Pick from the Past
Natural History, April 1917

Individuality, Temperament, and Genius in Animals

From such research we learn to appreciate human individuality, and to realize that any future
conscious control of human life must come through a study of the conditions under which
varied types of temperament will develop the highest character and the greatest genius.




Skirrl, a crab-eating monkey (Macaca irus), had an inexhaustible interest in objects that he could play with or manipulate, such as a hammer and nails.
XPERIMENTAL studies of animal behavior, pursued for the solution of definite problems of sense, instinct, or habit, frequently yield as by-products interesting and important information concerning individual, sex, species, and race differences. Such observations commonly fail to get recorded because of the primary importance to the observer of the problem on which his attention is focused. In preparing his results for publication he would gladly report everything of significance, were it not that exigencies of time and space render this either impracticable or impossible. It is largely because of our conviction that certain of the unrecorded by-products of our investigations are in various respects more important than the data which we have published, that we are attempting to write on evidences of individuality in various organisms.

In this field of naturalistic and experimental observation, there is always the risk of confusing age, sex, or race differences with those which are truly individual. The casual observer readily overlooks the fact that his pet canaries, kittens, or dogs, differ by several weeks in age or are otherwise not suitable for comparison, for as a naturalist he is less concerned with strict comparability than with that knowledge which will lead to sympathetic insight. But to those who are trained in critical and well-controlled observation, it is an easy task to eliminate such sources of error and to obtain fairly comparable data concerning individuality. Field naturalists and the born lovers of animals know by intimate acquaintance that important individual differences exist in many species of organism, but experimentalists are less generally aware of this fact, for their attention tends to be monopolized by problems of species characteristics and of general organic functions or reactive capacity.

Field naturalists and the born lovers of animals know by intimate acquaintance that important individual differences exist in many species of organism, but experimentalists are less generally aware of this fact, for their attention tends to be monopolized by problems of species characteristics and of general organic functions or reactive capacity.

Even in invertebrates individuality becomes conspicuous with familiarity. Among earthworms we have observed that specimens, comparable in all essential points and existing under the same conditions of observation, exhibit surprisingly different modes of response. Thus, one individual adapts itself to the demands of a situation, works smoothly, steadily—as it were willingly; another slowly and haltingly meets the experimenter’s requirements. Its tendency to do the wrong thing seemingly amounts to perversity or stubbornness. And so the observer gains the feeling that the two organisms are quite as different in reactive tendency as are two men.

It has often been remarked that the individuals of a human race with which one is unfamiliar look alike. This we always discover to be due to our failure to notice marked individual differences. As our familiarity with the type increases, these individual traits become increasingly obvious. Now precisely what is true in our experience with our fellow men is still more true of other types of organism. We note at first only the species or racial differences, or perhaps if they be equally conspicuous, certain age and sex differences, but as we continue to live with the organisms and to observe them carefully day by day, we come to appreciate those qualitative and quantitative peculiarities which constitute individuality. As far as we can see, there is no significant difference in degree of individuality between earthworm and man, ant and monkey.

Even in invertebrates individuality becomes conspicuous with familiarity. Among earthworms we have observed that specimens, comparable in all essential points and existing under the same conditions of observation, exhibit surprisingly different modes of response.

Intimacy of relation with a wide range of organic types has served, among other things, to convince us that temperament, character, and genius are terms, which, like individuality, may be used quite as appropriately in connection with one type of organism as with another. We wish especially, in this paper, to report certain of our observations concerning these aspects of life. Temperament we have come to think of as the sum of fundamental, inborn reactive tendencies,—they are sometimes called primary instincts; character, as these same tendencies organized through environmental contact or experience into a complex and more or less highly adaptive system of behavior; genius, as exceptionally strong or well-marked temperamental traits of a particular order. The conventional and ancient classification of temperaments according to strength and duration of response as choleric, melancholic, sanguine, and phlegmatic seems unduly simple in the light of our observations, for there are at least several important ingredients or constituents of temperament which apparently vary independently or in groups with respect to strength and duration of response, and possibly also in other important ways. We may not here further dwell upon definitions, but we shall hope to render these suggestions more significant by the facts which we have to record.

