The Mental Life of Monkeys and Apes - Robert M. Yerkes
The Mental Life of Monkeys and Apes:
A Study of Ideational Behavior
ROBERT M. YERKES
Harvard University
BEHAVIOR MONOGRAPHS
Volume 3, Number 1, 1916
Serial Number 12
Edited by JOHN B. WATSON
The Johns Hopkins University
WITH SIX PLATES AND FIVE TEXT FIGURES
CONTENTS
I. Interests, opportunity and materials
II. Observational problems and methods
III. Results of multiple-choice experiments:
1. Skirrl, _Pithecus irus_
2. Sobke, _Pithecus rhesus_
3. Julius, _Pongo pygmaeus_
IV. Results of supplementary tests of ideational behavior:
1. Julius, _Pongo pygmaeus_:
Box stacking experiment
Box and pole experiment
Draw-in experiment
Lock and key test
2. Skirrl, _Pithecus irus_:
Box stacking experiment
Box and pole experiment
Draw-in experiment
Hammer and nail test
Other activities
3. Sobke, _Pithecus rhesus_:
Box stacking experiment
Draw-in experiment
Box and pole experiment
Other activities
V. Miscellaneous observations:
1. Right- and left-handedness
2. Instinct and emotion:
Maternal instinct
Fear
Sympathy
VI. Historical and critical discussion of ideational behavior in
monkeys and apes:
1. Evidences of ideation in monkeys
2. Evidences of ideation in apes
VII. Provision for the study of the primates and especially the monkeys
and anthropoid apes
VIII. Bibliography
I
INTERESTS, OPPORTUNITY AND MATERIALS
Two strong interests come to expression in this report: the one in the
study of the adaptive or ideational behavior of the monkeys and the
apes; and the other in adequate and permanent provision for the thorough
study of all aspects of the lives of these animals. The values of these
interests and of the tasks which they have led me to undertake are so
widely recognized by biologists that I need not pause to justify or
define them. I shall, instead, attempt to make a contribution of fact on
the score of each interest.
While recognizing that the task of prospecting for an anthropoid or
primate station may in its outcome prove incomparably more important for
the biological and sociological sciences and for human welfare than my
experimental study of ideational behavior, I give the latter first place
in this report, reserving for the concluding section an account of the
situation regarding our knowledge of the monkeys, apes, and other
primates, and a description of a plan and program for the thorough-going
and long continued study of these organisms in a permanent station or
research institute.
In 1915, a long desired opportunity came to me to devote myself
undividedly to tasks which I have designated above as "prospecting" for
an anthropoid station and experimenting with monkeys and apes. First of
all, the interruption of my academic duties by sabbatical leave gave me
free time. But in addition to this freedom for research, I needed
animals and equipment. These, too, happily, were most satisfactorily
provided, as I shall now describe.
When in 1913, while already myself engaged in seeking the establishment
of an anthropoid station, I heard of the founding of such an institution
at Orotava, Tenerife, the Canary Islands, I immediately made inquiries
of the founder of the station, Doctor Max Rothmann of Berlin, concerning
his plans (Rothmann, 1912).[1] As a result of our correspondence, I was
invited to visit and make use of the facilities of the Orotava station
and to consider with its founder the possibility of cooeperative work
instead of the establishing of an American station. This invitation I
gratefully accepted with the expectation of spending the greater part of
the year 1915 on the island of Tenerife. But the outbreak of the war
rendered my plan impracticable, while at the same time destroying all
reasonable ground for hope of profitable cooeperation with the Germans in
the study of the anthropoids. In August, 1915, Doctor Rothmann died.
Presumably, the station still exists at Orotava in the interests of
certain psychological and physiological research. So far as I know,
there are as yet no published reports of studies made at this station.
It seems from every point of view desirable that American psychologists
should, without regard to this initial attempt of the Germans to provide
for anthropoid research, further the establishment of a well equipped
American station for the study not only of the anthropoid apes but of
all of the lower primates.
[Footnote 1: See bibliography at end of report.]
In the early months of the war while I was making every effort to obtain
reliable information concerning conditions in the Canary Islands, I
received an urgent invitation from my friend and former student, Doctor
G. V. Hamilton, to make use of his collection of animals and laboratory
at Montecito, California, during my leave of absence from Harvard. This
invitation I most gladly accepted, and in February, 1915, I established
myself in Santa Barbara, in convenient proximity to Doctor Hamilton's
private laboratory where for more than six months I was able to work
uninterruptedly under nearly ideal conditions.
Doctor Hamilton without reserve placed at my disposal his entire
collection of animals, laboratory, and equipment, provided innumerable
conveniences for my work, and in addition, bore the entire expense of my
investigation. I cannot adequately thank him for his kindness nor make
satisfactory acknowledgment here of his generous aid. Thanks to his
sympathetic interest and to the courtesy of the McCormick family on
whose estate the laboratory was located, my work was done under wholly
delightful conditions, and with assistance from Ramon Jimenez and Frank
Van Den Bergh, Jr., which was invaluable. The former aided me most
intelligently in the care of the animals and the construction of
apparatus; and the latter, especially, was of very real service in
connection with many of my experiments.
The collection of animals which Doctor Hamilton placed at my disposal
consisted of ten monkeys and one orang utan. The monkeys represented
either _Pithecus rhesus_ Audebert (_Macacus rhesus_), _Pithecus irus_ F.
Cuvier (_Macacus cynomolgos_), or the hybrid of these two species
(Elliot, 1913). There were two eunuchs, five males, and three females.
All were thoroughly acclimated, having lived in Montecito either from
birth or for several years. The orang utan was a young specimen of
_Pongo pygmaeus_ Hoppius obtained from a San Francisco dealer in
October, 1914 for my use. His age at that time, as judged by his size
and the presence of milk teeth, was not more than five years. So far as
I could discover, he was a perfectly normal, healthy, and active
individual. On June 10, 1915, his weight was thirty-four pounds, his
height thirty-two inches, and his chest girt twenty-three inches. On
August 18 of the same year, the three measurements were thirty-six and
one-half pounds, thirty-three inches, and twenty-five inches.
For the major portion of my experimental work, only three of the eleven
animals were used. A growing male, _P. rhesus_ monkey, known as Sobke; a
mature male, _P. irus_, called Skirrl; and the young orang utan, which
had been named Julius. Plates I and II present these three subjects of
my experiments in characteristically interesting attitudes. In plate I,
figure 1, Julius appears immediately behind the laboratory seated on a
rock, against a background of live oaks. This figure gives one an
excellent idea of the immediate environment of the laboratory. Figure 2
of the same plate is a portrait of Julius taken in the latter part of
August. By reason of the heavy growth of hair, he appeared considerably
older as well as larger at this time than when the photograph for figure
1 was taken. In plate II, figure 3, Julius is shown in the woods in the
attitude of reaching for a banana, while in figure 4 of the same plate
he is represented as walking upright in one of the cages.
Likenesses of Sobke are presented in figures 5 and 6 of plate II. In the
latter of these figures he is shown stretching his mouth, apparently
yawning but actually preparing for an attack on another monkey behind
the wire screen. Figure 7 of this plate indicates Skirrl in an
interesting attitude of attention and with an obvious lack of
self-consciousness. The same monkey is represented again in figures 8
and 9 of plate II, this time in the act of using hammer and saw.
EXPLANATION OF PLATE II
FIGURE 3.--Orang utan, Julius, reaching for banana.
FIGURE 4.--Julius walking across his cage.
FIGURE 5.--_P. rhesus_, Sobke.
FIGURE 6.--Sobke stretching his jaws (yawn?) preparatory to a fight.
FIGURE 7.--_P. irus_, Skirrl.
FIGURE 8.--Skirrl using hammer and nail.
FIGURE 9.--Skirrl using a saw.
All of the animals except the orang utan had been used more or less for
experiments on behavior by Doctor Hamilton, but this prior work in no
way interfered with my own investigation. Doctor Hamilton has
accumulated a large mass of the most valuable and interesting
observations on the behavior of monkeys, and he more thoroughly
understands them than any other observer of whom I have knowledge. Much
to my regret and embarrassment in connection with the present report, he
has thus far published only a small portion of his data (Hamilton, 1911,
1914). In his most recent paper on "A study of sexual tendencies in
monkeys and baboons," he has given important information concerning
several of the monkeys which I have observed. For the convenience of
readers who may make use of both his reports and mine, I am designating
the animals by the names previously given them by Hamilton. The
available and essential information concerning the individuals is
presented below.
