The Dancing Mouse Part 3

NUMBER 4 P.M. 6 P.M. 8 P.M.

OF ANIMAL RIGHT LEFT RIGHT LEFT RIGHT LEFT

29 33 114 31 36 45 99 33 436 7 408 3 364 2 35 279 6 165 24 353 10 151 3 8 2 285 2 217 157 52 15 19 125 51 104 211 190 7 86 31 67 250 215 15 292 45 336 150 232 225 133 86 48 39 177 81 415 268 3 437 7 382 8 425 12 242 19 210 4 192

Averages 142.1 78.0 126.0 109.6 159.5 119.5

Gen. Av. 220.1 235.6 279.0

The most important results of this statistical study of turning are the demonstration of the existence of individual tendencies to turn in a particular direction, and of the fact that the whirling increases in amount from morning to evening.

In order to discover whether the distribution of the dancers among the three groups which have been designated as right, left, and mixed whirlers agrees in general with that indicated by Table 4 (approximately the same number in each group) I have observed the direction of turning in the case of one hundred dancers, including those of the foregoing tables, and have cla.s.sified them in accordance with their behavior as is indicated below.

RIGHT LEFT MIXED WHIRLERS WHIRLERS WHIRLERS

Males 19 19 12 Females 12 23 15

Totals 31 42 27

The left whirlers occur in excess of both the right and the mixed whirlers. This fact, together with the results which have already been considered in connection with the counts of turning, suggests that a tendency to whirl in a certain way may be inherited. I have examined my data and conducted breeding experiments for the purpose of ascertaining whether this is true. But as the results of this part of the investigation more properly belong in a special chapter on the inheritance of behavior (XVIII), the discussion of the subject may be closed for the present with the statement that the preponderance of left whirlers indicated above is due to a strong tendency to turn to the left which was exhibited by the individuals of one line of descent.

TABLE 4

AVERAGE NUMBER OF WHIRLS TO THE RIGHT AND TO THE LEFT FOR THE SIX INTERVALS OF TABLES 2 AND 3, WITH A CHARACTERIZATION OF THE ANIMALS AS RIGHT WHIRLERS, LEFT WHIRLERS, OR MIXED WHIRLERS.

AVERAGE NO. AVERAGE NO.

MALES AGE OF WHIRLS OF WHIRLS CHARACTERIZATION

2 12 mo. 136.7 3.3 Right whirler 30 2 mo. 109.7 2.5 Right whirler 34 2 mo. 4.3 96.5 Left whirler 36 2 mo. 197.5 23.5 Right whirler 152 6 mo. 6.8 137.0 Left whirler 156 1 mo. 73.2 12.8 Right whirler 210 3 mo. 78.7 62.0 Mixed whirler 220 4 mo. 74.3 40.2 Mixed whirler 410 3 mo. 10.3 139.0 Left whirler 420 3 mo. 12.2 241.3 Left whirler

Average 70.4 75.8 4 Right whirlers 4 Left whirlers 2 Mixed whirlers

FEMALES

29 2 mo. 23.7 53.2 Left whirler 33 2 mo. 362.7 2.7 Right whirler 35 2 mo. 208.5 19.2 Left whirler 151 6 mo. 5.0 140.0 Right whirler 157 1 mo. 47.0 51.2 Left whirler 211 3 mo. 109.7 61.5 Right whirler 215 3 mo. 57.8 234.5 Mixed whirler 225 4 mo. 103.2 56.3 Mixed whirler 415 3 mo. 216.7 3.5 Left whirler 425 3 mo. 17.0 225.2 Left whirler

Average 115.1 84.7 3 Right whirlers 4 Left whirlers 3 Mixed whirlers

The tendency of the dancer's activity to increase in amount toward evening, which the results of Tables 2, 3, and 4 exhibit, demands further consideration. Haacke (7 p. 337) and Kishi (21 p. 458) agree that the dancing is most vigorous in the evening; but Alexander and Kreidl (i p.

544) a.s.sert, on the contrary, that the whirling of the individuals which they observed bore no definite relation to the time of day and apparently was not influenced in intensity thereby. Since the results of my own observations contradict many of the statements made by the latter authors, I suspect that they may not have watched their animals long enough to discover the truth. The systematic records which I have kept indicate that the mice remain quietly in their nests during the greater part of the day, unless they are disturbed or come out to obtain food. Toward dusk they emerge and dance with varying intensity for several hours. I have seldom discovered one of them outside the nest between midnight and daylight. The period of greatest activity is between 5 and 10 o'clock P.M.

Zoth states that he has observed the adult dancer whirl 79 times without an instant's interruption, and I have counted as many as 110 whirls. It seems rather absurd to say that an animal which can do this is weak.

Evidently the dancer is exceptionally strong in certain respects, although it may be weak in others. Such general statements as are usually made fail to do justice to the facts.

The supposition that light determines the periodicity of dancing is not borne out by my observations, for I have found that the animals continue to dance most vigorously toward evening, even when they are kept in a room which is constantly illuminated. In all probability the periodicity of activity is an expression of the habits of the mouse race rather than of the immediate influence of any environmental condition. At some time in the history of the dancer light probably did have an influence upon the period of activity; but at present, as a result of the persistence of a well-established racial tendency, the periodicity of dancing depends to a greater extent upon internal than upon external conditions. During its hours of quiescence it is possible to arouse the dancer and cause it to whirl more or less vigorously by stimulating it strongly with intense light, a weak electric current, or by placing two individuals which are strangers to one another in the same cage; but the dancing thus induced is seldom as rapid, varied, or as long-continued as that which is characteristic of the evening hours.

One of the most interesting results of this study of the direction of turning, from the observer's point of view, is the demonstration of the fact that the truth concerning even so simple a matter as this can be discovered only by long and careful observation. The casual observer of the dancer gets an impression that it turns to the left more often than to the right; he verifies his observation a few times and then a.s.serts with confidence that such is the truth about turning. That such a method of getting knowledge of the behavior of the animal is worse than valueless is clear in the light of the results of the systematic observations which have just been reported. But, however important the progress which we may have made by means of systematic observation of the phenomenon of turning, it must not for one moment be supposed that the whole truth has been discovered. Continued observation will undoubtedly reveal other important facts concerning circling, whirling, and the periodicity of dancing, not to mention the inheritance of peculiarities of dancing and the significance of the various forms of activity.

CHAPTER IV

BEHAVIOR: EQUILIBRATION AND DIZZINESS

Quite as interesting and important as the general facts of behavior which we have been considering are the results of experimental tests of the dancer's ability to maintain its position under unusual spatial conditions--to climb, cross narrow bridges, balance itself on high places.

Because of its tendency to circle and whirl, to dart hither and thither rapidly and apparently without control of its movements, the study of the mouse's ability to perform movements which demand accurate and delicate muscular coordination, and to control its expressions of activity, are of peculiar scientific interest.

That observers do not entirely agree as to the facts in this field is apparent from the following comparison of the statements made by Cyon and Zoth (31 p. 174).

Cyon states that the dancer

Cannot run in a straight line, Cannot turn in a narrow s.p.a.ce, Cannot run backward, Cannot run up an incline, Cannot move about safely when above the ground, because of fear and visual dizziness, Can hear certain tones.

Zoth, on the contrary, maintains that the animal

Can run in a straight line for at least 20 cm., Can and repeatedly does turn in a narrow s.p.a.ce, Can run backward, for he has observed it do so, Can run up an incline unless the surface is too smooth for it to gain a foothold, Can move about safely when above the ground, and gives no signs of fear or dizziness, Cannot hear, or at least gives no signs of sensitiveness to sounds.

Such contradictory statements (and unfortunately they are exceedingly common) stimulated me to the repet.i.tion of many of the experiments which have been made by other investigators to test the dancer's behavior in unusual spatial relations. I shall state very briefly the general conclusions to which these experiments have led me, with only sufficient reference to methods and details of results to enable any one who wishes to repeat the tests for himself to do so. For the sake of convenience of presentation and clearness, the facts have been arranged under three rubrics: equilibrational ability, dizziness, and behavior when blinded. To our knowledge of each of these three groups of facts important contributions have come from the experiments of Cyon (9 p. 220), Alexander and Kreidl (1 p. 545), Zoth (31 p. 157), and Kishi (21 p. 482), although, as has been stated, in many instances their results are so contradictory as to demand reexamination. All in all, Zoth has given the most satisfactory account of the behavior and motor capacity of the dancer.

If the surface upon which it is moving be sufficiently soft or rough to furnish it a foothold, the dancer is able to run up or down inclines, even though they be very steep, to cross narrow bridges, to balance itself at heights of at least 30 cm. above the ground, and even to climb up and down on rods, as is shown by certain of Zoth's photographs which are reproduced in Figure 4. Zoth himself says, and in this I am able fully to agree with him on the basis of my own observations, "that the power of equilibration in the dancing mouse, is, in general, very complete. The seeming reduction which appears under certain conditions should be attributed, not to visual dizziness, but in part to excitability and restlessness, and in part to a reduced muscular power" (31 p. 161). The dancer certainly has far less grasping power than the common mouse, and is therefore at a disadvantage in moving about on sloping surfaces. One evidence of this fact is the character of the tracks made by the animal. Instead of raising its feet from the substratum and placing them neatly, as does the common mouse (Figure 5), it tends to shuffle along, dragging its toes and thus producing on smoked paper such tracks as are seen in Figure 6. From my own observations I am confident that these figures exaggerate the differences.

My dancers, unless they were greatly excited or moving under conditions of stress, never dragged their toes as much as is indicated in Figure 6.

However, there can be no doubt that they possess less power of grasping with their toes than do common mice. The animal is still further incapacitated for movement on inclined surfaces or narrow places by its tendency to move in circles and zigzags. The results of my own experiments indicate that the timidity of the adult is greater than that of the immature animal when it is placed on a bridge 1 or 2 cm. wide at a distance of 20 cm. from the ground. Individuals three weeks old showed less hesitation about trying to creep along such a narrow pathway than did full-grown dancers three or four months old; and these, in turn, were not so timid apparently as an individual one year old. But the younger animals fell off more frequently than did the older ones.

[Ill.u.s.tration: FIGURE 4.--Zoth's photographs of dancers crossing bridges and climbing rods. Reproduced from _Pfluger's Archiv_, Bd. 86.]

[Ill.u.s.tration: FIGURE 5--Tracks of common mouse Reproduced from Alexander and Kreidl's figure in _Pfluger's Archiv_, Bd 82]

[Ill.u.s.tration: FIGURE 6--Tracks of dancing mouse Reproduced from Alexander and Kreidl's figure in _Pfluger's Archiv_ Bd 82]

Additional support for these statements concerning equilibrational ability is furnished by the observations of Kishi (21 p. 482). He built a wooden bridge 60 cm. long, 1 cm. wide at one end, and 1/2 cm. at the other, and supported it at a height of 30 cm. above the ground by posts at the ends.

On this bridge ten dancers were tested. Some attempted to move sidewise, others began to whirl and fell to the ground; only one of the ten succeeded in getting all the way across the bridge on the first trial. The second time he was tested this individual crossed the bridge and found the post; and the third time he crossed the bridge and climbed down the post directly. The others did not succeed in descending the post even after having crossed the bridge safely, but, instead, finally fell to the floor from awkwardness or exhaustion. On the basis of these and other similar observations, Kishi says that the dancer possesses a fair degree of ability to orient and balance itself.