Imitation of Intentional Manipulatory Actions in Chimpanzees (Pan

Journal of Comparative Psychology
2000, Vol. 114, No. 4, 381-391
Copyrighl 2000 by the American Psychological Association, lnc.
0735-7036/00/$5.00 DOI: 10.1037//0735-7036.114.4.381
Imitation of Intentional Manipulatory Actions
in Chimpanzees (Pan troglodytes)
Masako Myowa-Yamakoshi and Tetsuro Matsuzawa
Kyoto University
In this study, the authors investigated the understanding of other's actions in 5 adult chimpanzees (Pan
troglodytes). A human demonstrated an attempt to open different containers. Each container required a
different motor pattern to open it. Along with the container, a 2nd object was made available. After a free
play period in which the chimpanzees' natural behaviors toward the objects were recorded, the authors
tested the following 2 phases: The demonstrator (a) tried but failed to open and (b) opened the container
successfully, with I of 2 alternative strategies, either using an "irrelevant tool" or by hand. The
chimpanzees did not reproduce the demonstrator's motor patterns precisely but did reproduce the
demonstrated strategies in both phases. These results suggest that chimpanzees anticipate the intentions
of others by perceiving the directionality and causality of object(s) as available cues.
Recently, there has been much controversy over whether non­
human animals imitate. Comparative psychologists have argued
that imitation has important implications for the biological and
phylogenetic foundations of cognitive complexity. Much of this
debate centers on how the act of imitation is defined (e.g., Byrne
& Russon, 1998; Byrne & Tomasello, 1995; Galef, 1988; Heyes,
1993, l995; Heyes & Galef, 1996; Tomasello, Kruger, & Ratner,
1993; Visalberghi & Fragaszy, 199 0; Whiten, 1998; Whiten &
Ham, 1992). However, it has proved difficult for researchers to
agree on a definition of imitation and to decide which nonhuman
animals can imitate.
In attempting to determine whether the capacity for imitation
exists in animals, comparative psychologists should consider it
from another point of view. The question that must be asked is,
how do imitators understand what the demonstrator did? In other
words, how does the imitator represent the actions of others? If we
see another's actions as a series of precise physical movements
(e.g., right arm moves up 20 cm, knees bend 30° , etc.), we might
be very puzzled and unable to predict what the other is doing. It is
clear that humans extract some component elements from the
actions of others within a psychological framework (e.g., intention,
desire; Baron-Cohen, 1995; Fodor, 1983; Tomasello, 1996). From
this cognitive perspective, researchers must determine what com­
ponents of the actions the imitators process.
In humans, nonverbal tests provide suggestive evidence that
infants as young as 14 to 18 months old understand something
about the intentions of others (Carpenter, Akhtar, & Tomasello,
l998; Meltzoff, 1995). Meltzoff investigated the understanding of
others' intentions in LS-month-old infants. He took advantage of
the natural human tendency to reproduce the actions of others
called the behavioral re-enactment procedure. This procedure
capitalizes on an infant's natural tendency to imitate adults' ac­
tions. In this experiment, infants watched the demonstrator try but
fail to reach the end state of an action (e.g., he tried to pull the ends
of a dumbbell outward, but his hands slipped off). After the
demonstration, when given an opportunity to manipulate the object
themselves, the infants could achieve the end state of the actions
spontaneously. They actually achieved success as often as infants
who saw a successful demonstration (i.e., the end state of the
action) and more often than infants in other control conditions.
Meltzoff concluded that 18-month-old infants are able to represent
the actions of others in a psychological framework through the
body movement used to achieve an underlying goal or intention.
Little is known about how nonhuman animals understand the
actions of others. Some researchers have attempted to demonstrate
that apes understand something of the intention of others. Premack
and Woodruff (1978) published the first study on a chimpanzee's
understanding of others' intentions. A language-trained chimpan­
zee named Sarah observed several problem-solving situations fea­
turing a human actor on videotape. For example, Sarah watched a
video of an acto.r looking up at an out-of-reach banana hanging
from the ceiling. After that, she was shown a pair of photographs,
one of which pictured the solution (e.g., the actor climbed on a box
under the banana). Sarah successfully chose the correct alterna­
tives well above the level of chance. Premack and Woodruff
argued that Sarah could attribute intention to the human actor.
Recent experimental research on apes' understanding of others'
actions has focused on discrimination between intentional and
Masako Myowa-Yamakoshi and Tetsuro Matsuzawa, Department of
Behavioral and Brain Sciences, Primate Research Institute, Kyoto Univer­
sity, lnuyama, Japan.
The present research was financed by Grants 071020 I O and 1230 I 006
from the Ministry of Education, Science, Sports, and Culture, Japan.
Preparation of the manuscript was supported in part by Research Fellow­
ship 2867 from the Japan Society for the Promotion of Science for Young
Scientists and by the Cooperation Research Program of the Primate Re­
search institute, Kyoto University. We gratefully acknowledge Masuo
Koyasu, Shozo Kojima, Masaki Tomonaga, Josep t:all, M.A. Huffman,
and Gen Yamakoshi for helpful comments on an early draft of this article
and for generous guidance throughout the project.
Correspondence concerning this article should be addressed to Masako
Myowa-Yamakoshi, Section of Language and intelligence, Deparanent of
Behavioral and Brain Sciences, Primate Research Institute, Kyoto Univer­
sity, lnuyama, Aichi, 484-8506, Japan. Electronic mail may be sent to
[email protected] or to [email protected].
381
MYOWA-YAMAKOSHJ AND MATSUZAWA
382
accidental actions (Call & Tomasello, 1998; Povinelli, 1991; Po­
vinelli, Perilloux, Reaux, & Bierschwale, 1998; Premack, 1986).
However, the results have been both positive (e.g., Po.vinelli, 1991;
Call & Tomasello, 1998) and negative (e.g., Povinelli et al., 1998;
Premack, 1986). It is not evident whether or how apes distinguish
intentional from accidental actions performed by others. These
researchers focused on determining whether nonhuman apes can
distinguish intentional actions from accidental actions in a dichot­
omous fashion. Moreover, the results may be interpreted in several
ways: (a) Chimpanzees have developmentally limited cognitive
skills; (b) the result depends on their history of interacting with
humans, including language training; or (c) there exist salient
setting cues for each defined action in apes that differ from the
humans' perspective.
Myowa-Yamakoshi and Matsuzawa ( l 999) investigated the fac­
tors that determine the degree of difficulty in chimpanzees' (Pcm
iroglodytes) imitation. We found that there are some constraints in
the. basic cognitive processes required to transform visual infor­
mation into matching motor acts when chimpanzees imitate human
actions. For example, chimpanzees are less sensitive to body
movement than to the manipulated objects involved in the dem­
onstrated actions. On the other hand, it has been suggested that
18-month-old human infants understand the mental states of others
through body movement (Butterworth, 1994; Meltzoff, 1995). If
chimpanzees have a limited capacity for processing visual-motor
information as it relates to body movement, their capacity to
understand intentions of others may be different from that of
humans.
Our aim in the presern study was to investigate how chimpan­
zees represent the action of a human demonstrator. This is the first
study to explore the understanding of others' actions with the use
of a nonverbal completion procedure (such as the one Meltzoff,
1995, used) to formulate action plans based on chimpanzees'
interpretation of the performance. We focused on the structure of
the demonstrated actions to find what kinds of cues are available
for chimpanzees to understand the intentions of others. The action
was designed to involve various motor patterns and strategies for
manipulating objects as independent variables. We expected that
the chimpanzees would pick up some information from the dem­
onstrator's actions in ways that differed from those of humans.
To begin with, chimpanzees observed a demonstrator who tried
but failed to perform the intended actions. Then, the chimpanzees
saw the demonstrator achieve the intended actions successfully
(Experiment I). The hypothesis was that chimpanzees would un­
derstand something about the intention of the demonstrator de­
pending mainly on the information about how the objects can be
used (strategies for manipulating objects), even though the dem­
onstrator's action failed. In this respect, the results would be
different from those of humans who read the intention of others
through their body movements.
Expe1iment 1
Method
Subjects
The subjects were S female chimpanzees from 12 to 20 years old housed
at the Kyoto University Primate Research Institute (KUPRJ). Ai (20 years
old at the time of this study) was born in the wild. Pendesa (18 years) and
Chloe (IS years) were born in captivity and have been reared by humans
since they were less than I year old. Popo (13 years) and Pan (12 years) are
full siblings that were born through artificial insemination and raised in a
nursery by humans at KUPRl starting within 24 hours of birth because their
biological mother did not exhibit sufficient maternal care. All of the
chimpanzees live together in a community of 11 chimpanzees in a semi­
natural environment enriched with u·ees and a su·eam. They were nm
deprived of food for testing. Before this experiment, the chimpanzees had
participated in different perception and cognitive capacity experiments
involving a variety of objects (Fujita & Matsuzawa, 1990; Kojima, 1990:
Matsuzawa, 1985; Matsuzawa, Kojima, & Shinohara, 1997; Tanaka, 1995:
Tomonaga, 1993) and in a task that involved reproducing several kinds of
actions (Kojima, 1991; Myowa-Yamakoshi & Matsuzawa, 1999).
Throughout this study, all the chimpanzees paid attention 10 the actions
demonstrated by the human. Then, when they were handed the objects,
they remained in one place and manipulated the objects with great interest.