Some years ago we undertook a comparative study of two strains of albino rat, the one closely inbred for many generations, the other outbred. Save for this difference, the individuals of the strains were entirely comparable. We attempted by various experimental means to discover peculiarities of behavior in these animals. Soon it became apparent that the inbred individuals adapted themselves less readily to new environmental demands. They proved less apt pupils in tests of habit formation. We were struck, as our observations progressed, by certain peculiarities of behavior which appeared to be characteristic of the strains rather than of individuals. Among them fear, timidity, savageness, curiosity, sociability were conspicuous. In general, the inbred rats seemed more timid, fearful, more likely to defend themselves by biting if disturbed,

One rat is extremely fearful of anything new or unusual, it shrinks timidly from the experimenter. It can only with difficulty be induced to run its way through the experimental apparatus. Another individual of the same age and sex, born in the same litter, is by contrast aggressive and exhibits marked initiative in new situations.
less ready to try new things, more suspicious of the experimenter, slower to acquire obviously profitable modes of response than were the outbred animals. These differences in behavior seemed to us to account for an apparent difference in intelligence, and we finally concluded that it is really quite beside the mark to contrast the two strains by saying that the one is the more or the less intelligent.

Subsequently, increasingly definite and well-controlled studies were made, in which were recorded observations concerning the preferred position of an individual in its cage or nest box; the relative positions at different hours of the two individuals, male and female, in a given cage; the degree or amount of activity; savageness, or the tendency to bite; and the quickness and amount of response to various stimuli. These and similar observations shortly indicated that savageness designates certain tendencies to reaction, as does also fear, timidity, or wildness, and that our only intelligible way of defining these terms is by enumerating the several types of activity. Wildness, for example, is indicated by attempts to hide in the cage or in the observer’s hand, random and excited running about with repeated attempts to escape, squeaking, and various other forms of response. Timidity, which seemingly is not identical with wildness or fear, involves the avoidance of the experimenter, a kind of chattering or gnashing of the teeth, cowering, or even trembling.

Although most of our studies have been concerned with relations of behavior to inbreeding or to the crossing of individuals which differ markedly in some trait, we have incidentally obtained abundant evidence of important individual differences of the temperamental sort. One rat for example, is extremely fearful of anything new or unusual, it shrinks timidly from the experimenter. It can only with difficulty be induced to run its way through the experimental apparatus. When cornered, it defends itself by biting the experimenter’s hand. Its wildness is indicated by persistent efforts to hide or to escape. It responds quickly and markedly to any sudden and unexpected stimulus; peculiarly startling stimuli at times cause it to tremble. Another individual of the same age and sex, born in the same litter, is by contrast aggressive and exhibits marked initiative in new situations. Its fear or timidity is readily overcome by its curiosity. It quickly becomes accustomed to the experimenter, and allows him to touch it or take it up in his hand without attempting to bite, and shortly without effort to escape. It responds slowly and only slightly to most stimuli and is disturbed only by strong stimulation. In a word, the two rats are temperamentally as different as any two human beings one is likely to meet. It is such observations as these, made on many different individuals, that have wholly convinced us of the desirability of a careful analysis of temperament and the reduction to terms of measured description of its chief constituents.

We once undertook to study experimentally the ideational behavior of pigs. For this purpose two young animals were chosen, the one a male, the other a female. They were observed daily, and for several hours each day, the whole of one summer. We became sufficiently well acquainted with their characteristics to appreciate alike their varying degrees of intelligence and their temperamental peculiarities. What we have not published in our report on the behavior of these creatures certainly would interest the general reader much more than our printed contribution toward the solution of our problem. We therefore venture to present certain of the fascinating byproducts of our summer’s work. That the differences which we are about to emphasize are not necessarily individual, we readily admit; that they are not age or species differences, we are certain. We expect that some, at least, are sex differences.

