Harvard Psychological Studies Part 71

Group I., representing 163 tests, shows 59 per cent. to the right, with a time interval of 10.8 seconds (_i.e._, the time occupied in turning). Group II. shows 77 per cent. to the right; and so throughout the table there is an increase in the number of returnings to the right. These figures at first sight seem to indicate the formation of a habit, but in such case we would expect, also, a shortening of the time of turning. It may be, however, that the animals were gradually developing a tendency to turn in the easiest manner, and that at the same time they were becoming more accustomed to the unusual position and were no longer so strongly stimulated, when placed on their backs, to attempt to right themselves.

All the subjects were measured and weighed in order to discover whether there were inequalities of the two sides of the body which would make it easier to turn to the one side than to the other. The chelae were measured from the inner angle of the joint of the protopodite to the angle of articulation with the dactylopodite. The carapace was measured on each side, from the anterior margin of the cephalic groove to the posterior extremity of the lateral edge. The median length of the carapace was taken, from the tip of the rostrum to the posterior edge, and the length of the abdomen was taken from this point to the edge of the telson. These measurements, together with the weights of three of the subjects, are given in the accompanying table.

TABLE IV.

MEASUREMENTS OF CRAWFISH.

Chelae. Carapace. Abdomen. Weight.

Left. Right. Left. Right. Median.

No. 2, 9.8 10.0 38.2 38.7 47.3 48.1 29.7 No. 4, 7.7 7.7 33.6 33.8 39.4 42.3 17.7 No. 16, 12.5 12.4 37.6 37.6 46.4 53.2 36.2

Since these measurements indicate slightly greater size on the right it is very probable that we have in this fact an explanation of the tendency to turn to that side.

To test the effect of a change in the conditions, No. 16 was tried on a surface slanted at an angle of 1 12'. Upon this surface the subject was each time so placed that the slant would favor turning to the right. Under these conditions No. 16 gave the following results in two series of tests. In the first series, consisting of 46 turns, 82.6 per cent. were to the right, and the average time for turning was 17.4 seconds; in the second series, of 41 tests, there were 97.5 per cent, to the right, with an average time of 2.5 seconds. We have here an immediate change in the animal's method of re-turning caused by a slight change in the conditions. The subject was now tested again on a level surface, with the result that in 49 tests only 59 per cent.

were toward the right, and the time was 15 seconds.

SUMMARY.

1. Experiments with crawfish prove that they are able to learn simple labyrinth habits. They profit by experience rather slowly, from fifty to one hundred experiences being necessary to cause a perfect a.s.sociation.

2. In the crawfish the chief factors in the formation of such habits are the chemical sense (probably both smell and taste), touch, sight and the muscular sensations resulting from the direction of turning.

The animals are able to learn a path when the possibility of following a scent is excluded.

3. The ease with which a simple labyrinth habit may be modified depends upon the number of experiences the animal has had; the more familiar the animal is with the situation, the more quickly it changes its habits. If the habit is one involving the choice of one of two pa.s.sages, reversal of the conditions confuses the subject much more the first time than in subsequent cases.

4. Crawfish right themselves, when placed on their backs, by the easiest method; and this is found to depend usually upon the relative weight of the two sides of the body. When placed upon a surface which is not level they take advantage, after a few experiences, of the inclination by turning toward the lower side.

* * * * *

THE INSTINCTS, HABITS, AND REACTIONS OF THE FROG.

BY ROBERT MEARNS YERKES.

PART I. THE a.s.sOCIATIVE PROCESSES OF THE GREEN FROG.

I. SOME CHARACTERISTICS OF THE GREEN FROG.

The common green frog, _Rana clamitans_, is greenish or brownish in color, usually mottled with darker spots. It is much smaller than the bull frog, being from two to four inches in length ordinarily, and may readily be distinguished from it by the presence of prominent glandular folds on the sides of the back. In the bull frog, _Rana catesbeana_, these folds are very small and indistinct. The green frog is found in large numbers in many of the ponds and streams of the eastern United States, and its peculiar rattling croak may be heard from early spring until fall. It is more active, and apparently quicker in its reactions, than the bull frog, but they are in many respects similar in their habits. Like the other water frogs it feeds on small water animals, insects which chance to come within reach and, in times of famine, on its own and other species of frogs. The prey is captured by a sudden spring and the thrusting out of the tongue, which is covered with a viscid secretion. Only moving objects are noticed and seized; the frog may starve to death in the presence of an abundance of food if there is no movement to attract its attention.