Test Materials
Eight pairs of objects were used as test stimuli (see Appendix, Table
A I). In each pair, one object was a container that required one of a variety
of different motor panerns to open (pushing, pulling, or twisting), and the
other was an itTelevanl tool that was not required to open the container.
With each of these eight pairs of objects, opening the container was
considered a successful action. No food was needed to sustain the interest
of the chimpanzees.
Experimental Design
Each chimpanzee participated in a total of eight experimental sessions:
one session was conducted with each pair of objects. Each session con­
sisted of two phases of demonstrati.on: (a) The demonstrator tried but failed
to open the container because one of his hands "slipped" off the pan of the
container to be opened (failure phase), and (b) the demonstrator success­
fully opened the container (success phase). ln both phases following each
demonstration, the chimpanzees were given a chance 10 interact with the
objects. Each of them experienced the two phases in the above-stated order
(we did not counterbalance the order of the two phases in this study
because we primarily focused on chimpanzees' understanding of the in­
tention of the demonstrator).
In each success and failure phase, the demonstrator manipulated the
container with one of two alternative strategies: trying to open the con­
tainer (a) with the irrelevant tool or (b) by hand. We counterbalanced the
subjects 10 examine the alternatives within each phase and session (see
Figure I).
Procedure
Each chimpanzee was rested individually in a familiar playroom
(5. 0 x 7.2 m with a ceiling at 3.0 m) at KUPRI. There were two
experimenters. One demonstrated the actions by sitting face to face with
the chimpanzee in the playroom. The other experimenter recorded the trials
on videotape from outside the room through an acrylic panel. All the
chimpanzees had previously learned to sit quietly on a wooden seat
whenever they panicipared in a cognitive test. Each session started with
free play followed by the two rest phases.
Free play. A pair of objects was presented to the chimpanzee for
approximately 3 min of free play. During this time, rhe subject interacted
in some way with each of the two objects. The demonstrator then retrieved
the objects and proceeded with the following two phases (see Figure 2).
Failure phase. The demonstrator showed an action in which he tried
but failed 10 open a container using one of rwo alternate strategies, either
with the itTelevant tool or by hand. Each action was demonstrated two or
three times 10 each chimpanzee lo ensure thar she paid attention 10 the
IMITATION OF INTENTIONAL ACTIONS IN CHIMPANZEES
383
{a)
(b)
Figure 1. A chimpanzee (Pendesa), siuing face to foce with a human demonstrator who is performing the
demonstrated ac1jons (Session 7) in Experiment 1. (a) The demonstrator is trying to open a box with an i1Televant
tool: (b) the chimpanzee's performance using the demonstrated strategy.
action. After the demonstration, the chimpanzee was then given the pair of
objects and allowed 3 min to manipulate them. After 3 min elapsed, the
objects were retrieved, and the chimpanzee received a piece of food in
exchange for the objects. No further verbal description or encouragement
was provided.
Success phase. After the failure phase, we proceeded with the success
phase: The demonstrator showed the chimpanzee how to open the con­
tainer successfully with one of the two alternate strategies. Following this
demonstration, the chimpanzee was given the pair.of objects and allowed
another 3 min lO manipulate them. The procedure in the success phase was
identical to that followed in the failure phase, except that the demonstrator
succeeded in opening the container.
One session consisted of free play and the two phases of the imitation
test. There were four sets of conditions (failure with the irrelevant tool,
failure by hand, success with the irrelevant tool, success by hand) in a total
of eight sessions. Each session incorporated two of the four conditions. The
order of conditions in each phase was counterbalanced across the subjects.
Each chimpanzee participated in two sessions each day, 3 or 4 days a
week. Even if the chimpanzees successfully opened the containers in the
free play period, we conducted both subsequent testing phases. All the
testing sessions were recorded on videotape (Sony, CCD-TR3000) in their
entirety for larer analysis.
Data Analysis
During free play before the two testing phases, the chimpanzees often
spontaneously opened the containers. Jn all cases, they opened the con­
tainers by hand, not by using the irrelevant tool. We excluded the cases of
these successes in free play from further analysis. We analyzed only the
sessions in which the chimpanzees failed to open the container in the free
play period and observed the experimenter's demonstration in the follow­
ing two test phases.
As for the period of chimpanzees' manipulation observed in the free play
period and the two phases, we defined a pe1for111c111ce as starting when the
384
MYOWA-YAMAKOSHI AND MATSUZAWA
Free-play
Manipulation of two objects by the chimpanzee
(3 minutes)
list which motor panerns they thought the chimpanzees were performing.
The observers were unaware of the hypotheses being tesr.ed and free to
choose any of the strategies for each performance regardless of any
previous nominations. lnterobserver agreement on the performances cal­
culated using the kappa statistic (designed to correct for chance agreement)
was high, k = .83, p < .01.
Results
Failure phase
Demonstration by experimenter
Trying to open a container but fails
G;)or
w i t h irrelev a n t tool
--------------- --------------------------------------·
i
i
----------------------·
Manipulation of two objects by the chimpanzee
(3 min)
Success phase
.-----------------------------------------------------------------------------Demonstration by experimenter
Trying to open a container with successes
C:=)-or
: ____________·
i
with irrelevant tool
-----------·------------------------
i
-----------------------
Manipulation of two objects by the chimpanzee
(3 m in)
Figure 2.
Experimental procedure in Experiment I.
chimpanzee touched the object(s) and ending when contact with the ob­
ject(s) ceased, whether or not the container was opened. ln addition, we
defined the categories of motor patterns for chimpanzees' performances.
Each performance was coded according to mutually exclusive categories.
A vocabulary for motor patterns was coined and is listed in Table I. Using
these definitions, we idemified 18 motor pattern categories in total.
Chimpanzees' spontaneous performances in free play were excluded
from further analysis. As a result, we identified 29 performances in the
failure phase and 33 performances in the success phase. Each of the
chimpanzee's performances were analyzed with regard to the strategies
used to manipulate the container-specifically, whether the chimpanzee
manipulated it alone or with the irrelevant tool.
Scoring
Two independent observers were initially given the definition of a
performance and received a list of 18 identified motor pattern categories
(see Table I). Then, the performances for each session (25% of the cases
for eight sessions) were presented 10 the observers in random order. They
noted whether the container was manjpulated alone or with the irrelevant
tool in each session. At the same time, they determined on the basis of the
Successful Performances
Table 2 lists the performances of each chimpanzee for all
sessions in each phase and the mean latency from first touch to
opening the container in free play. In 4 out of 19 cases, the
chimpanzee successfully opened the container after observing the
demonstrator.
In the failure phase, the chimpanzees successfully opened the
containers in 2 cases ( I I% of a total of l 9 cases). Pan opened the
container using a different strategy from that previously shown by
the demonstrator. Ai opened the container using the demonstrated
strategy. Both chimpanzees opened the containers by hand. The
latencies to their f"mt opening the containers were 9.8 s (Pan,
Session 3) and 8.7 s (Ai, Session 6).
ln the success phase, excluding the 2 cases in which chimpan­
zees had already opened the container in the preceding failure
phase, they succeeded in opening the container in 2 of 17 cases
(12%). Both chimpanzees reproduced the demonstrated strategy.
The latencies to open the container were 26.2 s (Ai, Session 2)
and 9.4 s (Chloe, Session 4).
There were no clear differences between the chimpanzees' suc­
cesses in the failure and success phases. When they succeeded in
opening the container in either of those phases, the latency was
much shorter than the mean latency for the cases in which the same
container was opened in free play (Sessions 2, 3, 4, and 6, M =
109, 77, l07, and 63 s, respectively). The chimpanzees did not
engage in extensive trial and error; rather, they opened the con­
tainer directly.
There were no performances in which the container was directed
toward the tool in the two phases; the chimpanzees always ma­
nipulated the tool alone or directed it toward the container.
Reproducing the Strategy for Manipulating Objects
We analyzed the performances in each phase from two perspec­
tives. Fir t, did the chimpanzees manipulate the container with the
irrelevant tool or by hand? Second, did they use the demonstrated
strategies? In other words, when the demonstrator manipulated the
container with his hands, did they also manipulate the container
with their hands without using the irrelevant tool? When the
demonstrator manipulated the container using the irrelevant tool,
did they also m<1nipulate the container using the irrelevant tool?
Table 3 shows the frequencies of performances by subjects for
each type of demonstration in both phases. We found that chim­
panzees used the demonstrated strategy significantly more often
than an alternative strategy: Wilcoxon signed rank test, T(N =
5) = 0, p < .05, 72% in the failure phase; T(N = 5) = 0, p < .05,
88% in the success phase (see Figure 3). However, the difference
in the alternative strategies in the failure phase versus the success
phase was not significant: T(N = 4) = 5, ns.
IMlTATION OF INTENTIONAL ACTIONS JN CHJMPANZEES
385
Table l
Vocabulary Used for Identifying Mo/Or Panerns Directed at Objects
Object
Container
Container &
Irrelevant tool
Motor
pattern
Hold
Mouth
Put
Give
Touch
Press
Pull
Hit
Push
Rub
Move
Twist
Poke
Pinch
Poke
Put
Move
Attach
Push
Hit
Insen
Press
Pull
Stick
Rub
Hold
Give
Turn
Touch
Twist
Nore.
Definition
Hold container in hands or feet. Container is lifted.
Bring container into contact with lips and nose while holding it with the hand.