_List of animals in collection_
Skirrl. _Pithecus irus_. Adult male.
Sobke. _P. rhesus_. Young adult male.
Gertie. _P. irus-rhesus_. Female. Born November, 1910.
Maud. _P. rhesus_. Young adult female.
Jimmy II. _P. irus_. Adult male.
Scotty. _P. irus_ (?). Adult male.
Tiny. _P. irus-rhesus_. Female. Born August, 1913.
Chatters. _P. irus_. Adult eunuch.
Daddy. _P. irus_. Adult eunuch.
Mutt. _P. irus_. Young adult male. Born August, 1911.
Julius. _Pongo pygmaeus_. Male. Age, 4 years to 5 years.
When I arrived in Santa Barbara, Doctor Hamilton was about to remodel,
or rather reconstruct, his animal cages and laboratory. This gave us
opportunity to adapt both to the special needs of my experiments. The
laboratory was finally located and built in a grove of live oaks. From
the front it is well shown by figure 10 of plate III, and from the rear,
by figure 11. Its location was in every way satisfactory for my work,
and in addition, the spot proved a delightful one in which to spend
one's time.
[Illustration: FIGURE 12.--Ground plan of Montecito laboratory and
cages. Scale 1/120
L, laboratory; C, cages; A, experiment room in which multiple-choice
apparatus was installed; B, E, additional rooms for research; D, store
room and shop; Z, large central cage communicating with the eight
smaller cages 1-8.]
Figure 12 is a ground plan, drawn to scale, of the laboratory and the
adjoining cages, showing the relations of the several rooms of the
laboratory among themselves and to the nine cages. Although the
construction was throughout simple, everything was convenient and so
planned as to expedite my experimental work. The large room A, adjoining
the cages, was used exclusively for an experimental study of ideational
behavior by means of my recently devised multiple-choice method.
Additional, and supplementary, experiments were conducted in the large
cage Z. Room D served as a store-room and work-shop.
The laboratory was forty feet long, twenty-two feet wide, and ten feet
to the plate. Each small cage was six, by six, by twelve feet deep,
while the large compartment into which each of the smaller cages opened
was twenty-four feet long, ten feet wide, and twelve feet deep.
II
OBSERVATIONAL PROBLEMS AND METHODS
My chief observational task in Montecito was the study of ideational
behavior, or of such adaptive behavior in monkeys and apes as
corresponds to the ideational behavior of man. It was my plan to
determine, so far as possible in the time at my disposal, the existence
or absence of ideas and the role which they play in the solution of
problems by monkeys and apes. I had in mind the behavioristic form of
the perennial questions: Do these animals think, do they reason, and if
so, what is the nature of these processes as indicated by the
characteristics of their adaptive behavior?
My work, although obviously preliminary and incomplete, differs from
most of the previous studies of the complex behavior of the infrahuman
primates in that I relied chiefly upon a specially devised method and
applied it systematically over a period of several months. The work was
intensive and quantitative instead of more or less incidental, casual,
and qualitative as has usually been the case. Naturally, during the
course of my special study of ideational behavior observations were made
relative to various other aspects of the life of my subjects. Such, for
example, are my notes on the use of the hands, the instincts, the
emotions, and the natural aptitudes of individuals. It is, indeed,
impossible to observe any of the primates without noting most
interesting and illuminating activities. And although the major portion
of my time was spent in hard and monotonous work with my experimental
apparatus, I found time each day to get into intimate touch with the
free activities of my subjects and to observe their social relations and
varied expressions of individuality. As a result of my close
acquaintance with this band of primates, I feel more keenly than ever
before the necessity of taking into account, in connection with all
experimental analyses of behavior, the temperamental characteristics,
experience, and affective peculiarities of individuals.
The light which I have obtained on the general problem of ideation has
come, first, through a method which I have rather inaptly named the
multiple-choice method, and second, and more incidentally, through a
variety of supplementary methods which are described in Section IV of
this report. These supplementary methods are simple tests of ideation
rather than systematic modes of research. They differ from my chief
method, among other respects, in that they have been used by various
investigators during the past ten or fifteen years. It was not my aim to
repeat precisely the observations made by others, but instead to verify
some of them, and more especially, to throw additional light on my main
problem and to further the analysis of complex behavior.
What has been referred to as the multiple-choice method was devised by
me three years ago as a means of obtaining strictly comparable objective
data concerning the problem-solving ability of various types and
conditions of animals. The method was first tried with human subjects in
the Psychopathic Hospital, Boston, with a crude keyboard apparatus
which, however, proved wholly satisfactory as a means of demonstrating
its value. It has since been applied by means of mechanisms especially
adapted to the structure and activities of the organisms, to the study
of the behavior of the crow, pig, rat, and ringdove (Yerkes, 1914;
Coburn and Yerkes, 1915; Yerkes and Coburn, 1915). The method has also
been applied with most gratifying results to the study of the
characteristics of ideational behavior in human defectives,--children,
and adults,--and in subjects afflicted with various forms of mental
disease. It is at present being tried out as a practical test in
connection with vocational guidance and various forms of institutional
examination, such as psychopathic hospital and court examinations.
As no adequate description of the method has yet been published to which
I can here refer, it will be necessary to present its salient
characteristics along with a description of the special form of
apparatus which was found suitable for use with monkeys and apes.
The method is so planned as to enable the observer to present to any
type or condition of organism which he wishes to study any one or all of
a series of problems ranging from the extremely simple to the complex
and difficultly soluble. All of the problems, however, are completely
soluble by an organism of excellent ideational ability. For the human
subject, the solution of the easiest problem of all requires almost no
effort, whereas even moderately difficult problems may require many
repetitions of effort and hours or days of application to the task. In
each case, the solution of the problem depends upon the perception of a
certain constant relation among a series of objects to which the subject
is required to attend and respond. Such relations are, for example,
secondness from one end of the group, middleness, simple alternation of
ends, or progressive movement by constant steps from one end of a group
to the other.
It is possible to present such relational problems by means of
relatively simple reaction-mechanisms. In their essential features, all
of the several types of multiple-choice apparatus designed by the writer
and used either by him or by his students and assistants are the same.
They consist of a series of precisely similar reaction-devices, any one
or all of which may be used in connection with a given observation.
These reaction-mechanisms are so chosen as to be suited to the structure
and action-system of the animal to be studied. For the human being the
mechanism consists of a simple key and the total apparatus is a bank of
keys, with such electrical connections as are necessary to enable the
observer to obtain satisfactory records of the subject's behavior. Let
us suppose the bank of keys, as was actually the case in my first form
of apparatus, to consist of twelve separate reaction-mechanisms; and let
us suppose, further, the constant relation (problem) on the basis of
which the subject is required to react to be that of middleness. It is
evident that in successive trials or experiments the keys must be
presented to the subject in odd groups, the possibilities being groups
of 3, 5, 7, 9, or 11. If for a particular observation the experimenter
wishes to present the first three keys at the left end of the keyboard,
he pushes back the remaining nine keys so that they cannot be operated
and requires the subject to select from the group of three keys the one
which on being pressed causes a signal to appear. It is of course the
clearly understood task of the subject to learn to select the correct
key in the group on first trial. This becomes possible only as the
subject observes the relation of the key which produces the desired
effect to the other keys in the group. On the completion of a subject's
reaction to the group of three keys, a group of seven keys at the
opposite end of the keyboard may, for example, be presented. Similarly,
the subject is required to discover with the minimum number of trials
the correct reaction-mechanism. Thus, time after time, the experimenter
presents a different group of keys so that the subject in no two
successive trials is making use of the same portion of the keyboard. It
is therefore impossible for him to react to spatial relations in the
ordinary sense and manner, and unless he can perceive and appropriately
respond to the particular relation which constitutes the only constant
characteristic of the correct reaction-mechanism for a particular
problem, he cannot solve the problem, or at least cannot solve it
ideationally and on the basis of a small number of observations or
trials.