Most green frogs can be fed in captivity by swinging pieces of meat in front of them, and those that will not take food in this way can be kept in good condition by placing meat in their mouths, for as soon as the substance has been tasted swallowing follows.

The animals used for these experiments were kept in the laboratory during the whole year in a small wooden tank. The bottom of this tank was covered with sand and small stones, and a few plants helped to purify the water. An inch or two of water sufficed; as it was not convenient to have a constant stream, it was changed at least every other day. There was no difficulty whatever in keeping the animals in excellent condition.

Of the protective instincts of the green frog which have come to my notice during these studies two are of special interest: The instinctive inhibition of movement under certain circ.u.mstances, and the guarding against attack or attempt to escape by 'crouching' and 'puffing.' In nature the frog ordinarily jumps as soon as a strange or startling object comes within its field of vision, but under certain conditions of excitement induced by strong stimuli it remains perfectly quiet, as do many animals which feign death, until forced to move. Whether this is a genuine instinctive reaction, or the result of a sort of hypnotic condition produced by strong stimuli, I am not prepared to say. The fact that the inhibition of movement is most frequently noticed after strong stimulation, would seem to indicate that it is due to the action of stimuli upon the nervous system.

What appears to be an instinctive mode of guarding against attack and escaping an enemy, is shown whenever the frog is touched about the head suddenly, and sometimes when strong stimuli are applied to other parts of the body. The animal presses its head to the ground as if trying to dive or dodge something, and inflates its body. This kind of action is supposed to be a method of guarding against the attack of snakes and other enemies which most frequently seize their prey from the front. It is obvious that by pressing its head to the ground the frog tends to prevent any animal from getting it into its mouth, and in the few instants' delay thus gained it is able to jump. This is just the movement necessary for diving, and it is probable that the action should be interpreted in the light of that instinctive reflex.

The 'puffing' also would seem to make seizure more difficult. Another fact which favors this interpretation is that the response is most commonly given to stimuli which seem to come from the front and which for this reason could not easily be escaped by a forward jump, while if the stimulus is so given that it appears to be from the rear the animal usually jumps away immediately. We have here a complex protective reaction which may be called a forced movement. It is, so far as one can see, very much like many reflexes, although it does not occur quite so regularly.

The machine-like accuracy of many of the frog's actions gives a basis for the belief that the animal is merely an automaton. Certain it is that one is safe in calling almost all the frog's actions reflex or instinctive. During months of study of the reaction-time of the frog I was constantly impressed with the uniformity of action and surprised at the absence of evidences of profiting by experience. In order to supplement the casual observations on the a.s.sociations of the green frog made in the course of reaction-time experiments, the tests described in this paper were made. They do not give a complete view of the a.s.sociative processes, but rather such a glimpse as will enable us to form some conception of the relation of the mental life of the frog to that of other animals. This paper presents the outlines of work the details of which I hope to give later.

II. EXPERIMENTAL STUDY OF HABITS.

A. The Chief Problems for which solutions were sought in the following experimental study were: (1) Those of a.s.sociability in general, its characteristics, and the rapidity of learning; (2) of discrimination, including the parts played in a.s.sociative processes by the different senses, and the delicacy of discrimination in each; (3) of the modifiability of a.s.sociational reactions and general adaptation in the frog, and (4) of the permanency of a.s.sociations.