Put container on the floor with hands or feer.
Give container 10 the demonstrator with hands or feet.
Touch container with the hand or foot. Container is on the floor.
Press container against the floor with the hand. Container is on the floor.
Pull at pan of container with fingers. Container is touched by the other hand.
Hit container with hands.
Push part of container with fingers. Container is held with the other hand.
Rub container with the hand in continuous contact. Container is held in hands.
Move container aside with hands or feet.
Twist container with the hand. Container is held in hands.
Poke at container with an index finger in continuous contact.
Pinch container with fingers. Container is lifted.
Poke at container with the other object held in hand or foot in cominuous contact.
Put the other object on container with hands or feet.
Move container with the other object on top aside with hands or feet.
Attach the other object to container with hands or feet.
Push part of container using the other object held in hand. Container is touched by the other hand.
Hit container with the other object held in hand.
Insert the other object into a hole in container with the hand.
Press container against the floor with the other object held in the hands. Container is on the floor.
Pull at part of container using the other object held in hand. Container is touched by the other hand.
Stick the other object into pan of container with hands.
Rub container with the other object held in hand in continuous contact. Another object is placed flat
on the smface of container.
Hold container and the other object with hands or feet. Both objects are lifted.
Give container and the other object to the demonstrator with hands or feet.
Turn container using the other object held in the hands.
Touch container and the other object with hands or feet. Both objects are on the !loor.
Twist container using the other object held in hand. Container is held in the other hand.
The motor patterns are listed according to the order of frequencies in the subjects' performances.
In the success phase, the demonstrator used either the identical
or a different strategy to open the container as he used in the failure
phase. As an example of the latter, the demonstrator manipulated
the container with his hands in the preceding failure phase and
used the irrelevant tool in the success phase (also see Figure 2). No
difference was found in the choice of strategy used between both
cases: T(N = 5) = 5, ns. The chimpanzees significantly tended to
use the demonstrated strategy, irrespective of whether the demon­
strator used the same strategy in both phases.
Reproduction of Motor Patterns
Table 4 presents the proportion of the performances reproduced
by the chimpanzees in each demonstrated motor pattern in each
phase. Wilcoxon's signed rank test showed that there was no
significant difference in the proportion of reproduction of the
motor patterns between the two phases: T(N = 4) = 3, ns; M =
17% in the failure phase; M = 15% in the suc�ss phase. As for the
proportion of reproduction of each type of motor patterns, the
Friedman test showed that there was no significant difference
among the three types, x2(2, N = 5) = 6.23, ns; pulling, M = 9%
in Sessions 1, 4, and 7; pushing, M = 27% in Sessions 2, 3, 6, and
8; twisting, M = 0% in Session 5. The chimpanzees did not
accurately reproduce the motor patterns involved in the demon­
strator's actions in both phases.
Discussion
The results show that the chimpanzees reproduced some
aspects of the demonstrated actions in the failure and success
phases. They manipulated the objects using the demonstrated
strategy significantly more often than other strategies. Even
though they saw the demonstrator fail to open the container,
they tried to use the strategies demonstrated in the failure phase.
On the other hand, they did not appear to reproduce precisely
the demonstrator's body movements involved in the demonstra­
tions in both phases.
Experiment 2
In Experimeqt 2, we presented the sets of objects during free
play and then tested the chimpanzees using the order of success
phases followed by failure phases. The hypothesis was that once
chimpanzees had seen the demonstrator succeed in the success
phase, they would understand how to open the container. As a
result, even though the demonstrator showed the alternative strat­
egy in the later failure phase, chimpanzees would not try the
strategies demonstrated in the failure condition, which would
differ from the results of Experiment 1.
MYOWA-YAMAKOSHJ AND MATSUZAWA
386
100 1
Table 2
Cases by Chim.pam:.ees for Each Session in the Failure and
Success Phase in i::,xperiment I
Subject
Session
Ai
Chloe
Pendesa
Popo
Pan
x
x
x
x
x
x
-
g
2
3
4
5
6
7
8
2
3
4
5
6
7
8
x
-
•
-
•
-
-
-
x
x
x
x
x
x
-
-
x
•
x
-
x
-
Mean
latency'''
"'
x
x
x
x
x
-
•
-
-
x
x
x
x
x
x
• With irrelevant tool
II
60
v
8
109
77
107
25
63
72
75
v
"'
v
v
Success Phase
x
I
DBy hand
v
Failure Phase
x
80
I
I
40
20
0
Failure
x
Phase
Success
Figa.re 3. Percentage of performances ( + SE) in which the chimpanzees
performed using the identical strategies shown by the demonstrator in both
phases in Experiment I .
x
x
No1e. • = chimpanzee opened the container: x = chimpanzee failed to
open the container; - = chimpanzee already opened the container in free
play period prior to the success phase and failure phase.
