INDIVIDUAL, AGE, AND s.e.x DIFFERENCES IN BEHAVIOR

All dancers are alike in certain important respects, but to the trained observer of animal behavior their individual peculiarities are quite as evident, and even more interesting than their points of resemblance.

Omitting consideration of the structural marks of individuality, we shall examine the individual, age, and s.e.x differences in general behavior, rapidity of learning, memory, and discrimination, which have been revealed by my experiments. Observations which bear on the subject of differences are scattered through the preceding chapters, but in no case have they been given sufficient prominence to force them upon the attention of those who are not especially interested in individual peculiarities. It has seemed worth while, therefore, to a.s.semble all the available material in this chapter for systematic examination and interpretation.

In the pages which follow, individual, age, and s.e.x peculiarities are discussed in turn. Within each of these three groups of differences I have arranged in order what Royce has appropriately named the facts of discriminating sensitiveness, docility, and initiative. Individuals of the same age and s.e.x no less than those which differ in s.e.x or age exhibit important differences in ability to discriminate among sense impressions ("discriminative sensitiveness"), in ability to profit by experience ("docility"), and in ability to try new kinds of behavior ("initiative").

Individual differences in sensitiveness to visual, auditory, tactual, and olfactory stimuli have been revealed by many of my experiments. The brightness discrimination tests conclusively proved that a degree of difference in illumination which is easily detectable by one dancer may be beyond the discriminating sensitiveness of another. Both the tests with gray papers and those with the Weber's law apparatus furnished striking evidence of individual differences in the kind of visual sensitiveness which throughout this book has been called brightness vision. I suspect that certain of the differences which were observed should be referred to the experience of the individuals rather than to the capacity of the visual organs, for training improves visual discrimination to a much greater extent than would ordinarily be thought possible. To the truth of this statement the results of the Weber's law experiments with No. 51 bear witness. Likewise in color discrimination there are individual differences, examples of which may be discovered by the examination of the results given in Chapters IX and X.

No differences in auditory sensitiveness appeared in my adult dancers, for in none of them was there definite response to sounds, but among the young individuals differences were prominent. I may call attention to the data on this subject which Table 5, p. 89, contains. The mice in four out of twelve litters gave no indications of hearing any sounds that I was able to produce; the remaining individuals responded with varying degrees of sensitiveness. I made no attempt to measure this sensitiveness, but it obviously differed from mouse to mouse. I feel justified, therefore, in stating that the young dancers exhibit extreme individual difference in sensitiveness to sounds.

My observations of differences in sensitiveness to other forms of stimulation were made in connection with training tests, and although they are not quant.i.tative, I venture to call attention to them. Indeed, I am led by the results of my study of various aspects of the dancer's behavior to conclude that the race exhibits individual differences in discriminating sensitiveness to a far greater extent than do most mammals, not excepting man. The importance of this fact (for I am confident that any one who carefully examines the detailed results of the various experiments which are described in this book will agree that it is an established fact) cannot be overlooked. It alters our interpretation of the results of training, memory, heredity, and discrimination experiments, and it leads us to suspect that the dancing race is exceedingly unstable.

I do not venture to make comparison of my own observations of the dancer's sense equipment with those of Cyon, Rawitz, Zoth, and Kishi, for the differences are too great in many instances to be thought of as other than species or variety peculiarities. It has seemed fairer to compare only individuals of the same breed, or, as I have done and shall continue to do throughout this chapter, of two lines of descent.

With respect to docility individual differences are prominent. We need only turn to the various tables of results to discover that in modifiability of behavior, in memory, in re-learning, not to mention other aspects of docility, dancers of the same s.e.x and age differed strikingly.

Let me by way of ill.u.s.tration cite a few cases of difference in docility.