B. Simple a.s.sociations, as studied in connection with reaction-time work, show that the green frog profits by experience very slowly as compared with most vertebrates. The animals have individual peculiarities in reaction which enable one in a short time to recognize any individual. To these characteristic peculiarities they stick tenaciously. One, for instance, always jumps upward when strongly stimulated; another has a certain corner of the tank in which it prefers to sit. Their habits are remarkably strong and invariable, and new ones are slowly formed. While using a large reaction box I noticed that the frogs, after having once escaped from an opening which could be made by pushing aside a curtain at a certain point in the box, tended to return to that place as soon as they were again put into the box. This appeared to be evidence of an a.s.sociation; but the fact that such stimuli as light and the relation of the opening to the place at which the animals were put into the box might in themselves be sufficient to direct the animals to this point without the help of any a.s.sociations which had resulted from previous experience, makes it unsatisfactory. In addition to the possibility of the action being due to specific sensory stimuli of inherent directive value, there is the chance of its being nothing more than the well-known phenomenon of repet.i.tion. Frogs, for some reason, tend to repeat any action which has not proved harmful or unpleasant.

For the purpose of more carefully testing this kind of a.s.sociation, a small box with an opening 15 cm. by 10 cm. was arranged so that the animal could escape from confinement in it through the upper part of the opening, the lower portion being closed by a plate of gla.s.s 10 cm.

by 10 cm., leaving a s.p.a.ce 5 cm. by 10 cm. at the top. One subject placed in this box escaped in 5 minutes 42 seconds. After 5 minutes'

rest it was given another trial, and this time got out in 2 minutes 40 seconds. The times for a few subsequent trials were: Third, 1 minute 22 seconds; fourth, 4 minutes 35 seconds; fifth, 2 minutes 38 seconds; sixth, 3 minutes 16 seconds. Although this seems to indicate some improvement, later experiments served to prove that the frogs did not readily form any a.s.sociations which helped them to escape. They tended to jump toward the opening because it was light, but they did not learn with twenty or thirty experiences that there was a gla.s.s at the bottom to be avoided. Thinking that there might be an insufficient motive for escape to effect the formation of an a.s.sociation, I tried stimulating the subject with a stick as soon as it was placed in the box. This frightened it and caused violent struggles to escape, but instead of shortening the time required for escape it greatly lengthened it. Here was a case in which the formation of an a.s.sociation between the appearance of the upper part of the clear s.p.a.ce and the satisfaction of escape from danger would have been of value to the frog, yet there was no evidence of adaptation to the new conditions within a reasonably short time. There can be little doubt that continuation of the training would have served to establish the habit. This very clearly shows the slowness of adaptation in the frog, in contrast with the rapidity of habit formation in the cat or chick; and at the same time it lends additional weight to the statement that instinctive actions are all-important in the frog's life. A few things it is able to do with extreme accuracy and rapidity, but to this list new reactions are not readily added. When put within the box described, an animal after having once escaped would sometimes make for the opening as if it knew perfectly the meaning of the whole situation, and yet the very next trial it would wander about for half an hour vainly struggling to escape.

A considerable number of simple experiments of this kind were tried with results similar to those just given. The frog apparently examines its surroundings carefully, and just when the observer thinks it has made itself familiar with the situation it reacts in such a way as to prove beyond doubt the absence of all adaptation. In all these experiments it should be said, for the benefit of any who may be trying similar work, that only animals of exceptional activity were used. Most green frogs when placed in the experiment box either sit still a great part of the time or jump about for only a short time. It is very important for studies of this kind, both on account of time saving and the obtaining of satisfactory records, to have animals which are full of energy and eager to escape when in confinement. By choosing such subjects one may pretty certainly avoid all unhealthy individuals, and this, it seems to me, counterbalances the disadvantage of taking animals which may be unusually quick in learning.

C. Complex a.s.sociations.

1. _Labyrinth Habits_.--A more thorough investigation of the a.s.sociative processes, sensory discrimination and the permanency of impressions has been made by the labyrinth method. A wooden box, 72 cm. long, 28 cm. wide and 28 cm. deep, whose ground plan is represented by Fig. 1, served as the framework for a simple labyrinth.

