The Mental Life of Monkeys and Apes - Robert M. Yerkes
3. Julius, _Pongo pygmaeus_
_Problem 1. First at the Left End_
The orang utan, Julius, was gentle, docile, and friendly with the
experimenter throughout the period of investigation. He at no time
showed inclination to bite and could be handled safely. As contrasted
with Skirrl and even with Sobke, he adapted himself to the
multiple-choice apparatus very promptly, and only slight effort on the
part of the observer was necessary to prepare him, by preliminary
trials, for the regular experiments. But in order to facilitate work, he
was familiarized with the apparatus by means of regular route training
and feeding in the several boxes from April 5 to April 9.
On April 10 the apparatus was painted white as has been stated
previously, and on the following Monday, April 12, Julius when again
introduced to it gave no indications of fear, uneasiness, or dislike,
but worked as formerly, making his round trips quickly and eagerly
entering any box which happened to be open, in order to obtain the
reward of food.
The regular experimentation was undertaken on April 13, and the results
of the first series of trials with Julius are sharply contrasted with
those obtained with the monkeys in that fewer choices were necessary.
Instead of the expected ratio of right to wrong first choices, 1 to 2.5,
the orang utan gave a ratio of 1 to 1. An additional markedly different
result from that obtained with the monkeys is indicated below in the
total time required for a series of trials. As examples, the data for
the first, second, fifth, and tenth series are presented.
TIME FOR SERIES OF TRIALS
1st series 2nd series 5th series 10th series
Skirrl 35 min. 20 min. 14 min. 10 min.
Sobke 14 " 17 " 10 " 9 " (8th series)
Julius 12 " 11 " 14 " 9 "
It is also noteworthy that Julius in the presence of visitors or under
other unusual conditions worked steadily and well, whereas the monkeys,
and especially Sobke, tended to be distracted and often refused to work
at all.
Almost from the beginning of his work on problem l, Julius began to
develop the tendency to enter immediately the open door nearest the
starting point. In case the group of open doors lay to the right of the
middle of the apparatus, this method naturally yielded success; whereas
if the group included doors to the left of the middle, it resulted in
failure. Obviously it was a most unsatisfactory method, and although it
enabled him to make more right than wrong first choices, it prevented
him from increasing the number of right choices, and as table 1
indicates, it maintained the ratio of 1 right to .67 wrong first choices
for eight successive days.
On April 23 a break occurred in which the number of correct choices was
reduced from six to five. Julius worked very rapidly and with almost no
hesitation in choosing. My notes record "he seems to miss the point
wholly. It is doubtful whether the punishment is sufficiently severe."
At this time he was being punished by thirty seconds confinement in each
wrong box, the interval having been held fairly steadily from the first
series of experiments. On April 26 it was increased to sixty seconds, in
an effort to break him of the habit of choosing the "nearest" door. But
he became extremely restless under the longer confinement and tried his
best to raise the entrance and exit doors. Since there was at this time
no mechanism for locking them when closed, it was difficult for the
experimenter to prevent him from escaping by way of the entrance door or
from raising the exit door sufficiently to obtain the food. Indeed, the
longer confinement worked so unsatisfactorily that on the following day
I substituted for it the punishment of forcing him to raise the entrance
door of the wrong box in order to escape for a new choice. He was
rewarded with food in the alleyway H, beside door 15 (figure 17), only
when he chose correctly on first attempt.
This method discouraged him extremely and proved wasteful of time.
Consequently, in a second series on the same date return was made to the
former method, and he was rewarded with food whenever he found the right
box. But on April 28, the two methods were again employed, the first in
the initial series and the second in a final series of trials. The
animal's persistent attempts to raise the doors gave the experimenter so
much trouble that on April 29 barbed wire was nailed over the windows of
the entrance doors with the hope that it might prevent him from working
at them. But he quickly learned to place his fingers between the barbs
and raise the doors as effectively as ever.
On April 30 the reward of food was given only when the first choice was
that of the right box and in that event it was placed in the alleyway H
as stated above.
As it seemed absolutely essential to break the unprofitable habit of
choosing the nearest door, on May 3 a new series of settings was
presented, in which only the doors to the left of the middle of the row
of nine boxes were used as right doors. That is, in this new series,
doors 1 to 4 occur as right doors; 5 to 9 do not. As punishment for
wrong choices on this date, Julius was confined in the wrong box from
one to five minutes. It was difficult to keep him in, but by means of
cords which had been attached to the doors, this was successfully
accomplished. Yet another and slightly different series of settings was
employed on May 4, and this, proving satisfactory, was continued in use
until the end of the experiment, with punishment ranging from sixty to
one hundred and twenty seconds for each mistake.
Naturally the modification of settings introduced May 3 greatly
increased the proportion of wrong first choices. Indeed, as appears in
table 8, the ratio of right to wrong immediately changed from 1:0.67 to
1:4.00. Between May 3 and May 10, no steady and consistent improvement
in method or in the number of correct first choices occurred, and on the
last named date, Julius chose correctly only three times in his ten
trials. At this time there was, as my notes record, no satisfactory
indication of progress, and the status of the experiment seemed
extremely unsatisfactory in as much as in spite of the experimenter's
best efforts to break up the habit of choosing the nearest door, the
orang utan still persisted, to a considerable extent, in the use of this
method. The only encouraging feature of the results was an evident
tendency to choose somewhat nearer the left end of a group than
previously.
TABLE 8
Results for Orang utan in Problem 1
========+===========+==============+==============+==============+==============+==============+==============+==============+==============+==============+==============+===+===+===+===+========
| No. | S.1 | S.2 | S.3 | S.4 | S.5 | S.6 | S.7 | S.8 | S.9 | S.10 | | | | | Ratio
Date | of | | | | | | | | | | | R | W | R | W | of
| trials | 1.2.3 | 8.9 | 3.4.5.6.7 | 7.8.9 | 2.3.4.5.6 | 6.7.8 | 5.6.7 | 4.5.6.7.8 | 7.8.9 | 1.2.3 | | | | | R to W
--------+-----------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+---+---+---+---+--------
| | | | | | | | | | | | | | | |
April | | | | | | | | | | | | | | | |
13 | 1- 10 | 3.1 | 8 | 4.3 | 7 | 4.2 | 7.6 | 5 | 4 | 7 | 3.1 | 5 | 5 | 5 | 5 | 1:1.00
14 | 11- 20 | 3.2.1 | 8 | 4.3 | 7 | 4.4.2 | 6 | 5 | 4 | 7 | 3.1 | 6 | 4 | 6 | 4 | 1:0.67
15 | 21- 30 | 3.2.1 | 8 | 4.3 | 7 | 4.5.5.2 | 6 | 5 | 4 | 7 | 3.1 | 6 | 4 | 6 | 4 | 1:0.67
16 | 31- 40 | 3.1 | 8 | 4.3 | 7 | 4.2 | 6 | 5 | 4 | 7 | 3.2.1 | 6 | 4 | 6 | 4 | 1:0.67
17 | 41- 50 | 3.2.1 | 8 | 4.3 | 7 | 4.2 | 6 | 5 | 4 | 7 | 3.1 | 6 | 4 | 6 | 4 | 1:0.67
19 | 51- 60 | 3.1 | 8 | 4.3 | 7 | 4.2 | 6 | 5 | 4 | 7 | 3.1 | 6 | 4 | 6 | 4 | 1:0.67
20 | 61- 70 | 2.1 | 8 | 4.3 | 7 | 5.3.2 | 6 | 5 | 4 | 7 | 3.1 | 6 | 4 | 6 | 4 | 1:0.67
21 | 71- 80 | 3.1 | 8 | 4.3 | 7 | 5.4.3.2 | 6 | 5 | 4 | 7 | 3.2.1 | 6 | 4 | 6 | 4 | 1:0.67
22 | 81- 90 | 3.1 | 8 | 5.3 | 7 | 6.3.2 | 6 | 5 | 4 | 7 | 3.2.1 | 6 | 4 | 6 | 4 | 1:0.67
23 | 91-100 | 3.2.1 | 8 | 5.3 | 7 | 4.3.2 | 6 | 5 | 5.4 | 7 | 3.2.1 | 5 | 5 | 5 | 5 | 1:1.00
24 | 101-110 | 3.2.1 | 8 | 4.3 | 7 | 4.3.2 | 6 | 5 | 4 | 7 | 3.1 | 6 | 4 | 6 | 4 | 1:0.67
26 | 111-120 | 3.1 | 8 | 4.3 | 7 | 4.3.2 | 6 | 5 | 5.4 | 7 | 3.1 | 5 | 5 | 5 | 5 | 1:1.00
27 | 121-130 | 3.2.1 | 8 | 4.3 | 7 | 4.3.2 | 6 | 6.5 | 5.8.6.4 | 7 | 3.3.3.1 | 4 | 6 | | |
" | 131-140 | 3.1 | 8 | 4.3 | 7 | 4.3.2 | 6 | 5 | 4 | 7 | 3.2.1 | 6 | 4 |10 |10 | 1:1.00
28 | 141-150 | 3.2.1 | 8 | 3 | 7 | 5.4.2 | 6 | 5 | 4 | 7 | 3.1 | 7 | 3 | | |
" | 151-160 | 3.1 | 8 | 3 | 7 | 3.2 | 6 | 5 | 4 | 7 | 3.2.1 | 7 | 3 |14 | 6 | 1:0.43
29 | 161-170 | 3.1 | 8 | 4.3 | 7 | 4.3.2 | 6 | 5 | 4 | 7 | 3.2.1 | 6 | 4 | | |
" | 171-180 | 3.2.1 | 8 | 4.3 | 7 | 4.2 | 6 | 5 | 4 | 7 | 3.2.1 | 6 | 4 |12 | 8 | 1:0.67
| | | | | | | | | | | | | | | |
30 | 181-190 | 3.1 | 8 | 4.3 | 7 | {4.5.6.4 | 6 | 5 | 4 | 7 | 3.1 | 6 | 4 | | |
| | | | | | {5.6.4.2 | | | | | | | | | |
| | | | | | | | | | | | | | | |
" | 191-200 | 3.1 | 8 | 4.5.6.7.3 | 7 | 4.5.3.2 | 6 | 5 | 4 | 7 | 3.2.1 | 6 | 4 |12 | 8 | 1:0.67
May | | | | | | | | | | | | | | | |
1 | 201-210 | 3.1 | 8 | 4.3 | 7 | 3.2 | 6 | 5 | 4 | 7 | 3.1 | 6 | 4 | | |
" | 211-220 | 3.2.1 | 8 | 4.3 | 7 | 4.2 | 6 | 5 | 4 | 7 | 3.1 | 6 | 4 |12 | 8 | 1:0.67
--------+-----------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+---+---+---+---+--------
| | | | 2.3.4.5 | | | | | | | 2.3.4.5 | | | | |
| | 1.2.3 | 3.4.5.6.7 | 6.7.8 | 1.2.3 | 3.4.5.6.7 | 4.5.6.7.8.9 | 2.3.4.5 | 1.2.3 | 4.5.6.7.8.9 | 6.7.8.9 | | | | |
--------+-----------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+---+---+---+---+--------
3 | 221-230 | 3.1 | 4.3 | 4.2 | 3.1 | 4.3 | 4 | 4.3.2 | 3.1 | 4 | 4.3.2 | 2 | 8 | 2 | 8 | 1:4.00
--------+-----------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+---+---+---+---+--------
| | | | 2.3.4.5 | 4.5.6.7 | | | | | 2.3.4.5 | | | | | |
| | 1.2.3 | 3.4.5.6.7 | 6.7.8 | 8.9 | 2.3.4.5 | 3.4.5.6 | 1.2.3 | 4.5.6.7.8 | 6.7.8.9 | 1.2.3 | | | | |
--------+-----------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+---+---+---+---+--------
4 | 231-240 | 3.2.1 | 4.3 | 4.2 | 4 | 4.3.2 | 4.3 | 3.2.1 | 4 | 3.2 | 3.2.1 | 2 | 8 | 2 | 8 | 1:4.00
5 | 241-250 | 2.1 | 3 | 3.2 | 4 | 3.2 | 4.3 | 3.2.1 | 4 | 3.2 | 3.2.1 | 3 | 7 | 3 | 7 | 1:2.33
6 | 251-260 | 2.1 | 3 | 2 | 4 | 3.2 | 3 | 2.1 | 4 | 3.2 | 2.1 | 5 | 5 | 5 | 5 | 1:1.00
7 | 261-270 | 2.1 | 3 | 3.2 | 4 | 3.2 | 3 | 2.1 | 4 | 4.2 | 2.1 | 4 | 6 | 4 | 6 | 1:1.50
8 | 271-280 | 2.1 | 4.3 | 4.3.2 | 4 | 3.2 | 4.3 | 3.1 | 4 | 3.2 | 2.1 | 2 | 8 | 2 | 8 | 1:4.00
10 | 281-290 | 1 | 4.3 | 4.2 | 4 | 3.2 | 4.3 | 2.1 | 4 | 3.2 | 2.1 | 3 | 7 | 3 | 7 | 1:2.33
11 | 291-300 | 1 | 3 | 2 | 4 | 2 | 3 | 1 | 4 | 2 | 1 |10 | 0 |10 | 0 | 1:0.00
--------+-----------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+---+---+---+---+--------
| | | | | | | | | | | | | | | |
| | 1.2.3 | 8.9 | 3.4.5.6.7 | 7.8.9 | 2.3.4.5.6 | 6.7.8 | 5.6.7 | 4.5.6.7.8 | 7.8.9 | 1.2.3 | | | | |
--------+-----------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+---+---+---+---+--------
12 | 301-310 | 1 | 8 | 3 | 7 | 2 | 6 | 5 | 4 | 7 | 1 |10 | 0 |10 | 0 | 1:0.00
========+===========+==============+==============+==============+==============+==============+==============+==============+==============+==============+==============+===+===+===+===+========
A series of correct first choices was obtained on May 11, greatly to the
surprise of the experimenter, for no indication had previously appeared
of this approaching solution of the problem. It seemed possible,
however, that the successes were accidental, and it was anticipated that
in a control series Julius would again make mistakes. But on the
following day, May 12, the presentation of the original series of ten
settings, which, of course, differed radically from the settings used
from May 4 to May 11 was responded to promptly, readily, and without a
single mistake. Julius had solved his problem suddenly and, in all
probability, ideationally.
Only three reactive tendencies or methods appeared during Julius's work
on this problem: (a) choice of the open door nearest to the starting
point (sometimes the adjacent boxes were entered); (b) a tendency to
avoid the "nearest" door and select instead one further toward the left
end of the group; (c) direct choice of the first door on the left.
The curve of learning plotted from the daily wrong choices and presented
in figure 18, had it been obtained with a human subject, would
undoubtedly be described as an ideational, and possibly even as a
rational curve; for its sudden drop from near the maximum to the base
line strongly suggests, if it does not actually prove, insight.
Never before has a curve of learning like this been obtained from an
infrahuman animal. I feel wholly justified in concluding from the
evidences at hand, which have been presented as adequately as is
possible without going into minutely detailed description, that the
orang utan solved this simple problem ideationally. As a matter of fact,
for the solution he required about four times the number of trials which
Sobke required and twice as many as were necessary for Skirrl. Were we
to measure the intelligence of these three animals by the number of
trials needed in problem 1, Sobke clearly would rank first, Skirrl
second, and Julius last of all. But other facts clearly indicate that
Julius is far superior to the monkeys in intelligence. We therefore must
conclude that _where very different methods of learning appear, the
number of trials is not a safe criterion of intelligence._ The
importance of this conclusion for comparative and genetic psychology
needs no emphasis.
_Problem 2. Second from the Right End_
Julius was given four days' rest before being presented with problem 2.
He was occasionally fed in the apparatus, but regular continuation of
training was not necessary to keep him in good form. During this rest
interval, locks were attached to the doors of the apparatus so that the
experimenter by moving a lever directly in front of him could fasten
either one or both of the doors of a given box by a single movement. On
May 13 Julius was given opportunity to obtain food from each of the
boxes in turn, and trial of the locks was made in order to familiarize
him with the new situation. He very quickly discovered that the doors
could not be raised when closed, and after two days of preliminary work,
he practically abandoned his formerly persistent efforts to open them.
The locks worked satisfactorily from a mechanical point of view as well
as from that of the adaptation of the animal to the modified situation.
Problem 2 was regularly presented for the first time on May 17, on which
day a single series was given. The period of punishment adopted was
twenty seconds, and for each successful choice a small piece of banana
was given as a reward. After the first trial in this series, in which
Julius repeatedly entered the first box at the left, that is box 7,
there was but slight tendency to reenter the first box at the left of
the group. Instead, Julius developed the method of moving box by box
toward the right end of the group. The choices were made promptly, and
their systematic character enabled the animal to obtain his reward
fairly quickly, in spite of the large number of mistakes.
In the second series, the orang utan developed the interesting trick of
quickly dodging out of the wrong box before the experimenter could lower
the door behind him. This he did only after having been punished for
many wrong choices to the point of discouragement. The trick was easily
broken up by the sudden lowering of the entrance door as soon as he had
passed under it.
There appeared on May 21 an unfavorable physical condition which
manifested itself, first of all through the eyes which appeared dull and
bloodshot. On the following day they were inflamed and the lids nearly
closed. Julius refused to eat, and experimentation was impossible. Until
June 2 careful treatment and regulation of diet was necessary. He passed
through what at the time seemed a rather startling condition, but
rapidly regained his usual good health, and on June 3, although somewhat
weak and listless, he again worked fairly steadily.
Since it was now possible to lock the doors and confine the animal for
any desired period, on June 5 the interval of punishment was made sixty
seconds, and a liberal quantity of banana, beet, or carrot was offered
as reward. No increase in the number of successful choices appeared, and
Julius showed discouragement. Sawdust had been strewn on the floor, and
in the intervals between trials as well as during confinement in wrong
boxes, he took to playing with the sawdust. He would take it up in one
hand and pour it from hand to hand until all had slipped through his
fingers, then he would scrape together another handful and go through
the same process. Often he became so intent on this form of amusement
that even when the exit door was raised, he would not immediately go to
get the food.
The reactive tendencies which appeared in the work on problem 2 will now
be presented in order, since I shall have to refer to them repeatedly,
and the list will be more useful to the reader at this point than at the
conclusion of the presentation of daily results. The following is not an
exhaustive list but includes only the most important and conspicuous
tendencies or methods together with the dates on which they were most
apparent.
(a) May 17, choice of first box at left of group or near it, then the
next in order, and so on, until the second from the right was reached.
This method with irregularities and certain definite skipping was used
at various times, sometimes over periods of several days, during the
course of the work.
(b) June 3, preference for number 3 and number 4 developed immediately
after the orang utan's illness and when he was working rather
listlessly.
On June 9 and 10, the original tendency (a) reappeared and persisted for
a number of series.
(c) June 14, a tendency to choose the box at or near the right end of a
group, and then the one next to it. In connection with this tendency,
which of course required only two choices in any given trial, interest
in playing with the sawdust on the floor developed.
Again on June 21, the animal returned to the use of tendency (a).
(d) June 29, movement to box at right end of group, hesitation before
it, and turning through a complete circle so that the second box from
the right was faced. This, the correct box, was often promptly entered.
This method, if persisted in, would obviously have yielded solution of
the problem.
(e) July 5, approach to and pretense to enter the box next to the right
end (right one), and then choice of some other box. This _feint_ is
peculiarly interesting, and its origin and persistence are difficult to
account for.
(f) In connection with the tendency to pretend that he was going to
enter the second box from the right end, Julius developed also the
tendency to turn around in front of the box at the right end, starting
sometimes to back into it, and then to enter, instead, the box second
from the end.