The ability to monitor and control one’s own cognitive states, metacognition, is crucial for effective learning and problem solving. Although the literature on animal metacognition has grown considerably during last 15 years, there have been few studies examining whether great apes share such introspective abilities with humans. Here, we tested whether four gorillas could meet two criteria of animal metacognition, the increase in escape responses as a function of task difficulty and the chosenforced performance advantage. During testing, the subjects participated in a series of object choice memory tests in which a preferable reward (two grapes) was placed under one of two or three blue cups. The apes were required to correctly select the baited blue cup in this primary test. Importantly, the subjects also had an escape response (a yellow cup), where they could obtain a secure but smaller reward (one grape) without taking the memory test. Although the gorillas received a relatively small number of trials and thus experienced little training, three gorillas significantly declined the memory tests more often in difficult trials (e.g., when the location of the preferred reward conflicted with side bias) than in easy trials (e.g., when there was no such conflict). Moreover, even when objective cues were eliminated that corresponded to task difficulty, one of the successful gorillas showed evidence suggestive of improved memory performance with the help of escape response by selectively avoiding trials in which he would be likely to err before the memory test actually proceeded. Together, these findings demonstrate that at least some gorillas may be able to make optimal choices on the basis of their own memory trace strength about the location of the preferred reward.

1. ORIGINAL PAPER
Gorillas’ use of the escape response in object choice memory tests
Chikako Suda-King • Amanda E. Bania •
Erin E. Stromberg • Francys Subiaul
Received: 7 May 2012 / Revised: 31 July 2012 / Accepted: 7 August 2012 / Published online: 25 August 2012
Ó Springer-Verlag 2012
Abstract The ability to monitor and control one’s own
cognitive states, metacognition, is crucial for effective
learning and problem solving. Although the literature on
animal metacognition has grown considerably during last
15 years, there have been few studies examining whether
great apes share such introspective abilities with humans.
Here, we tested whether four gorillas could meet two cri-
teria of animal metacognition, the increase in escape
responses as a function of task difficulty and the chosen-
forced performance advantage. During testing, the subjects
participated in a series of object choice memory tests in
which a preferable reward (two grapes) was placed under
one of two or three blue cups. The apes were required to
correctly select the baited blue cup in this primary test.
Importantly, the subjects also had an escape response (a
yellow cup), where they could obtain a secure but smaller
reward (one grape) without taking the memory test.
Although the gorillas received a relatively small number of
trials and thus experienced little training, three gorillas
significantly declined the memory tests more often in dif-
ficult trials (e.g., when the location of the preferred reward
conflicted with side bias) than in easy trials (e.g., when
there was no such conflict). Moreover, even when objective
cues were eliminated that corresponded to task difficulty,
one of the successful gorillas showed evidence suggestive
of improved memory performance with the help of escape
response by selectively avoiding trials in which he would
be likely to err before the memory test actually proceeded.
Together, these findings demonstrate that at least some
gorillas may be able to make optimal choices on the basis
of their own memory trace strength about the location of
the preferred reward.
Keywords Metacognition Á Ape Á Gorillas Á Escape
response
Introduction
Metacognition, the ability to reflect on our knowledge, is
essential for learning and problem solving (Nelson and
Narens 1990, 1994; Dunlosky and Metcalfe 2009). Imagine
a student preparing for a test. While reading the textbook,
the student will know what material is familiar and known
and what material requires more time for studying, allo-
cating her time and effort accordingly. During the test, the
student might skip difficult questions for which she does not
remember correct answers and might try to complete easy
questions first. After the test, the student will be able to
accurately judge the accuracy of her answers and might try
to improve her future grade by spending more time studying
the topics where she performed poorly. Such introspective
cognitive monitoring and control is called metacognition, or
C. Suda-King Á A. E. Bania Á E. E. Stromberg Á F. Subiaul
Think Tank at the Smithsonian’s National Zoological Park,
Washington, DC, USA
C. Suda-King (&)
Center for Animal Care Sciences, Smithsonian Conservation
Biology Institute, PO Box 37012, MRC 5507,
Washington, DC 20013-7012, USA
e-mail: chimpkako@hotmail.com; kingch@si.edu
F. Subiaul
Department of Speech and Hearing Science, GW Mind-Brain
Institute and Institute for Neuroscience, The George Washington
University, Washington, DC, USA
F. Subiaul
Department of Anthropology, Center for the Advanced Study
of Human Paleobiology, The George Washington University,
Washington, DC, USA
123
Anim Cogn (2013) 16:65–84
DOI 10.1007/s10071-012-0551-5

2. thinking about thinking (Dunlosky and Metcalfe 2009;
Nelson and Narens 1990, 1994). Adult humans can accu-
rately judge their own memory prospectively, simulta-
neously, and retrospectively. They are also able to employ
appropriate strategies (e.g., seeking for information) on the
basis of such awareness to achieve better task performance
(Dunlosky and Metcalfe 2009). This skill, however,
develops gradually throughout childhood continuing well
into adolescence (Schneider 2008); though, children as
young as preschool age may have implicit awareness of
their own memory (e.g., Balcomb and Gerken 2008). The-
oretically speaking, the human metacognition model con-
sists of the object level and the meta-level, with the latter
monitoring and controlling the former (Nelson and Narens
1990, 1994). Because the model assumes that the meta-level
acts as executive functioning on the basis of secondary
representations (metarepresentations) of object-level rep-
resentations, metacognitive abilities have been associated
with both consciousness and self-awareness (e.g., Shea and
Heyes 2010).
Given that subjective introspection implies such higher
cognitive faculties, a fascinating question is whether non-
human animals share metacognitive skills with humans.
Since the pioneering work by Smith et al. (1995) exam-
ining a dolphin’s use of the uncertainty response in an
auditory discrimination task, comparative psychologists
have developed innovative nonverbal procedures in order
to assess subjects’ metacognitive abilities. In many com-
parative metacognition paradigms, animals are tested on a
primary task such as a perceptual discrimination or mem-
ory task. During testing, subjects are required to make a
perceptual (e.g., respond to largest item) or memory (e.g.,
respond to familiar item) response. Importantly, the sub-
jects are also allowed to make a secondary response, the
metacognitive response. Here, they can decline the current/
future test, seek for task-related information before taking
the test, or make a retrospective confidence judgment about
their own performance. The logic is this: Subjects should
use the metacognitive response based on the awareness of
their own cognitive states, such as uncertainty or the
strength of the memory trace. Accordingly, two standard
criteria inferring animal metacognition are as follows: (1)
Subjects should use the metacognitive response more
during difficult than during easy trials (for retrospective
judgments, subjects should rate difficult trials as low con-
fidence while rating easy ones as high confidence), and (2)
the subjects’ primary task performance should be better
when the subjects are given the option to make the meta-
cognitive response than when they are not (i.e., are forced
to take the test) (Hampton 2001; Inman and Shettleworth
1999; Sutton and Shettleworth 2008). Rhesus monkeys,
orangutans, and rats have met both of these criteria
(Hampton 2001; Hampton et al. 2004; Call and Carpenter
2001; Call 2005; Suda-King 2008; Foote and Crystal
2007), while the dolphin in Smith et al.’s (1995) study has
produced evidence meeting the first criterion only. Capu-
chin monkeys and birds have yielded mixed results. Results
suggest that these species’ functional use of the metacog-
nitive response might be restricted to specific circum-
stances (Fujita 2009; Beran and Smith 2011; Paukner et al.
2006; Basile et al. 2009; Inman and Shettleworth 1999;
Sutton and Shettleworth 2008; Nakamura et al. 2011). So
far, dogs have failed to show any convincing evidence of
metacognitive abilities (Bra¨uer et al. 2004; McMahon et al.
2010).
Although the literature on animal metacognition con-
tinues to grow, the question of how to interpret the previous
data on animal metacognition remains a subject of consid-
erable debate, and alternative nonmetacognitive interpre-
tations have been proposed for the animals’ apparently
effective use of the metacognitive response (e.g., Hampton
2009). First, in many previous studies, observable cues have
been correlated with task difficulty (e.g., Smith et al. 1997).
Consequently, subjects may associate these cues with an
optimal response strategy that resembles metacognition. For
instance, in perceptual discrimination tasks, which typically
include numerous testing trials, the subjects might learn that
stimuli near their perceptual threshold result in poor per-
formance and consequently might avoid those stimuli
without any understanding of their own uncertainty. Sec-
ond, some studies have presented the primary responses
with the metacognitive response simultaneously, putting
these responses in direct competition with one another (e.g.,
Beran et al. 2006). As a result, subjects may learn which
primary response is most attractive and might learn to opt
for the metacognitive response when the degrees of attrac-
tiveness are roughly equal among primary responses. In
other words, the subjects could learn to select the secondary
response when they find themselves wavering between the
primary responses. Similarly, the response strength model
argues that the metacognitive response creates constant
attractiveness when the secondary response is plotted
against the test stimulus continuum, whereas each of the
primary responses produces different degrees of response
strength across the continuum, with the degree of attrac-
tiveness determined through previous reinforcement his-
tory. According to this model, the animals compare the
degrees of response strength for all the available responses
(both primary and secondary) and choose the one that is
most attractive without any awareness of their uncertainty
or confidence (Smith et al. 2008; Crystal and Foote 2009).
In an attempt to control for these confounds, various
experimental modifications have been introduced to animal
metacognition research. These modifications are the fol-
lowing: (1) introducing a small number of testing trials to
minimize learning effect (Hampton et al. 2004; Call and
66 Anim Cogn (2013) 16:65–84
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3. Carpenter 2001; Suda-King 2008), (2) designing a primary
task so that its difficulty is determined by subjective states
(such as memory strength) rather than objectively acces-
sible cues (i.e., perceptual discrimination cues) to avoid
associative learning (Hampton 2001; Kornell et al. 2007),
(3) arranging the presentation of the metacognitive
response temporarily distant from (either before or after)
the primary responses to avoid response competition
(Hampton 2001; Suda-King 2008; Kornell et al. 2007;
Smith et al. 2006; Fujita 2009; Son and Kornell 2005), (4)
introducing different tasks or new sets of stimuli to
examine whether the animals’ effective use of the meta-
cognitive response transfers without additional training to
further exclude the possibility of associative learning
(Kornell et al. 2007; Smith et al. 2010), (5) dissociating
reinforcement histories from task performance to refute the
response strength model (Smith et al. 2006), and (6) using
the pure escape response that simply generates a new trial
without direct rewards (Beran et al. 2006). Overall, accu-
mulating evidence has shown that rhesus monkeys continue
to effectively use the metacognitive response once all these
controls have been adopted (see a review by Terrace and
Son 2009). In contrast, there have been only three studies
examining great apes’ metacognitive abilities (Call and
Carpenter 2001; Call 2005; Suda-King 2008), only one of
which tested gorillas as subjects. Furthermore, this study
examined only the first criterion of animal metacognition,
that is, whether the gorillas would seek for information
more often in difficult trials (when they did not know the
location of hidden food reward) than in easy ones (when
they knew where the food was) (Call 2005). It is therefore
necessary to test whether this species of great ape can also
meet the second criterion of the chosen-forced performance
advantage. The investigation of gorillas’ potentially meta-
cognitive skills seems to be especially important because
gorillas were once considered to show little evidence of
mirror self-recognition (Suarez and Gallup 1981; Ledbetter
and Basen 1982; Shillito et al. 1999), a skill associated with
self-awareness (Gallup 1998). Although it has been even
argued that this species of great ape has lost their capacity
for self-recognition after splitting from a common ancestor
with the other great ape species (Povinelli 1993; Gallup
1997), more recent studies have revealed that at least some
gorillas may recognize themselves in mirrors (Parker 1994;
Patterson and Cohen 1994; Swartz and Evans 1994; Posada
and Colell 2007). Evidence of metacognition in gorillas
may provide additional evidence consistent with self-
awareness in this species and help paint a more complete
picture of the origins of these skills in great apes.
Here, we examined whether gorillas could decline dif-
ficult trials on the basis of their own memory trace strength
about the location of a food reward in a relatively small
number of trials. We tested four gorillas using the proce-
dure developed by Suda-King (2008). Using this paradigm
Suda-King (2008) demonstrated orangutans’ functional use
of a metacognitive ‘‘escape’’ response in a series of object
choice spatial memory tasks. In this study, orangutans met
the first criterion of metacognition, declining the test more
often in difficult trials than in easy trials. One of the
orangutans tested also met the second criterion of meta-
cognition, performing significantly better in a memory test
that she had agreed to take (i.e., free-choice trials) than
when forced to take the memory test (i.e., forced trials).
The gorillas’ primary task in the current study was to
remember the location of a preferred reward (i.e., two
grapes) hidden under one of several blue cups. On some
trials, the subjects also had an option of declining trials and
gaining a secure, but less preferable reward (i.e., one grape)
placed inside a yellow cup. We tested the two specific
predictions of metacognition: (1) The subjects should use
the escape response more often during difficult trials than
during easy trials, and (2) subjects should be more accurate
in the primary task during free-choice trials where they can
choose to take the test or not than during forced trials
where they are forced to take the test.
In order to examine the first criterion, we adjusted the
task difficulty by manipulating the visibility of baiting
(Visible vs. Hidden conditions in Experiment 1) and the
subjects’ visual access to the testing stimuli (Cover vs. No
Cover tests in Experiment 3). To test the second criterion,
we manipulated the availability of the escape response and
examined whether the gorillas were more accurate in the
memory test in free-choice trials than in forced-choice
trials (Free vs. Forced conditions in Experiments 2 and 3).
Also, in an attempt to rule out the response competition
account, we presented the escape response prior to the final
configuration of the memory test (in Experiments 2 and 3)
and introduced a new test in which the test stimuli (the blue
cups) were temporarily occluded from the subjects while
they decided to either take the test or escape (in Experi-
ment 3).
Due to the small sample size, we used nonparametric
tests for all subsequent analyses. All analyses were two-
tailed. All P values were exact, and the probability of a
type I error was maintained at 0.05.
Experiment 1
The purpose of this experiment was to examine whether
gorillas could effectively escape a memory test based on
the presence/absence of memory about the location of
preferred reward. The gorillas were tested using the same
procedure used in Suda-King (2008, Experiment 2). The
Anim Cogn (2013) 16:65–84 67
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4. subjects were first presented with a pair of identical blue
cups. The experimenter then hid a highly preferred reward
(two grapes) beneath one of them. The baiting procedure
was visible in half of the trials. In the other half, cups were
baited behind an occluder. The experimenter then placed a
yellow cup—the escape response—between the blue cups.
If it was chosen, subjects would obtain a guaranteed but
less preferred food reward (i.e., one grape). Selecting a
blue cup resulted in either a highly preferred food reward
or no reward at all. If the gorillas could tell when they had
not seen the location of the preferred reward, they should
choose the escape response more often in difficult trials,
when the baiting procedure is hidden, than in easy trials
when the baiting is visible.
Methods
Subjects
Four gorillas housed at the Smithsonian’s National Zoo-
logical Park served as subjects: one adult male (Baraka,
16 years old at the time of testing), two juvenile males
(Kojo and Kwame, 7 and 9 years old, respectively), and
one adult female (Mandara, 26 years old). The female is
the mother of the two juveniles. Although the adult male
was related to neither the female nor her offspring, the
female gorilla adopted him shortly after his birth and raised
him. When this male was 12 years old, he was transferred
to another facility for breeding, but a few years later
returned to the National Zoo as an adult during the course
of the current study. The subjects had previously partici-
pated in a computerized memory task (Subiaul unpublished
data), but had never participated in metacognition research.
The subjects lived with two other adult females in a social
group in indoor and outdoor compounds. These subjects
showed little interest in research and thus did not partici-
pate in the current study. Diets consisted of monkey chow,
fruits, greens, and vegetables, and several smaller enrich-
ment feedings were offered throughout the day, including
various types of browse and forage type foods. Testing
sessions occurred during normal daily separations of the
animals from their group, when individuals were separated
in their indoor enclosures for the afternoon diets. Water
was available ad libitum, and the subjects were not food
deprived during the testing.
Apparatus
The apparatus used in the current study were identical to
those used by Suda-King (2008, Experiment 2). Two opaque
blue plastic cups (7 cm top diameter 9 6 cm bottom diam-
eter 9 10 cm in height) and two opaque yellow plastic cups
(7 cm top diameter 9 6 cm bottom diameter 9 10 cm in
height) were used as hiding locations of rewards. The blue
cups were paired with two blue plastic dishes (9 cm top
diameter 9 7.5 cm bottom diameter 9 1.5 cm in height),
so that two grapes could be placed on a dish and covered with
a cup positioned upside down. One of the yellow cups was
filled with grapes and covered with a yellow plastic lid
(7.8 cm diameter 9 0.7 cm in height). This procedure pre-
vented the subjects from seeing the contents in the cups. The
other yellow cup was paired with a yellow dish (9 cm top
diameter 9 7.5 cm bottom diameter 9 1.5 cm in height),
so that one grape could be placed on it and covered with the
cup. In addition, two green plastic dishes (9 cm top diame-
ter 9 7.5 cm bottom diameter 9 1.5 cm in height) were
used to present two grapes and one grape in order to assess
the subjects’ reward preference. A four-sided wooden barrier
(34.5 cm width 9 19.5 cm depth 9 20 cm in height) was
used to visually block the baiting procedure from the subjects
on some trials (see below). The cups were placed on a
Plexiglas platform (94 cm length 9 25.5 cm width 9
2.5 cm in height), which rested on a wooden table (94 cm
length 9 61 cm width 9 83–92 cm in height) that was
positioned in front of a Plexiglas panel (either 112 cm 9
92 cm, 135 cm 9 92 cm, or 145 cm 9 92 cm, depending
on the cage used during testing; cages were at different
heights from the floor). The removable panel was attached to
the cage mesh with hooked metal bolts prior to the start of
testing. Each panel had three circular holes (3.3 cm in
diameter and 18.5 cm apart from each other) lined up near
the bottom, so that the subjects could indicate their choices
by inserting their fingers through one of them. The platform
had two plastic handles that allowed the experimenter to
easily slide it back and forth on the table. The height of the
table could be adjusted appropriately such that the platform
was positioned just below the holes in the Plexiglas panel.
All the testing trials were filmed with a Panasonic video
camera. The experimenter wore an earphone metronome to
count the length of each testing trial, and she also wore a
baseball cap throughout testing in an attempt to prevent the
subjects from using any social cues. When the subjects
were making their choice, she looked down at the center of
the platform such that the cap partially blocked her upper
face from their perspective.
Procedure
The procedure of the current experiment was identical to
those of Suda-King (2008, Experiment 2).
Training Prior to the testing, the experimenter trained the
subjects to point at a grape that was placed in front of a
hole in the Plexiglas panel. The subjects readily learned to
request a reward by inserting their finger through a corre-
sponding hole.
68 Anim Cogn (2013) 16:65–84
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5. Pretest Having completed the training phase, the subjects
proceeded to a pretest phase. The pretest phase consisted of
three tests: (a) Preference Test, (b) Low-Value Food
Association, and (c) Memory Test. The Preference Test was
conducted to make sure that the gorillas preferred two
grapes over one. The experimenter stood in front of each
subject, positioned the platform about 36 cm away from
the Plexiglas panel, and presented a pair of the green dishes
side by side on the center of the platform. While making
sure that the subject was paying attention, she placed two
grapes and one grape, respectively, on the dishes. She then
slid the dishes from the center to opposite sides of the
platform and positioned them in front of the far right and
left holes in the Plexiglas panel. Finally, the experimenter
pushed the platform against the Plexiglas panel, so that the
subject could choose one of the dishes. The subjects
received the contents of the selected dish. The locations of
the two different quantities were counterbalanced and
randomized across trials with a restriction that two grapes
did not appear in the same location in more than two
consecutive trials. The subjects were given 4–10 sessions
of 24 trials each, and they all came to prefer the larger
quantity of reward ([83 %).
The Low-Value Food Association was conducted to
facilitate the subjects’ learning of the contingency between
choosing the yellow cup and receiving one grape. The
experimenter presented the yellow cup with a lid filled with
grapes in front of one of the holes in the Plexiglas. Once the
subject chose the yellow cup, the experimenter lifted the lid,
picked up one grape, and gave it to the subject. Subjects
received 24 trials and always pointed at the yellow cup to
receive one grape.
The Memory Test was conducted to assess the subjects’
performance in a memory task in which two grapes were
hiddenunderneathone ofthe two bluecups. Theexperimenter
placed a pair of the blue dishes side by side on the center of the
platform and covered them with a pair of the upside-down
blue cups. While making sure that the subject was paying
attention, she then lifted one of the cups, placed two grapes on
the dish underneath, and hid the reward by covering it with the
cup. The experimenter slid the cups from the center to the
opposites of the platform such that the cups came in front of
the far right and left holes in the Plexiglas panel. Finally, she
pushed the platform against the Plexiglas panel to allow the
subject to make a choice. The subject received two grapes
only when it selected the baited blue cup. One session con-
sisted of a random mixture of 18 memory trials, six preference
trials, and six low-value association trials. All subjects pro-
ceeded to testing as they reached the criteria of performing
significantly above chance in the memory trials (C14/18,
P 0.031, Binominal test) within two sessions.
Test Experiment 1 had the following four conditions. The
(?) and (-) in the condition names indicate whether the
contents of the escape option were revealed or remained
concealed before the subjects made a choice (see below).
Visible (-) The procedure was identical to the Memory
Test described above where the experimenter hid two
grapes underneath one of the blue cups and placed them in
front of the far right and left holes. She then placed the
covered yellow cup filled with grapes in front of the center
hole in the Plexiglas panel such that it was located between
the blue cups. Finally, the experimenter pushed the plat-
form against the Plexiglas panel to allow the subject to
choose one of the cups. The subject received the corre-
sponding contents of the selected cup. The subject received
two grapes only upon choosing the baited blue cup,
whereas the selection of the empty blue cup led to no
reward. If the subject chose the yellow cup instead, the
experimenter lifted its lid and gave one grape to the sub-
ject. The yellow cup thus served as the escape response
with which the subject could receive a less preferable but
secure reward. Each trial was timed by the metronome and
lasted about 10 s from the moment when the experimenter
covered two grapes with the blue cup until the platform
touched against the Plexiglas panel (retention inter-
val = about 10 s).
Hidden (-) This condition was identical to the Visible
(-) condition except that the baiting of two grapes was
hidden from the subject. After presenting the two blue cups
with a dish underneath each on the center of the platform,
the experimenter covered the test stimuli with the wooden
box such that its opening sides faced down and toward the
experimenter. She then showed two grapes to the subject,
and surreptitiously hid the grapes under one of the blue
cups out of the subject’s view by lifting each cup behind
the barrier. The experimenter removed the box, and the rest
of the procedure was identical to the Visible (-) condition.
Each trial was timed by the metronome and lasted about
10 s from the moment when the experimenter removed the
wooden barrier until the platform touched against the
Plexiglas panel (retention interval = about 10 s).
Visible (?) The procedure was the same as in the Visible
(-) condition except that the subjects could see the con-
tents of the escape response. After hiding two grapes under
one of the blue cups and placing them in front of the far
right and left holes of the Plexiglas panel, the experimenter
presented the yellow dish containing one grape in front of
the center hole of the panel such that it was located
between the two blue cups. She then immediately covered
the grape with the yellow cup. Finally, the experimenter
pushed the platform against the Plexiglas panel to allow the
Anim Cogn (2013) 16:65–84 69
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6. subject to choose one of the cups. The subject received the
corresponding contents of the selected cup.
Hidden (?) The procedure was the same as in the Hidden
(-) condition except that the subject could see the contents
of the escape response. After removing the wooden barrier
and sliding the blue cups from the center to opposite sides
of the platform, the experimenter placed the yellow dish
containing one grape between the blue cups such that it was
located in front of the center hole of the Plexiglas panel.
She then immediately covered the grape with the yellow
cup and pushed the platform against the Plexiglas panel to
allow the subject to choose one of the cups. The subject
received the corresponding contents of the selected cup.
We conducted 24 trials for each condition per subject.
Besides the testing trials, 48 pretest preference trials were
interspersed among the trials of the four testing conditions
in order to assess the gorillas’ preference for the larger
quantity of grapes and maintain the subjects’ motivation
throughout the testing. Each subject received eight sessions
consisting of 18 trials each, for a total of 144 trials. Each
session was a random mixture of 12 testing trials (three
trials per condition) and six preference trials. Each session
started with three low-value food association trials, which
was designed to be as warm-up and a reminder of the
contingency between selecting the lidded yellow cup and
receiving one grape. The position of two grapes was
counterbalanced and randomized with a restriction that
they did not appear in the same position in more than two
consecutive trials.
Results
Memory performance and escape response
Table 1 presents the performance of each subject in the
four testing conditions and preference trials. All subjects
showed a significant preference for two grapes over one
grape in the preference trials, P 0.001, Binomial test.
When taking the memory test (i.e., when choosing a blue
cup), all subjects performed significantly above chance in
both of the Visible conditions, whereas none of them sig-
nificantly exceeded chance level in either of the Hidden
conditions (Visible conditions, P 0.005; Hidden condi-
tions, P [ 0.15, Binomial test). Therefore, the gorillas
were skillful at selecting a baited blue cup when they could
see the experimenter placing two grapes underneath one of
the blue cups, whereas they guessed the location of the
preferred reward when they had not seen the baiting.
In order to examine whether the gorillas were able to use
the escape response differentially among the four testing
conditions, we conducted a Cochran’s Q test on each
individual. Only one juvenile male, Kojo, differentially
used the escape option at a statistically significant level
(P = 0.001). Overall, he selected the yellow cup 10 times
in the Hidden conditions and 6 times in the Visible con-
ditions. Post hoc paired comparisons revealed that Kojo
chose the yellow cup significantly more often in the Hidden
(?) condition than in the Visible (-) condition (P = 0.008,
McNemar test). Kojo’s results of the both tests remained
significant even when the P values were corrected for
multiple comparisons (P = 0.004, P = 0.032, Sˇida´k cor-
rections). No other significant differences were found for
the other comparisons examining the effect of baiting
visibility (Visible (?) vs. Hidden (?), P = 0.69, Visible
(-) vs. Hidden (-), P = 0.50, Visible (?) vs. Hidden (-),
P = 0.13, McNemar tests). Hence, there was some evi-
dence that Kojo avoided the memory test more often when
he had not seen the location of the preferred reward. Two
other subjects, Baraka and Kwame, also used the escape
response more often in the Hidden conditions than in the
Visible conditions although these differences only
approached statistical significance (P = 0.063, P = 0.32,
respectively, Cochran’s Q tests). Baraka selected the yel-
low cup 3 times in the Hidden conditions, whereas he never
used it in the Visible conditions. Kwame declined the
memory test 12 times in the Hidden condition, whereas he
only declined the memory test 6 times in the Visible con-
ditions. The remaining subject, