Robert M. Yerkes.

The Mental Life of Monkeys and Apes A Study of Ideational Behavior online

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the piece of banana which was near the opening, then pick up the second
pole, which had not previously been noticed, and after a number of
attempts, push it into and through the box, looking after it and then
pulling it out and looking into the box. Having done this he again came
to my end of the cage, and from there returned to try once more with the
pole which he had first used. He pushed this pole all the way through,
then walked to the other end of the box, looked in and reaching in,
obtained the banana which had been pushed far enough along to be within
his grasp. Figures 29, 30 and 31 of plate V show stages of this process.

Julius had worked twenty-four minutes with relatively little lost time
before succeeding. He had shown almost from the start the idea of using
the pole as an instrument, and his sole difficulty was in making the
pole serve the desired purpose.

The experiment was rendered still more crucial on May 5 by the placing
of the two poles upright in opposite corners of the large cage. For a
few minutes after he entered the cage, Julius did not see them, and his
time was spent pulling and gnawing at the box. Then he discovered one of
the poles, seized it, and pushed it into the box. He tried four times,
then went and got the other pole and pushed it into the opposite end of
the box. Twice he did this, then he returned to the original pole,
bringing the second one with him. He pushed it in beside the first, and
as it happened, shoved the banana out of the opposite end of the box.
But he did not see this, and only after several seconds when he happened
to walk to that end of the box did he discover the banana. The total
time until success was fifteen minutes.

Subsequently the ape became very expert in using the pole to obtain the
banana, and often only a minute or two sufficed for success. It was not
possible for him to direct the stick very accurately, for when he was in
such a position that he could look through the box, he could not work
the stick itself. It was, therefore, always a matter of chance whether
he obtained the banana immediately or only after a number of trials.

Although it is possible that the use of the poles in this experiment was
due to observation of human activities, it seems probable in the light
of what we know of the natural behavior of the anthropoid apes that
Julius would have solved this problem independently of human influence.
It was the expectation of the experimenter that the pole would be used
to push the banana through the box, but as a matter of fact the ape used
it, first of all, to pull the food toward him, thus indicating a natural
tendency which is important in connection with the statements just made.
Subsequently he learned that the banana must be pushed through and
obtained at the farther end of the box. I am not prepared to accept the
solution of this problem as satisfactory evidence of ideation, but I do
know that few observers could have watched the behavior of the orang
utan without being convinced that he was acting ideationally.

_Draw-in Experiment_

An interesting contrast with the box and pole test is furnished by what
may be called the draw-in experiment. This was planned as a simple test
of Julius's ability to use a stick to draw things into his cage from
beyond the wire side. A board was placed, as is shown in figure 34 of
plate VI, with sides to hold a banana, carrot, or some other bit of
food, in position. In the actual test either a carrot or a banana was
placed about two feet from the wire netting and a stick two feet long
was then put into the cage with the ape.

When this situation was first presented to Julius, he looked at the
banana, reached for it, and failing, picked up a bag from the floor of
the cage and tried to push it through the wire mesh toward the banana.
He also used a bit of wire in the same way, but was unable thus to get
the food. As soon as a stick was placed in his cage, he grasped it and
used it in a very definite, although unskillful, way to pull the banana
toward him. He was extremely eager and impatient, but nevertheless
persistent in his efforts, and within five minutes from the beginning of
the first trial, he had succeeded in getting two pieces of banana, using
always his left hand to manipulate the stick. This test was repeated a
number of times with similar results. He had from the first the ability
to use a stick in this way, and the only difficulty with the test as a
means of obtaining evidence of ideational behavior is that the
possibility of imitation of man cannot be certainly excluded.

_Lock and Key Test_

By my assistant it was reported on May 5 that the orang utan had been
seen to place a splinter of wood in a padlock which was used on the
cages and to work with it persistently. It looked very much like
imitation of the human act of using the key, and I therefore planned a
test to ascertain whether Julius could readily and skillfully use a key
or could learn quickly to do so by watching me.

The first test was made on May 15 with a heavy box whose hinged lid was
held securely in position by means of a hasp and a padlock. The key,
which was not more than an inch in length, was fastened to a six inch
piece of wire so that Julius could not readily lose it. With the animal
opposite me, I placed a piece of banana in the box, then closed the lid
and snapped the padlock. I next handed Julius the key. He immediately
laid it on the floor opposite him and began biting the box, rolling it
around, and occasionally biting also at the lock and pulling at it.
During these activities he had pulled the box toward his cage. Now he
suddenly looked up to the position where the banana had been suspended
in the box experiment. Evidently the box had suggested to him the
banana. For thirty minutes he struggled with the box almost
continuously, chewing persistently at the hinges, the hasp, or the lock.
Then he took the key in his teeth and tried to push it into one of the
hinges, then into the crack beneath the lid of the box.

Subsequently I allowed him to see me use the key repeatedly, and as a
result, he came to use it himself now and then on the edge of the box,
but he never succeeded in placing it in the lock, and the outcome of the
experiment was total failure on the part of the animal to unfasten the
lock of his own initiative or to learn to use the key by watching me do
so. I did not make any special attempt to teach him to use the key, but
merely gave him opportunity to imitate, and it is by no means impossible
that he would have succeeded had the key been larger and had the
situation required less accurately coordinated movements. However, it is
fair to say that the evidence of the idea of using the key in the lock
was unconvincing. My assistant's observation was, perhaps, misleading in
so far as it suggested that idea. It may and probably was purely by
accident that the animal used the splinter on the padlock.

2. Skirrl, _Pithecus irus_

_Box Stacking Experiment_

The monkey Skirrl was tested by means of the box stacking experiment
much as Julius had been. On August 23, with a carrot suspended six feet
from the floor of the large cage and three boxes in distant corners, the
animal was admitted and his behavior noted.

The boxes, which were made of light, thin material, ranged in size from
one six inches in its several dimensions to one twenty inches long,
thirteen inches wide, and eleven inches deep. Only by using at least two
of these boxes was it possible for the animal to reach the carrot.

Immediately on admission to the cage, Skirrl began to gnaw at the boxes,
trying with all his might to tear them to pieces. After some thirty
minutes of such effort, interrupted by wanderings about the cage and
attempts to get at the other monkeys, he suddenly went to the largest
box of all, set it up on end almost directly under the carrot, mounted
it, and looked up at the food. It was still beyond his reach and he made
no effort to get it, but instead, he reached from his perch on the big
box for the next smaller box, which was approximately sixteen inches, by
fourteen, by twelve. This he succeeded in pulling toward him, at the
same time raising it slightly from the floor, but his efforts caused the
large box to topple over and he quit work. The experiment was
discontinued after a few minutes, the total period of observation having
been thirty-five minutes.

Skirrl handled the boxes with ease and with evident pleasure and
interest. He also noticed the carrot at various times during the
interval, but his attention was fixed on it only for short periods.

The test was continued on August 24 when, instead of a carrot, a half
banana was used as bait. It was placed only five feet from the floor,
and three boxes were as formerly placed in distant corners of the cage.
When admitted, Skirrl looked at the banana, then pulled one of the boxes
toward it, but instead of mounting, he went to the smallest box and
began to gnaw it. Shortly, he mounted the middle sized box and looked up
toward the banana, but the box was not directly under the bait, and in
any event, it would have been impossible for him to reach it. He next
went to the largest box, gnawed it vigorously, turned it over several
times, and then abandoned it for the middle sized box, from which by
skillful use of his teeth and hands, he quickly tore off one side.

By this time, apparently without very definitely directed effort on the
part of the monkey, all three of the boxes were in the center of the
cage and almost directly beneath the banana. Skirrl climbed up on the
largest box and made efforts to pull the middle sized one up on to it,
the while looking at the banana every few seconds. He did not succeed in
getting the boxes properly placed, and after a time began moving them
about restlessly.

His behavior plainly indicated that hunger was not his chief motive. He
was more interested in playing with things or in working with them than
in eating, and the satisfaction of tearing a box to pieces seemed even
greater than that of food. It is especially noteworthy that when Skirrl
attempts to dismember a box, instead of starting at random, he searches
carefully for a favorable starting point, a place where a board is
slightly loosened or where a slight crack or hole enables him to insert
his hand or use his teeth effectively. Many times during this experiment
he was observed to examine the boxes on all sides in search of some weak
point. If no such weak point were found, he shortly left the box; but if
he did find a favorable spot, he usually succeeded, before he gave up
the attempt, in doing considerable damage to the box.

Following the behavior described above, Skirrl returned to the middle
sized box, placed it on end under the banana, mounted, and looked upward
at the bait, but as it was a few inches beyond his reach, he made no
attempt to get it, but instead, after a few seconds, went to the
smallest box, and finding a weak point, began to tear it to pieces.

Later he rolled what was left of the smallest box close to the other two
boxes, nearly under the banana, and the remainder of his time was spent
gnawing at the boxes and playing with pieces which he had succeeded in
tearing from them. During the remainder of the thirty minute interval of
observation, no further attention was given the bait.

Again, on August 25, the test was tried, but this time with boxes whose
edges had been bound with tin so that it was impossible for the monkey
to destroy them. He spent several minutes searching for a starting point
on the middle sized box, but finding none, he dragged it under the
banana, looked up, mounted the box, but, as previously, did not reach
for the bait because it was beyond his reach. He then played with the
boxes for several minutes. Finally he worked the two smaller boxes to a
position directly under the banana, put the middle sized one on end,
mounted it, and looked at the bait, but again abandoned the attempt
without reaching.

During the thirty minutes of observation he made no definite effort to
place one box upon another. Three times he mounted one or another of the
boxes when it was under the banana or nearly so, but in no case was it
possible for him to reach the bait.

From the above description of this monkey's behavior, it seems fairly
certain that with sufficient opportunity, under strong hunger, he would
ultimately succeed in obtaining the bait by the use of two or more
boxes. For his somewhat abortive and never long continued efforts to
drag two boxes together or to place the one upon the other clearly
enough indicate a tendency which would ultimately yield success. The
possibility of imitation is not excluded, for Skirrl had opportunities
to see Julius and the experimenter handle the boxes.

Because of the other work which seemed more important at the time, this
experiment was not continued further. The results obtained suggest the
desirability of testing thoroughly the ability of monkeys to use objects
as only the anthropoid apes and man have heretofore been thought capable
of using them.

_Box and Pole Experiment_

Skirrl was first tested with the box and pole experiment on August 12.
As in the case of Julius, a half banana was placed in the middle of the
long box and the attention of the monkey was attracted to the bait by
small pieces of carrot placed near each open end. Two poles were placed
near the box on the floor of the cage. When admitted to the cage Skirrl
went almost directly to the ends of the box, took the pieces of carrot
which were in sight, but apparently failed to perceive the bait in the
middle of the box. For a while he played with the locks on the box,
shoved it about, and amused himself with it, showing no interest in
obtaining the food. Later he looked through the box and saw the banana.
He then dragged the box about, apparently trying to get it into his
cage, but he gave no attention to the poles nor did he make any evident
effort to obtain the banana which was easily visible in the center of
the box. The period of observation was only twelve minutes.

On August 24 this experiment was repeated with an important modification
of the apparatus in that the wooden lid of the long box had been
replaced by a wire cover through which the animal could see the bait.
Two poles were as formerly on the floor of the cage, not far from the
box. Skirrl almost immediately noticed the banana and tried to get it by
gnawing at the box. He did not once reach in at the ends of the box, but
he did handle the poles, throwing them about and pounding with them.
There was not the slightest attempt to use them in obtaining the bait.

This experiment was later repeated three times at intervals of a number
of days, but in no case did Skirrl show any tendency to use the poles as
means of obtaining the food.