Number 1000 learned to discriminate white from black more quickly and retained his habit longer than any other dancer with which I have experimented. I should characterize him as an exceptionally docile individual. Table 44 offers several examples. Numbers 403 and 407, though they were born in the same litter and were alike in appearance and in conditions of life, acquired the white-black habit with a difference in rapidity which is expressed by the indices of modifiability 50 and 100. In other words, it took No. 407 twice as long to acquire this habit as it took No. 403. Similarly the ladder-climbing tests revealed important individual differences in ability to profit by experience. In the tables of labyrinth tests (38, 39, 40) individual differences are too numerous to mention. It required forty-nine tests to establish in No. 50 a labyrinth-C habit which was approximately equal in degree of perfection to that which resulted from twenty-two tests in the case of No. 52. The figures in this and other instances do not exaggerate the facts, for repeatedly I have tested individuals of the same litter, the same s.e.x, and, so far as I could judge, of the same stage of development, and obtained results which differ as markedly as do those just cited. If s.p.a.ce limits permitted, I could present scores of similar differences in docility which the problem, labyrinth, and discrimination methods have revealed.

In examining the detailed individual results of the various tables for differences of this sort, it is important to bear in mind that s.e.x, age, and descent should be taken into account, for with each of them, as will be shown clearly later in this chapter, sensitiveness, docility, and initiative vary. I have therefore based my statements concerning individual differences in docility upon the results of comparison of mice of the same litter, s.e.x, and age. It is safe to say that human beings similarly selected for comparison do not exhibit greater differences in ability to profit by experience than did these dancing mice.

The facts concerning individual differences in initiative which I have discovered are not less definite than those of the preceding paragraphs.

From the beginning of my study of the dancer I observed that what one individual would readily learn of his own initiative another never learned. For example, in the ladder-climbing experiment No. 1000 distinguished himself for his initiative, whereas Nos. 4 and 5 never acquired the habit of escaping from confinement by using the ladder. I noticed, in this test of the animal's ability to learn, that while one individual would be scurrying about trying all ways of escape, investigating its surroundings, looking, sniffing, and dancing by turns, another would devote all its time to whirling, circling, or washing itself. One in the course of its activity would happen upon the way of escape, the other by reason of the limited scope of its activity, not the lack of it, would fail hour after hour to discover even the simplest way of getting back to its nest, to food, and to its companions. Hundreds of times during the past three years I have noticed important individual differences in initiative in connection with the discrimination experiments. The swinging wire doors which one dancer learned to push open before he had been in the box five minutes, another might not become familiar with through his own initiative for hours or days. In fact, it was not seldom that I had to teach an individual to pa.s.s from one compartment to the other by gently pushing him against the door until it opened sufficiently to allow him to squeeze through. Occasionally a mouse learned to pull the doors open so that he could pa.s.s through the openings in either direction with facility. This was a form of individual initiative which I had not antic.i.p.ated and did not especially desire, so I did not encourage its development, but, nevertheless, at least one fourth of the mice which I experimented with in the discrimination box learned the trick. The other three fourths, although they were used in the box day after day sometimes for weeks, never discovered that they might return to the nest-box by pulling the swing-door through which they had just pa.s.sed as well as by entering one of the electric-boxes.

Another indication of individual initiative in action appeared in the tendency of certain mice to climb out of the experiment boxes or labyrinths. It would have been extremely easy for any of the mice to escape from the labyrinths by scaling the walls of the alleys, for they were only 10 cm. in height, and when a dancer stood on its hind legs it could easily reach the top with its nose. But, strange though it will seem to any one who has not worked with the dancer, not more than one in ten of the animals which I observed made any attempt to escape in this manner.

They lacked initiative. That it was not due to a lack of the power to climb, I abundantly demonstrated by teaching a few individuals that a scramble in one corner meant easy escape from the maze of paths. I do not think any one of the mice was physically incapable of climbing, but I am confident that they differed markedly, not only in the willingness to try new modes of action, but in the readiness with which they could climb. I have already said that individuals differ noticeably in the scope of their activity. By this statement I mean that they try a varying number of kinds of activity. As in the case of men, so in mice, one individual will do a greater number of things in a few hours than another will in weeks or months. The dancers differ in versatility, in individual initiative, as do we, albeit not so markedly.