Nip and Tuck, for thus we early decided to designate our subjects, soon made us feel their individuality. Both, under the spur of the hunger motive, worked remarkably well toward the solution of ideational problems, and their success in this work fully justified the popular impression that the pig is one of the more intelligent among mammals. Nip, the male, was less active and energetic than his sister, Tuck. He also was less greedy, and showed rather less initiative and a more limited range of reactions. Tuck it was who usually led if there was opportunity for competition, while Nip followed. Both quickly became accustomed to the experiment, but Nip showed more persistent wariness, timidity, and suspicion than did Tuck. She, however, was much quicker in response, more alert, curious, and quick to discover new opportunities for pig satisfaction. When at work on experimental problems, Nip much more easily discouraged and tended earlier than Tuck to give up his search for the reward of success. Tuck constantly acquired new and profitable tricks, which as a rule sooner or later appeared in Nip also, sometimes spontaneously and again by reason of the imitative tendency.

One summer we removed a brood of four young crows from their nest just before they were able to fly. We could not identify the sexes at the time, so the differences we observed may be either sex or individual, but at any rate, the four specimens were as sharply contrasted in temperament as are the children of any household.

As day after lay we observed these two specimens of a mammalian type whose life under domestication gives its intelligence slight opportunity for display, we were strongly impressed, as we had been in the case of rats also, by the importance of temperamental reactive tendencies in responses to any experimentally arranged situation. The experimenter who ignores individuality or temperament in his subjects runs a grave risk of misunderstanding or wrongly evaluating his results. Our descriptions sound anthropomorphic, but that, the alert reader will appreciate, is due to our avoidance of stilted and unnatural terminology. We are attempting to describe in an intelligible way, and briefly, behavior which, if we should restrict ourselves to wholly objective terms, would require pages of unusual behavioristic statement.

Among the birds, there is probably no more interesting object of study than the crow. Its species characteristics are notably alluring to the psychologist, but to us, as a result of varied observations in the corn fields of Pennsylvania and on the wooded hills of New Hampshire, sex, age, and individual differences are no less fascinating. One summer we removed a brood of four young crows from their nest just before they were able to fly. We could not identify the sexes at the time, so the differences we observed may be either sex or individual, but at any rate, the four specimens were as sharply contrasted in temperament as are the children of any household.

We set about rearing these birds by hand, the while taming them for experimental purposes. Within a few days, one of the four began to exhibit the characteristic fear reaction of its species, and at once it became extremely difficult to feed. For a few days we persisted in our attempts, and then as he or she, as the case may have been, was no less persistent, we decided to devote our time and energies to his three companions. Thus, at the very outset, temperamental peculiarities, perhaps amounting to nothing more than exceptionally strong and persistent fear reactions, served to eliminate one of the individuals from our collection.

Our space will not permit us to recite in detail, as we are tempted to do, the peculiarities which these birds exhibited during a memorable summer. We must content ourselves with the simple statement that in reactions which may be designated as those of wildness, fear, timidity, curiosity, suspicion, initiative, sociability, the individuals differed most obviously and importantly. We hope sometime, in justice to the problem of crow temperament, to devote a summer to the intensive study of sex and individual differences in these extremely intelligent birds.

Concerning temperament, character, and genius in the Primates, our materials are at once abundant and to us absorbing. Everyone who knows anything about primates, high or low, realizes that in them individuality is more conspicuous for the human observer than in most other organisms. But our results do not justify the conclusion that temperamental differences are more obvious or more important in monkeys, anthropoid apes, or man, than in crows, pigs, or rats. We have come to suspect that the popular opinion concerning the matter is due chiefly to similarity of structure and behavior—in a word, to felt kinship. It is simply because we are more like monkeys and apes that we more readily notice and more highly value their individual characteristics.

Not so very long ago, we had a splendid opportunity to become intimately acquainted with two adult male monkeys of the species Pithecus irus [Macaca irus]. The one, we shall call Skirrl; the other, Jimmie. It would be easier to tell what these individuals had in common than to enumerate their differences. Their temperamental divergences constantly amazed us. But here we must content ourselves with an account of a few of the most remarkable differences in behavior.