_Draw-in Experiment_

This also was arranged in the same manner as for Julius, and on each of
five days Skirrl was allowed at least thirty minutes to work for the
bait. Either a banana or a carrot was each day placed on the board well
beyond his reach, and one or two, usually two, small sticks were put
into his cage. Not once during the several periods of observation did
Skirrl make any attempt to use a stick or any other object as a means of
drawing the food to him. Instead, he reached persistently with his arm,
pulled and gnawed at the wires which were in his way, and occasionally
picked up and gnawed or pounded with the sticks in the cage. His
attention every now and then would come back to the food, but it tended
to fluctuate rather rapidly, and in the regular period of observation,
thirty minutes, it is unlikely that he attended to the bait itself for
as much as five minutes. In this respect as well as many others,
Skirrl's behavior contrasts sharply with that of the orang utan.

The results of this experiment indicate the lack in the monkey of any
tendency or ability, apart from training, to use objects as means of
obtaining food. Ways of using objects as tools which apparently are
perfectly natural to the anthropoid apes and to man are rarely employed
by the lower primates.

_Hammer and Nail Test_

One day I happened to observe Skirrl playing with a staple in his cage.
He had found it on the floor where it had fallen and was intently
prodding himself with the sharp points, apparently enjoying the unusual
sensations which he got from sticking the staple into the skin in
various portions of his body, and especially into the prepuce.

A few days later I saw him playing in similar fashion with a nail which
he had found, and still later he was seen to be using a stick to pound
the nail with. This suggested to me the hammer and nail test.

A heavy spike was driven into an old hammer to serve as an
indestructible handle. This hammer, along with a number of large wire
nails and a piece of redwood board, was then placed in the monkey's
cage. Skirrl immediately took up the hammer, grasping the middle of the
handle with his left hand, and with his right hand taking up a nail. He
then sat down on the board, examined the nail, placed the pointed end on
the board, and with well directed strokes by the use of the head of the
hammer drove the nail into the board for the distance of at least an
inch. He then tried to pull it out, but was forced to knock it several
times with the hammer before he could do so.

This performance, during the next few minutes, was repeated several
times with variations. Often the side of the hammer was used instead of
the head, and occasionally, as is shown in figure 8 of plate II, he
seized the hammer well up toward the juncture of the same with the
spike. This figure does justice to the performance. At the moment the
picture was taken, Skirrl's attention had been attracted by a monkey in
an adjoining cage, and he had momentarily looked up from his task, the
while holding nail and hammer perfectly still.

This test was repeated on various days, and almost uniformly Skirrl
showed intense interest in hammer and nails and used them more or less
persistently in the manner described. Occasionally, apparently for the
sake of variety, he would put the blunt end of the nail on the board and
hammer on the point. Again, he would try persistently to drive the nail
into the cement floor, and once by accident, when hammer and nails were
left in his cage over night, he succeeded in making several holes in the
bottom of his sheet iron water pan. There was no doubting the keen
satisfaction which the animal took in this form of activity.

It is impossible to say that the behavior was not imitative of man, for
Skirrl, along with all of the other monkeys, had had abundant
opportunity to see carpenters working. But this much can be said against
the idea of imitation, - no one of the other animals, not excepting the
orang utan, showed any interest whatever in hammer and nails.
Occasionally they would be played with momentarily or pushed about, but
Sobke, Jimmie, Gertie, Julius, although given several opportunities to
exhibit any ability which they might have to drive nails, made not the
least attempt to do so. Evidently we must either conclude that Skirrl
had a peculiarly strong imitative tendency in this direction, or
instead, a pronounced disposition or instinct for the use of objects as
tools. It would seem fair to speak of it as an instinct for mechanical

Under this same heading may be described Skirrl's reactions to such
objects as a handsaw, a padlock, and a water faucet. The saw was given
to him in order to test his ability to use it in human fashion, for if
he could so expertly imitate the carpenter driving nails, it seems
likely that he might also imitate the use of the saw.

As a matter of fact, he showed no tendency to use the saw as we do.
Instead, he persistently played with it in various ways, at first using
it as a sort of plane to scrape with, later often rubbing the teeth over
a board so that they cut fairly well, but never as effectively as in the
hands of a man. After two or three days' practice with the saw, Skirrl
hit upon a method which is, as I understand, used by man in certain
countries, namely, that of placing the saw with the teeth up, holding it
rigid, and then rubbing the object which is to be sawed over it. This
Skirrl succeeded in doing very skillfully, for he would sit down on the
floor of the cage, grip with both feet the handle of the saw, with the
teeth directed upward, then holding either end in his hands, he would
repeatedly rub a stick over the teeth. In this way, of course, he could
make the saw cut fairly well. But still more to his liking was the use
of a spike instead of a stick as an object to rub over the teeth, for
with this he was able to make a noise that would have satisfied even a
small boy.

Further light is shed on the force of the tendency to imitate man by the
saw test. After Skirrl had been given an opportunity to show what he
could do with the tool spontaneously, I demonstrated to him the approved
human way of sawing. Often he would watch my performance intently as
though fascinated by the sound and motion, but when given the tool he
invariably followed his own methods. Although I repeated this test of
imitation several times on three different days, the results were wholly

_Other Activities_

One day as Skirrl was being returned to his own cage by way of the
larger cage, he picked up an unfastened padlock and carried it into the
cage with him. For more than an hour he amused himself almost without
interruption by playing with this lock. The things which he did with it
during that time would require pages to describe. His interest in it was
very similar to that which he had exhibited in hammer and nails, saw,
and indeed any objects which he could play with. The lock was pounded in
various ways, bitten, poked with nails, hooked into the wires of the
cage, used to pull on, pounded with a stick, used to hammer on the floor
of the cage with, and in fine, manipulated in quite as great a variety
of ways as a human being could have discovered. Finally it was hooked to
the side of the cage and snapped shut, and as Skirrl was unable to
dislodge it from this position, he shortly gave up playing with it.

At the end of the large cage and just outside the wire netting was a
faucet to which a hose was usually attached. The valve could be opened
by turning a wheel-shaped hand piece. Both Skirrl and Julius learned to
turn this wheel in order to get water to play with, but usually the
former's strength was not sufficient to turn on the water. The latter
could do it readily. The indications are that both animals profited by
seeing human beings turn on the water. This unquestionably attracted
their attention to the faucet, and probably by playing with it they
accidentally happened upon the proper movement. At any rate, Skirrl's
behavior was significant in this connection, for he would pick up the
hose to see if water were flowing, and if it were not, he would throw it
down, go directly to the faucet, and try to turn the wheel. The
association of the wheel with the desired flow of water was therefore
definitely established. Shall we describe the act as ideational? It
seems the natural thing to do.

3. Sobke, _Pithecus rhesus_

_Box Stacking Experiment_

For this test, in the case of Sobke, three light boxes made of redwood
about one-third of an inch thick were used. The smallest, box 1, was six
inches in each direction, the next larger, box 2, was twelve inches, and
the third, box 3, eighteen inches. As in the case of the other animals,
bait, either banana or carrot, was suspended from the middle of the roof
of the large cage at such distance from the floor as to be reached by
the animal only by the use of the boxes.

The first observations on Sobke were made on June 14. The three boxes
had been placed in the form of a pyramid directly under the banana,
which hung about eighteen inches above the uppermost box. Sobke's
attention while in his cage had been attracted to the bait by seeing me
fastening it in position, but when admitted to the large cage, he simply
glanced at it and then wandered about the cage, picking up bits of food
and struggling to get at the other monkeys. This he did for about five
minutes. He then went to the boxes, placed his hands on top of the
bottom one, but did not climb up on it. A few minutes later he returned
to the box again, climbed up, and readily reached the food, which he ate
while resting on boxes 1 and 2.

I now replaced the bait and gave the monkey a second chance to obtain

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Online LibraryRobert M. YerkesThe Mental Life of Monkeys and Apes A Study of Ideational Behavior → online text (page 15 of 19)