Important differences which may with certainty be described as age differences are not so obvious as are such marks of individuality as have been set forth in the preceding pages. I have noted few changes in discriminative sensitiveness, other than those with regard to auditory sensitiveness, which could be correlated with age. In certain instances adults appeared to be able to discriminate more accurately and more easily than young mice, but it is difficult to say whether this change belongs under sensitiveness or docility. I have not made an ontogenetic study of the senses, and I am therefore unable to describe in detail the course of their development and decline. Of one important fact I am certain, that discriminative sensitiveness increases up to a certain point with age and with training.

Differences in docility which are obviously to be correlated with age abound. In the prime of its life (from the second to the tenth month) the dancer is active, full of energy, quick to learn; in its senility (during the second year) it is inactive, but at times even more docile than during the period of greatest physical development. Frequently I have noticed in connection with labyrinth tests that individuals of the age of a year or more learn much more quickly than do individuals of the age of two or three months. But, on the other hand, I have contradictory observations, for now and then I obtained just the opposite result in experiments to test docility. Evidently this is a matter which demands systematic, quant.i.tative investigation. Casual observation may suggest conclusions, but it will not justify them.

Early in my investigation of the behavior of the dancer I conceived the idea of determining the relation of modifiability of behavior (docility) to age. The question which was foremost in my mind and for which I first sought an answer may be stated thus: can the dancer acquire a given habit with the same facility at different ages? Since the visual discrimination experiment seemed to be well suited for the investigation of this problem I planned to train, in the white-black discrimination experiment, five pairs of dancers at the age of one month, and the same number for each of the ages four, seven, ten, thirteen, sixteen, and nineteen months.[1]

[Footnote 1: I have not been able thus far to determine the average length of the dancer's life. The greatest age to which any of my individuals has attained is nineteen months.]

To test the same individuals month after month would be the ideal way of obtaining an answer to our question, but I could devise no satisfactory way of doing this. The effects of training last so long, as the results of the previous chapter proved, and the uncertainty of their entire disappearance is so serious, that the same training process cannot be used at successive ages. The use of different methods of training is even more unsatisfactory because it is extremely difficult to make accurate quant.i.tative comparison of their results. It was these considerations that forced me to attempt to discover the relation of docility to age by carrying out the same experiments with groups of individuals of different ages.

As my plan involved the execution of precisely the same set of tests with at least seventy individuals whose age, history, and past experience were accurately known, and of which some had to be kept for nineteen months before they could be trained, the amount of labor and the risk of mishap which it entailed were great. To make possible the completion of the investigation within two years, I acc.u.mulated healthy individuals for several months without training any of them. In March, 1907, I had succeeded in completing the tests for the age of one month, and I had on hand for the remaining tests almost a hundred individuals, whose ages ranged from a few days to eighteen months. Had everything gone well, the work would have been finished within six months. Suddenly, and without discoverable external cause, my mice began to die of an intestinal trouble, and despite all my efforts to check the disease by changing food supply and environment, all except a single pair died within a few weeks.

Thus ended a number of experiments whose final results I had expected to be able to present in this volume. However, the work which I have done is still of value, for the single pair of survivors have made possible the continuance of my tests with other individuals of the same line of descent as those which perished, and I have to regret only the loss of time and labor.

As I have on hand results for ten individuals of the age of one month, and for four individuals of the age of four months, it has seemed desirable to state the problem, method, and incomplete results of this study of the relation of modifiability to age. The indices of modifiability for these two groups of dancers differ so strikingly that I feel justified in persisting in my efforts to obtain comparable data for the seven ages which have been mentioned.

TABLE 52

PLASTICITY (RELATION or MODIFIABILITY TO AGE)

Number of Errors in Successive Daily Series of Ten White-Black Tests, with Dancers Four Months Old

SERIES MALES FEMALES

NO. 76 NO. 78 AV. NO. 75 NO. 77 AV. GENERAL AV.

A 7 7 7.0 4 8 6.0 6.50 B 8 6 7.0 6 5 5.5 6.25

1 5 5 5.0 5 5 5.0 5.00 2 5 4 4.5 2 2 2.0 3.25 3 4 5 4.5 2 5 3.5 4.00 4 3 4 3.5 1 1 1.0 2.25 5 5 2 3.5 0 1 0.5 2.00 6 3 2 2.5 1 0 0.5 1.50 7 2 1 1.5 1 2 1.5 1.50 8 5 1 3.0 0 0 0 1.50 9 1 3 2.0 0 0 0 1.00 10 1 2 1.5 1 0 0.5 1.00 11 1 1 1.0 0 0 0.50 12 1 1 1.0 0 0 0.50 13 0 0 0 0 0 0 14 0 0 0 0 15 0 0 0 0

[Ill.u.s.tration: FIGURE 33.--Plasticity curves. In the left margin are given the indices of modifiability (the number of tests necessary for the establishment of a perfect habit). Below the base line the age of the individuals is given in months. Curve for males, --?--?--?--; curve for females, - - - -; curve for both males and females,----. When these three plasticity curves are completed, they will represent the indices of modifiability as determined for ten individuals at the age of 1 month, and similarly for the same number of individuals at each of the ages, 4, 7, 10, 13, 16, and 19 months.]

The detailed results for the one-month old individuals appear in Table 43; those for the four-month individuals in Table 52. The general averages for the former are to be found in the third column of Table 46, under the heading "10 tests per day"; those for the latter in the last column of Table 52. Mere inspection of these tables reveals the curious s.e.x difference which goes far towards justifying the presentation of this uncompleted work. The index of modifiability for the ten one-month individuals is 88 (that is, 88 tests were necessary for the establishment of a habit); for the four-month individuals it is 102.5. The heavy solid line of Figure 33 joins the points on the ordinates at which these values are located. Apparently, then, the dancer acquires the white-black discrimination habit less readily at the age of four months than at the age of one month.

Further a.n.a.lysis of the results proves that this statement is not true.

When the averages for the two s.e.xes are compared, it appears that the males learned much less quickly at four months than at one month, whereas just the reverse is true of the females. The dash and dot line of the figure extends from the index of modifiability of the one-month males (72) to that of the four-month males (120); and the regularly interrupted line similarly joins the indices of the one-month (104) and the four-month (85) females. In seeking to discover age differences in docility or ability to profit by experience we have stumbled upon what appears to be an important s.e.x difference. Perhaps I should add to this presentation of partial results the following statement. Since there are only four individuals in the four-month group, two of each s.e.x, the indices are not very reliable, and consequently too much stress should not be laid upon the age and s.e.x differences which are indicated.

In view of this impressive instance of the way in which averages may conceal facts and lead the observer to false inferences, I wish to remark that my study of the dancer has convinced me of the profound truth of the statement that the biologist, whether he be psychologist, anthropologist, physiologist, or morphologist, should work with the organic individual and should first of all deal with his results as individual results. Averages have their place and value, but to ma.s.s data before their individual significance has been carefully sought out is to conceal or distort their meaning. Too many of us, in our eagerness for quant.i.tative results and in our desire to obtain averages which shall justify general statements, get the cart before the horse.

Figure 33 presents the beginning of what I propose to call plasticity curves. When these three curves are completed on the basis of experiments with five dancers of each s.e.x for each of the ages indicated on the base line of the figure, they will indicate what general changes in plasticity, modifiability of behavior, or ability to learn (for all of these expressions have been used to designate much the same capacity of the organism) occur from the first month to the nineteenth in the male and the female dancer, and in the race without respect to s.e.x. So far as I know, data for the construction of plasticity curves such as I hope in the near future to be able to present for the dancing mouse have not been obtained for any mammal.

At present it would be hazardous for me to attempt to state any general conclusion concerning the relation of docility to age.

The initiative of the dancer certainly varies with its age. In scope the action system rapidly increases during the first few months of life, and if the animal be subjected to training tests, this increase may continue well into old age. The appearance of noticeable quiescence does not necessarily indicate diminished initiative. Frequently my oldest mice have shown themselves preeminent in their ability to adjust their behavior to new conditions. However, I have not studied individuals of more than eighteen months in age. One would naturally expect initiative to decrease in senility. All that I can say is that I have seen no indications of it.

We may now briefly consider the princ.i.p.al s.e.x differences which have been revealed by the experiments. In sensitiveness I have discovered no difference, but it should be stated that no special attention has been given to the matter. In docility the males usually appeared to be superior to the females. This was especially noticeable early in my visual discrimination tests. The males almost invariably acquired a perfect habit quicker than the females. I may cite the following typical instances.

Number 14 acquired the black-white habit with 40 tests; No. 13, with 60 (Table 10, p. 109). Of the five pairs of individuals whose records in white-black training appear in Table 43, not one contradicts the statement which has just been made. It is to be noted, however, that under certain conditions of training, for example, 20 tests per day, the female is at an advantage. Recently I have with increasing frequency obtained measures of docility which apparently favor the female. That this difference in the results is due to a difference in age is probable.

In labyrinth tests the female is as much superior to the male as the male is to the female in discrimination tests. From the tables of Chapter XIII I may take a few averages to indicate the quant.i.tative nature of this difference. A degree of proficiency in labyrinth B attained by the males after 7.0 trials was equaled by the females after 6.2 trials. In labyrinth C the males acquired a habit as a result of 18.7 trials; the females, as a result of 13.8. And similarly in labyrinth D, 6.1 trials did no more for the males than 5.9 did for the females.

That at the age of about one month the male dancer should be able to acquire a visual discrimination habit more rapidly than the female, whereas the female can acquire a labyrinth habit more readily than the male, suggests an important difference in the nature of their equipment for habit formation. One might hazard the suggestion that the male depends more largely upon discrimination of external conditions, whereas the female depends to a greater extent than does the male upon the internal, organic changes which are wrought by acts. At any rate the female seems to follow a labyrinth path more mechanically, more accurately, more easily, and with less evidence of sense discrimination than does the male.

Finally, in concluding this chapter, I may add that in those aspects of behavior which received attention in the early chapters of this volume the dancers differ very markedly. Some climb readily on vertical or inclined surfaces to which they can cling; others seldom venture from their horizontally placed dance floor. Some balance themselves skillfully on narrow bridges; others fall off almost immediately. My own observations, as well as a comparison of the accounts of the behavior of the dancer which have been given by Cyon, Zoth, and other investigators, lead me to conclude that there are different kinds of dancing mice. This may be the result of crosses with other species of mice, or it may be merely an expression of the variability of an exceptionally unstable race.

I can see no satisfactory grounds for considering the dancer either abnormal or pathological. It is a well-established race, with certain peculiarities to which it breeds true; and no pathological structural conditions, so far as I have been able to learn, have been discovered.

I have presented in this chapter on differences a program rather than a completed study. To carry out fully the lines of work which have been suggested by my observations and by the presentation of results would occupy a skilled observer many months. I have not as yet succeeded in accomplishing this, but my failure is not due to lack of interest or of effort.

CHAPTER XVIII

THE INHERITANCE OF FORMS OF BEHAVIOR

In a general way those peculiarities of behavior which suggested the name dancing mouse are inherited. Generation after generation of the mice run in circles, whirl, and move the head restlessly and jerkily from side to side. But these forms of behavior vary greatly. Some individuals whirl infrequently and sporadically; others whirl frequently and persistently, at certain hours of the day. Some are unable to climb a vertical surface; others do so readily. Some respond to sounds; others give no indications of ability to hear. I propose in this chapter to present certain facts concerning the inheritance of individual peculiarities of behavior, and to state the results of a series of experiments by which I had hoped to test the inheritance of individually acquired forms of behavior.

My study of the nature of the whirling tendency of the dancer has revealed the fact that certain individuals whirl to the right almost uniformly, others just as regularly to the left, and still others now in one direction, now in the other. On the basis of this observation, the animals have been cla.s.sified as right, left, or mixed whirlers. Does the dancer transmit to its offspring the tendency to whirl in a definite manner?