* Mean latencies are the times (in seconds) chimpanzees needed to open
the container in the free play_period.
Method
Subjects
The subjects in Experiment 2 were the same 5 chimpanzees who were
tested in Experiment I.
Test Materials
Four other pairs of objects were used as test stimuli (see Appendix,
Table A2). In each pair, one object was a container, and the other was an
irrelevant tool that was not required to open the container.
with each pair of objects. After the free play period, the experimenter
first demonstrated the success phase and then the failure phase. Fur­
thennore, in the success phase followed by the failure phase, the
demonstrator showed alternative strategies. That is, two demonstrated
conditions existed: trying to open the container (a) with the irrelevant
tool in the success phase and by hand in the failure phase, or (b) by hand
in the success phase and with the irrelevant tool in the failure phase.
The order of the conditions in each session was counterbalanced. All
sessions were videotaped.
The tapes were analyzed by the same method in Experiment I. Exclud­
ing performances that the chimpanzees spontaneously did in free play, we
identified 37 performances in the success phase and 38 performances in the
failure phase in total.
lnterobserver agreement by two independent observers of the perfor­
mances calculated using the kappa statistic was high, k = .89, p < .0 I.
Results
Procedure, Data Analysis, and Scoring
The procedure was similar to that in Experiment I but differed in the
order of presentation of the failure and success phases. Each chimpan­
zee participated in a total of four sessions. One session was conducted
Successful Pe,formances
There was only one case in which the chimpanzees could open
the containers in the success and failure phases. Jn the success
Table 3
Frequency of Types of Pe,formances Used by Chimpanzees After the Dem.onstrator
Attempted 10 Open the Contain.er
Manipulation by human demonsu-ator
By hand
With irrelevant tool
Manipulation by subject
Failure
Success
Total
Failure
Success
Total
By hand
With irrelevant tool
7
5
13
I
20
3
14
4
15
29
6
7
387
IMlTATION OF INTENTIONAL ACTIONS IN CHIMPANZEES
Table 4
Proportion of Pe,formances Reproduced by Chimpanzees in Each Demonslra.ted
Molar Pattern in Each Phase
Motor patterns
Phase
Pulling
(Sessions I, 4, 7)
Pushing
(Sessions 2, 3, 6, 8)
Twisting
(Session 5)
Failure
Success
7.0 (1/14)
10.5 (2/19)
28.6 (4/14)
23. I (3/13)
0.0 (0/1)
0.0 (0/1)
Nore. The first number in parentheses represents the number of the performances that subjects reproduced from
the observed motor patterns. The second number represents the total numbers of subjects' pe1formances in three
types of sessions.
phase, 1 chimpanzee, Pan, successfully opened the containers by
hand (7% of a total of 15 cases). Pan opened it using the strategy
that had previously been shown by the demonstrator. The latency
to open the container was 42 s (Session l).
Reproduction of the Strategy or Manipulating Objects
Table S illustrates the frequencies of chimpanzees' perfor­
mances for each rype of demonstration in the success and failure
phases as a function of the strategies shown by the demonstrator.
We found that the chimpanzees used the demonstrated strategy
significantly more often than an alternative strategy in the success
phase: Wilcoxon's signed rank test, T(N = 6) = 0, p < .OS, 97%
in the success phase.
On the other hand, in the failure phase, the chimpanzees signif­
icantly used the strategy that had previously been shown in the
success phase in both conditions: T(N = 5) = 0, p < .OS, 92% in
the failure phase. No difference was found in the choice of strategy
between two conditions in the two phases: T(N = 4) = 6, ns, in the
success phase; T(N = 3) = 4, ns, in the failure phase. The
chimpanzees significantly tended to use the demonstrated strategy
in the success phase, even though they saw the alternative strate­
gies in the failure phase. Figure 4 presents the percentage of
performances in which the chimpanzees performed using identical
strategies shown by the demonstrator in both phases. There were
no performances in which the container was directed to the tool;
the chimpanzees always manipulated the tool alone or directed it
toward the container.
phases. However, the chimpanzees reproduced some aspects of the
demonstrated actions only in the success phases. When the chim­
panzees saw the demonstrator succeed in opening the comainers,
they used the identical su·ategy in the success phase. On the other
hand, once they had seen the demonstrator succeed, chimpanzees
did not use the alternative strategy demonstrated in the failure
phase.
Experiment 3
In previous experiments, the chimpanzees produced a few more
actions in the failure and success phases compared with the free
play condition. The findings suggest that the chimpanzees bene­
fited from the demonstrator's action in the two phases. However,
the longer the chimpanzees manipulated the containers, the more
likely they would be LO discover how Lo open them. In Experi­
ment 3, we explored the effect of extended practice on the chim­
panzees by presenting some sets of objects in a free play situation
that lasted as long as the combined free play, failure, and success
phases.
Method
Subjects
The subjects in Experiment 3 were the same 5 chimpanzees who were
tested in Experiments l and 2.
Test Materials
Discussion
There were few successes in which the chimpanzees could open
the containers after observing the demonstration in the two test
Four additional pairs of objects were used as rest stimuli (see Appendix,
Table A3). In each pair, one object was a container, and the other was an
irrelevant tool that was not required to open the container.
Table S
Frequency of Types of Pe,formances Used by Chimpanzees After the Demonstrator Auempled 10
Open the Con1ainer in Each Phase
Subjects' manipulation
Failure phase
Success phase
Demonstrator's
manipulation
By hand
By hand
With irrelevant tool
2
17
With
irrelevant tool
18
By hand
With
irrelevant tool
15
0
21
388
MYOWA-YAMAKOSHl AND MATSUZAWA
100
DBy hand
• With irrelevant tool
� 80
60
<I)
40
<I)
20
0
Failure
Success
Phase
Percentage of performances ( + SE) in which the chimpanzees
performed using the identical strategies shown by the demonsu·aror in both
phases in Experiment 2.
Figure 4.
Procedure
Each chimpanzee participated in a total of four sessions; one session
was conducted with each pair of objects. Each session consisted of three
phases of free play. A pair of objects was presented 10 the chimpanzees
for approximately 3 min of free play in each phase. During this time,
they interacted in some way with each of the two objects. The demon­
strator then retrieved the objects and gave a piece of food to the
chimpanzees in exchange for the objects. This procedure was repeated
two times.
Results and Discussion
There were seven cases in which the chimpanzees opened the
containers. When they succeeded in opening the containers in
Experiment 3, the mean latency to open them was within 3 min. ff
a chimpanzee had failed to open the container in the first free play
period, she did not open it in the later two free play periods (3
minutes X 3 times). These results of Experiment 3 indicate that
extended practice to manipulate the containers did not affect the
later opening of containers.
General Discussion
How Do Chimpanzees "Understand" Demonstrated
Actions?
In this study, we used a completion procedure to investigate
whether chimpanzees understand the intentions of other indi­
viduals. Expeiiment I showed that the chimpanzees were rarely
able to open the containers in the failure phase (less than l l %
of total cases). Overall, there was no clear difference in the
success of actually opening the container between the failure
and success phases. The chimpanzees did not appear to be more
successful, even after being shown how to open the container.
How might we interpret the results of Experiment I? Did the
chimpanzees understand any of what the demonstrator tried LO
impart? There are two possible hypotheses. One is that the chim­
panzees already understood something about the intention of the
demonstrator in the failure phase. The other is that the chimpan­
zees could not understand the demonstrator's intention, even after
observing the outcomes of the demonstrated actions.
Although the chimpanzees did not open the container very often
after observing the demonstration, when they did open it, they
reproduced the demonstrated strategy in both phases in Experi­
ment 1. Myowa-Yamakoshi and Matsuzawa ( 1999) found that
chimpanzees were less sensitive to imitating a demonstrator's
body movements compared with imitating the manipulations of the
objects involved in the demonstrated action. Therefore, although
the chimpanzees might have understood the demonstrator's inten­
tion in the failure phase, it might be difficult for them to actually
reproduce opening the container from the view of an all-or-none
issue.
These results could be easily explained in terms of simple
stimulus enhancement, in which seeing some act done on a par­
ticular object would increase the observer's probability of inter­
acting with that object (Spence, 1937). However, when we com­
pare the results of Experiments I and 2, we can rule out stimulus
enhancement as an explanation. Chimpanzees tried to complete the
demonstrator's alternative strategies in success phases when they
could not have understood how to open the containers in failure
phases (Experiment l ). On the other hand, once they had seen the
demonstrator succeed in success phases, they did not try to follow
the demonstrator's alternative strategies in failure phases (Exper­
iment 2). Chimpanzees seemed to understand that there was no
need to try alternative strategies because they already knew how to
open the container from the success phases. They learned from the
demonstrator some causal relationships between the tools used to
open the containers or the containers to be opened in the action.
(Tomasello, 1990). Namely, these findings would seem to go
beyond stimulus enhancement.