The monkey was provided with a suitable hammer, nails, and a board. He went to work immediately and although be exhibited no constructive ability, his skill, without tuition, in handling hammer and nails and in driving the latter into the board or elsewhere, according to his taste, more than equaled that of the unpracticed human.

Skirrl’s attitude toward the friendly experimenter was frankly aggressive, but not vicious. Jimmie was extremely vicious; he never could be trusted. Skirrl’s interest in objects which he could play with or in any wise manipulate proved inexhaustible, whereas Jimmie exhibited slight interest in other objects than the members of his species, his enemies, or foods. By a competent observer who had studied him carefully prior to our acquaintance, we were told that Skirrl was feeble-minded. And it certainly seemed so, when, as frequently happened, he sat before an experiment box, yawning repeatedly, and from time to time interrupting these expressions of ennui by half-hearted attempts to solve his problem. Whereas Skirrl rather quickly became accustomed to unusual experimental situations, Jimmie was so wary and distrustful that we finally gave up our attempts to observe his behavior under rigidly controlled conditions, and treated him merely as a visitor in the laboratory.

One day we noticed Skirrl pounding with a stick a nail which he had found in his cage. We were quick to follow this cue. The monkey was provided with a suitable hammer, nails, and a board. He went to work immediately and although be exhibited no constructive ability, his skill, without tuition, in handling hammer and nails and in driving the latter into the board or elsewhere, according to his taste, more than equaled that of the unpracticed human. In the presence of the same outfit of tools, Jimmie threw the hammer into one corner of his cage, scattered the nails about the floor, and proceeded to tear the board to pieces with his teeth. Never did he exhibit the least inclination to use hammer and nails independently or together as tools or implements.

When given a saw, rendered indestructible by metallic guards for the handle, and a heavy wooden block on which the saw might be used, Skirrl was manifestly pleased. He used the saw in quite as many and varied ways as might a boy of four or five years. By sawing before him at various times, the observer tried to teach him to use it in the conventional human way. But to this he paid scant attention, preferring, it seemed, to work out his own modes of amusement. Finally, he hit upon a way of using the saw which we have been told is in vogue among certain peoples of the East. Sitting on the floor, he held it teeth uppermost, his feet grasping the handle tightly and holding the saw firmly in position. He then grasped a nail by both ends and rubbed it rapidly over the teeth of the saw, thus producing a noise which evidently delighted his soul.

It is clear enough from the responses of other monkeys of the same and opposite sex (the same and other species) to saw, hammer, and nails, as well as to other implements, that Skirrl’s behavior must be described as highly individual or temperamental. Never have we observed anything comparable with it in any untaught Primate other than the human. We have agreed to call Skirrl’s behavior an expression of genius, for the more we consider the matter the more certain we feel that this particular individual possesses remarkably strong tendencies to react to certain objects as tools or mechanical devices. From our point of view, he possesses an unusual type of interest or the same to an unusual degree. Feeble-minded though he may be as far as most intellectual requirements are concerned, he is a genius in mechanical manipulation, and to him we feel indebted for a new point of view and for new insight into the meaning of genius.

The anthropoid apes are so manlike in appearance and behavior that we should be surprised were they not highly individualized and possessed of temperamental traits as well as forms of genius strikingly similar to our own. Our opportunities for intimate acquaintance with the higher apes have been disappointingly few, but with one young orang-utan whom we knew as Julius, we came into delightfully friendly relations. Julius was not born in captivity—few anthropoid apes have that advantage, or disadvantage—but he was captured young, and when we knew him in California, he was probably not far from five years old. Already we have recorded in print many interesting features of his behavior, as well as our strong conviction of the supreme importance to science and to other aspects of civilization of the thorough study of the anthropoid apes.

When threatened with punishment or actually punished, and when, out of sorts or ill, the young orang-utan behaved so like a child of two or three years that he caused the observers to feel uncomfortably sympathetic.

Julius one day was resting placidly in his good-size cage. A workman passing the cage stopped and offered him a banana. He hurried over to get the proffered food, but just as he reached out his hand for it, the man unkindly drew it away and started to walk off. Julius, evidently disappointed and seemingly resentful, turned, and by a series of somersaults rapidly rolled the whole length of his cage. Later, the same sort of behavior was observed in quite different situations. When, for instance, after working persistently to solve an experimental problem, he failed to obtain the desired reward of food, Julius would bring his head to the floor with a thump and turn a few somersaults. In thus expressing his feelings of disappointment and resentment, he seemed to relieve himself, for afterward be would go to work, sometimes with energy and a fair show of cheerfulness. It may be remarked, by the way, that similar modes of response have been observed in children of two to six years of age. We recall an instance in which a little boy who for some time had been working unsuccessfully on an ideational problem bumped his head several times, carefully it is true, against a wooden partition, and then remarked, by way of explanation, that he wished to stir things up.

When threatened with punishment or actually punished, and when, out of sorts or ill, the young orang-utan behaved so like a child of two or three years that he caused the observers to feel uncomfortably sympathetic. Many aspects of his behavior, which unhappily we may not now stop to describe, reminded us of our observations of children, and we found ourselves involuntarily comparing him with human subjects.

How surprising it is, as one stops to reflect on this matter of temperament, that in the same household, as children of the same parents, we find individuals who seem to be opposites in the most varied respects. The one child is sympathetic; the other tends to be cold, unresponsive, or even cruel. The one is frank, naturally honest; the other sly, secretive, and unreliable. The one kindly, good-natured; the other sour and resentful. As these children develop, their temperamental traits may be molded perhaps by educational influences into equally valuable types of character. But never by any chance can they come to possess similar temperamental characteristics.

Surely we shall do well to observe diligently and develop means of studying carefully and measuring the various constituents of temperament, and the factors which enter into character. We should study the constitution and varieties of genius, and especially the conditions which, as experience, operate upon temperamental traits to develop the responses of genius and to elaborate character. For in our efforts to control and direct human life knowledge of those aspects of individuality is of fundamental importance, and there are today unmistakable indications that the future will require of us a science of human behavior which shall consider as carefully the individual as the species. We live in the era of the biological sciences, and we look forward to an unprecedented development of the sciences of organic function, and especially of those which, like physiology, psychology, and sociology, attempt to inform us concerning phenomena of behavior. These sciences promise to become of supreme importance to civilization.

Robert M. Yerkes is assistant professor of comparative psychology at Harvard University, managing editor of the Journal of Animal Behavior, and editor of the Animal Behavior Series.


Bibliography

Basset, G. C. “Habit Formation in a Strain of Albino Rats of Less Than Normal Brain Weight.” Behavior Monographs 2: Serial No. 9, 1914.
Coburn, Charles A. “The Behavior of the Crow, Corvus americanus, Aud.” Journal of Animal Behavior 4:185–201, 1914.
Coburn, Charles A., and Robert M. Yerkes. “A Study of the Behavior of the Crow, Corvus americanus, Aud., by the Multiple–Choice Method.” Journal of Animal Behavior 5:75–114, 1915.
Utsurikawa, Nenozo. “Temperamental Differences Between Outbred and Inbred Strains of the Albino Rat.” Journal of Animal Behavior 2:111–129, 1917.
Yerkes, Ada W. “Comparison of the Behavior of Stock and Inbred Albino Rats.” Journal of Animal Behavior 6:267–296, 1916.
Yerkes, Robert M. “The Heredity of Savageness and Wildness in Rats.” Journal of Animal Behavior 3:286–296, 1913.
Yerkes, Robert M., and Charles A. Coburn. “A Study of the Behavior of the Pig, Sus scrofa, by the Multiple–Choice Method.” Journal of Animal Behavior 5:185–225, 1915.
Yerkes, Robert M. “The Mental Life of Monkeys and Apes: A Study of Ideational Behavior.” Behavior Monographs 3: Serial No. 12, 1916.
Yerkes, Robert M. “Provision for the Study of Monkeys and Apes.” Science N. S. 43:231–234, 1916.


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