfected by the destruction of those individuals (or of their descendants) that have not pierced the caterpillar in exactly the middle of a segment of the anterior ventral surface? It seems to me that the argument proves too much from the selectionist’s point of view. If the wasp pierced the caterpillar in the middle of its back, we should have passed over the act without comment; but since the injection is usually made on the ventral side, and since we know that the nervous system lies in this position, it has been assumed that the act is carried out in this way, in order that the poison may penetrate the nervous system more quickly. Yet a fuller knowledge may show that there is really no necessity for such precision.
A curious response is the so-called death-feigning instinct shown by a number of animals, especially by certain insects, but even by some mammals and birds. Certain insects, if touched, draw in their legs, let go their hold, and fall to the ground, if they happen to be on a plant. It is not unusual to meet with the statement that this habit has been acquired because it is useful to the insect, since it may often escape in this way from an enemy. This does not appear on closer examination to be always the case, and sometimes as much harm as good may result, or what is more probable, neither much advantage, nor disadvantage, is the outcome. This can, of course, only be determined in each particular case from a knowledge of the whole life of a species and of the enemies that are likely to injure it.
Hudson has recorded[[33]] a number of cases of this death-feigning instinct in higher animals, and attributes it to violent emotion, or fear, that produces a sort of swoon. He describes the gaucho boys’ method, in La Plata, of catching the silver-bill by throwing a stick or a stone at it, and then rushing toward the bird, “when it sits perfectly still, disabled by fear, and allows itself to be taken.” He also states that one of the foxes (Canis azaræ) and one of the opossums (Didelphys azaræ) “are strangely subject to the death-simulating swoon.”
[33]. “The Naturalist in La Plata.”
Hudson remarks that it seems strange that animals so well prepared to defend themselves should possess this “safeguard.” When caught or run down by dogs, the fox fights savagely at first, but after a time its efforts stop, it relaxes, and it drops to the ground. The animal appears dead, and Hudson states that the dogs are “constantly taken in by it.” He has seen the gauchos try the most barbarous tricks on a captive fox in this condition, and, despite the mutilations to which it was subjected, it did not wince. If, however, the observer draws a little away from the animal, “a slight opening of the eye may be detected, and finally, when left to himself, he does not recover and start up like an animal that has been stunned, but cautiously raises his head at first and only gets up when his foes are at a safe distance.” Hudson, coming once suddenly upon a young fox, saw it swoon at his approach, and although it was lashed with a whip it did not move.
The common partridge of the pampas of La Plata (Hothura maculosa) shows this death-feigning instinct in a very marked degree. “When captured, after a few violent struggles to escape, it drops its head, gasps two or three times, and to all appearance dies.” But if it is released it is off in an instant. The animal is excessively timid, and if frightened, may actually die simply from terror. If they are chased, and can find no thicket or burrow into which to escape, “they actually drop down dead on the plain. Probably when they feign death in their captor’s hand they are in reality very near to death.”
In this latter instance it must appear very improbable that we are dealing with an instinct that has been built up by slow degrees on account of the benefit accruing at each stage to the individual. In fact, it appears that the instinct is in this case of really no use at all to the animal, for there can scarcely be any question of an escape by this action. Yet so far as we can judge it is the same instinct shown by other animals, and it is not logical to account for its origin in one case on the grounds of its usefulness, when we cannot apply the explanation in the other cases. If this be admitted, we have another illustration of the importance of keeping apart the origin of an instinct or of a structure and the fact of its usefulness or non-usefulness to the organism. Thus under certain conditions this death-feigning instinct might really be of use to the animal, while under other conditions and in other animals it may be of no advantage at all, and in still other conditions it may be a positive injury to its possessor. Perhaps we need not go outside of our own experience to find a parallel case, for the state of fright into which imminent danger may throw an individual may deprive him for the moment of the proper use of those very mental qualities of which he stands in this crisis in greatest need.
The peculiar behavior of cattle caused by the smell of blood is another case of an instinct whose usefulness to its possessors is far from apparent. It is known that cattle and horses and several wild animals become violently excited by the smell of blood. Hudson gives a vivid account of a scene witnessed by himself, the animals congregating, “and moving around in a dense mass, bellowing continually.” Those animals that forced their way into the centre of the mass where the blood was “pawed the earth and dug it up with their horns, and trampled each other down in their frantic excitement.”
This action leads us to a consideration of the behavior of animals toward companions in distress. “Herbivorous animals at such times will trample and gore the distressed one to death. In the case of wolves and other savage-tempered carnivorous species the distressed fellow is frequently torn to pieces and devoured on the spot.” If any one will be bold enough to claim in this case that this habit has been acquired because of advantage to the pack, i.e. if it be imagined that the pack gains more by feeding on a weak member than by letting him take his chances of recovery, it may be pointed out in reply that cattle also destroy their weak or injured, but do not devour them, and the same statement holds for birds, where the same instinct has often been observed. Romanes has suggested that the instinct of destroying the weak or injured members is of use because such members are a source of danger to the rest of the herd; but Hudson points out that it is not so much the weak and sickly members of the herd that are attacked in this way, as those that are injured, and concludes, “the instinct is not only useless, but actually detrimental.” He suggests that these “wild abnormal movements of social animals” are a sort of aberration, so “that in turning against a distressed fellow they oppose themselves to the law of being.” Yet whether we gain anything by calling this action aberrant or abnormal, the important fact remains that it is a definite response under certain external conditions, and is shown by all the individuals of the species.