For the various infrahuman animals whose ideational behavior has been
studied by means of this method, it has been found eminently
satisfactory to use as reaction-mechanisms a series of similar boxes,
each with an entrance and an exit door. An incentive to the selection of
the right box in a particular test is supplied by food, a small quantity
of which is placed in a covered receptacle beyond the exit door of each
of the boxes. Each time an animal enters a wrong box, it is punished for
its mistake by being confined in that box for a certain period, ranging
from five seconds to as much as two minutes with various individuals or
types of organism. This discourages random, hasty, or careless choices.
When the right box is selected, the exit door is immediately raised,
thus uncovering the food, which serves as a reward. After eating the
food thus provided, the animal, according to training, returns to the
starting point and eagerly awaits an opportunity to attempt once more to
find the reward which it has learned to expect. With this form of the
apparatus, the boxes among which choice may be made are indicated by the
raising (opening) of the front door.
Since with various birds and mammals the box form of apparatus had
proved most satisfactory, I planned the primate apparatus along similar
lines, aiming simply to adapt it to the somewhat different motor
equipment and destructive tendencies of the monkeys. I shall now briefly
describe this apparatus as it was constructed and used in the Montecito
laboratory.
EXPLANATION OF PLATE IV
FIGURE 13.--Multiple-choice apparatus, showing observer's bench
and writing stand.
FIGURE 14.--Apparatus as seen from observer's bench.
FIGURE 15.--Entrances to multiple-choice boxes as seen from
the response-compartment.
FIGURE 16.--Apparatus as seen from the rear, showing exit
doors, food receptacles, and covers for same.
The apparatus was built in room A (figure 12), this room having been
especially planned for it with respect to lighting as well as dimensions
and approaches. It was unfortunately impossible to obtain photographs
showing the whole of the apparatus, but it is hoped that the four
partial views of plate IV may aid the reader who is unfamiliar with
previously described similar devices to grasp readily the chief points
of construction. In this plate, figure 13 shows the front of the
complete apparatus, with the alleyway and door by way of which the
experimenter could enter. The investigator's observation-bench and
record-table also appear in this figure, together with weighted cords
used to operate the various doors and the vertically placed levers by
means of which each pair of doors could be locked. Figure 14 is the view
presented to the observer as he stood on the bench or observation stand
of figure 13 and looked over the entire apparatus. Three of the entrance
doors are shown at the right of this figure as raised, whereas the
remainder of the nine entrance doors of the apparatus are closed. Figure
15 is a view of the entrance doors from below the wire roof of the
apparatus. Again, two of the doors are shown as raised, and three
additional ones as closed. The rear of the apparatus appears in figure
16, in which some of the exit doors are closed and others open. In the
latter case, the food receptacles appear, and on the lower part of the
raised doors of the corresponding boxes may be seen metal covers for the
food receptacles projecting at right angles to the doors, while on the
lower edge of each door is an iron staple used to receive a sliding bar
which could be operated from the observer's bench as a means of locking
the doors after they had been closed. The space beyond the exit doors
was used as an alleyway for the return of the animals to the starting
point.
It will be necessary at various points in later descriptions to refer to
these several figures. But further description of them will be more
readily appreciated after a careful examination of the ground plan of
the apparatus presented as figure 17 In accordance with the labelling of
this figure, the experimenter enters the apparatus room through doorway
16, passes thence through doorways 17 and 10 to the large cage Z, from
which he has direct access to the animals and can bring them into the
apparatus. The multiple-choice mechanism proper, consisting of nine
similar boxes (nine were used instead of twelve as a matter of
convenience of construction, not because this smaller number is
otherwise preferable) is labelled F. These boxes are numbered 1 to 9,
beginning at the left. This numbering was adhered to in the recording of
results throughout the investigation. The other important portions of
the apparatus are the runway D, from which the subject at the
experimenter's pleasure could be admitted through doorway 12 to the
large response-chamber E; the alleyways G, H, and I, by way of which
return to the starting point was possible; the observation bench C, with
its approach step 13; and the observer's writing table A.
In the construction of this large apparatus, it was necessary to make
provision for the extremely destructive tendencies of monkeys and
anthropoid apes,--hence the apparent cumbersomeness of certain portions.
It was equally necessary to provide for the protection of the observer
and the prevention of escape of the subjects by completely covering the
apparatus and alleyways with a heavy wire netting.