At one end was a small covered box, _A_, from which the frog was allowed to enter the labyrinth. This entrance pa.s.sage was used in order that the animal might not be directed to either side by the disturbance caused by placing it in the box. _E_, the entrance, marks a point at which a choice of directions was necessary. _P_ is a movable part.i.tion which could be used to close either the right or the left pa.s.sage. In the figure the right is closed, and in this case if the animal went to the right it had to turn back and take the left pa.s.sage in order to get out of the box. A series of interrupted electrical circuits, _IC_, covered the bottom of a portion of the labyrinth; by closing the key, _K_, the circuit could be made whenever a frog rested upon any two wires of the series. When the frog happened to get into the wrong pa.s.sage the key was closed and the animal stimulated. This facilitated the experiment by forcing the animal to seek some other way of escape, and it also furnished material for an a.s.sociation. Having pa.s.sed through the first open pa.s.sage, which for convenience we may know as the entrance pa.s.sage, the animal had to choose again at the exit. Here one of the pa.s.sages was closed by a plate of gla.s.s (in the figure the left) and the other opened into a tank containing water. The box was symmetrical and the two sides were in all respects the same except for the following variable conditions.

At the entrance the part.i.tion on one side changed the appearance, as it was a piece of board which cut off the light. On either side of the entrance there were grooves for holding card-boards of any desired color. The letters _R, R_ mark sides which in this case were covered with red; _W, W_ mark white s.p.a.ces. These pieces of cardboard could easily be removed or shifted at any time. At the exit the gla.s.s plate alone distinguished the sides, and it is not likely that the animals were able to see it clearly. We have thus at the entrance widely differing appearances on the two sides, and at the exit similarity.

The opening from _A_ into the large box was provided with a slide door so that the animal could be prevented from returning to _A_ after entering the labyrinth. The part.i.tions and the triangular division at the entrance extended to the top of the box, 28 cm., so that the animals could not readily jump over them.

[Ill.u.s.tration: FIG. 1. Ground Plan of Labyrinth. _A_, small box opening into labyrinth; _E_, entrance of labyrinth; _T_, tank containing water; _G_, gla.s.s plate closing one pa.s.sage of exit; _P_, part.i.tion closing one pa.s.sage at entrance; _IC_, interrupted electrical circuit; _C_, cells; _K_, key in circuit; _RR_, red cardboard; _WW_, white cardboard. Scale 1/12.]

The experiments were made in series of ten, with ten-minute intervals between trials. In no case was more than one series a day taken, and wherever a day was missed the fact has been indicated in the tables.

The only motive of escape from the box depended upon was the animal's desire to return to the water of the tank and to escape from confinement in the bright light of the room. The tank was one in which the frogs had been kept for several months so that they were familiar with it, and it was as comfortable a habitat as could conveniently be arranged. Usually the animals moved about almost constantly until they succeeded in getting out, but now and then one would remain inactive for long intervals; for this reason no record of the time taken for escape was kept. On account of the great amount of time required by experiments of this kind I have been unable to repeat this series of experiments _in toto_ on several animals in order to get averages, but what is described for a representative individual has been proved normal by test observations on other animals. There are very large individual differences, and it may well be that the subject of the series of experiments herein described was above the average in ability to profit by experience. But, however that may be, what is demonstrated for one normal frog is thereby proved a racial characteristic, although it may be far from the mean condition.

Before beginning training in the labyrinth, preliminary observations were made to discover whether the animals had any tendencies to go either to the right or to the left. When the colored cardboards were removed it was found that there was usually no preference for right or left. In Table I. the results of a few preliminary trials with No. 2 are presented. For these the colors were used, but a tendency to the right shows clearly. Trials 1 to 10 show choice of either the right or the red throughout; that it was partly both is shown by trials 11 to 30, for which the colors were reversed. This individual has therefore, to begin with, a tendency to the right at the entrance. At the exit it went to the right the first time and continued so to do for several trials, but later it learned by failure that there was a blocked pa.s.sage as well as an open one. In the tables the records refer to choices. It was useless to record time or to lay much stress upon the course taken, as it was sometimes very complicated; all that is given, therefore, is the action in reference to the pa.s.sages. _Right_ in every case refers to the choice of the open way, and _wrong_ to the choice of the blocked pa.s.sage. The paths taken improved steadily in that they became straighter. A few representative courses are given in this report. Usually if the animal was not disturbed a few jumps served to get it out of the labyrinth.

TABLE I.

PRELIMINARY TRIALS WITH FROG NO. 2.

Trials. Red on Right. White on Left.

1 to 10 10 times to red 0

Red on Left. White on Right.