Moreover, the fact that the chimpanzees produced a few more
actions in the failure and success phases compared with the free
play could be explained as an effect of extended exposure to the
containers. However, this explanation seems unlikely in light of
the findings of Experiment 3. In Experiment 3, we presented
chimpanzees sets of objects in a free play situation that lasted
as long as the combined free play, failure, and success phases in
the first two experiments. Even so, the mean latencies from the
first contact with the containers to opening them in Experi­
ment 3 were not different from those of free play in Experi­
ments I and 2,
We concluded that the chimpanzees understood something
about the intention of the demonstrator in the failure phase. How­
ever, they mainly observed the directionality of objects as cues
rather than the demonstrator's body movements (i.e., pulling,
pushing, and twisting). In that sense, chimpanzees may interpret
other's actions in different ways from humans. For that reason, our
results differ from the results for human infants (Carpenter et al.,
1998; Meltzoff, 1995)
IMITATION OF INTENTIONAL ACTIONS TN CHlMPANZEES
Role of Body Movement in Understanding the Actions of
Others
It is fruitful to compare our results with previous studies in
which researchers investigated chimpanzees' understanding of the
intentions of others. These studies have produced both positive and
negative results. We will review the tasks used in each study to
determine what kind of cues lead chimpanzees to understand
others' intentions.
As mentioned above, Myowa-Yamakoshi and Matsuzawa
(1999) suggested that chimpanzees might not be sensitive to the
details of the body movements of others. Chimpanzees may have
more difficulty than humans in understanding gestures consisting
of only body movements (see also Call & Tomasello, l99 5, for
examples involving orangutans). This view is congruent with the
negative results of experiments in which the subject was required
to discriminate between another's intentional and accidental
actions.
Povinelli et al. (1998) reported that 5- to 7-year-olcl chimpan­
zees did not understand the distinction between accidental and
intentional actions. In our view, this seems reasonable. The acci­
dental and intentional actions Povinelli et al. studied had physi­
cally equivalent outcomes (e.g., the actor's hand touched the cup,
and the juice in the cup spilled on the floor). The only difference
between the two actions was the actor's body movements defined
from a human perspective (e.g., the actor intended to pour a cup of
juice slowly, or the actor accidentally spilled it). We suggest that
it was difficult for chimpanzees to distinguish these actions be­
cause they paid more attention to the information in the physically
equivalent outcomes than to the actor's body movements.
The same interpretation may be applied to the study by Toma­
sello, Call, and Gluckman (1997). Young children (2.5 and 3.0
years old), chimpanzees, and orangutans observed a human exper­
imenter hide a reward in one of three opaque containers on a
wooden plank. Another experimenter played the role of commu­
nicator and helped the subject find the reward using one of three
signs: pointing, marker, and replica. Pointing involved placing a
hand directly above the correct container with the index fmger
oriented clown. Marking was when a wooden block was placed on
top of the correct container. Replica involved holding up a dupli­
cate of the correct container. The results showed that young
children understood all three communicative signs above the
chance level. On the other hand, apes that were not previously
trained to use the marker and pointing strategies did not score
above chance for any of the intentional communicative signs.
Tomasello et al. (1997) argued that the naive apes did not under­
stand the communicated intentions of others. However, although
the naive apes all performed at the chance level, their best perfor­
mance was with the marker, followed by the replica and pointing.
This result is also consistent with ours, because the apes may have
responded to the containers and objects (marker and replica) held
by the communicator rather than to the co111municator's body
movement (pointing).
We investigated whether chimpanzees could understand a dem­
onstrator's intention without seeing the outcome of the actions.
The results suggest that chimpanzees do not understand others'
intentions in the same way that humans do. Human infants under­
stand the intentions of others in a psychological framework
through body movements. On the other hand, chimpanzees antic-
389
ipate the forthcoming action of others by perceiving the direction­
ality and physical causality of objects as a more available cue
compared with the details of the body movements of the demon­
strator performing the manipulation. In our task, we can only
speculate whether it is more difficult for chimpanzees to distin­
guish between actions that have the same outcome or body move­
ments to understand another's intentions.
In the present ·study, we used a completion procedure to show
that chimpanzees can understand another's intentions. In the pub­
lished studies on whether chimpanzees understand the intention of
others, there may have been different levels of task difficulty. The
constraint on visual-motor information processing may have pro­
duced mixed results regarding the ability of chimpanzees to un­
derstand intention. Whereas humans interpret another's intentions
by perceiving both body movements and the directionality of
manipulated objects, chimpanzees depend mainly on the informa­
tion about the manipulated objects to anticipate the intentions of
others.
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Appendix
Table Al
The Eight Pairs of Objects Used and the Motor Patterns Needed
to Open Each Container in Experiment 1
Set of objects
Session
Container
Irrelevant tool
Motor pattern
1
2
3
4
Transparent box
Airtight jar
Fishing box
Chopstick case
Sprayer
Plastic basket
Key box
Case for spectacles
Spatula
Wooden handle
Brush
Screwdriver
Tongs
Wooden spoon
Pin
Cap opener
Pulling
Pushing
Pushing
Pulling
Twisting
Pushing
Pulling
Pushing
5
6
7
8
•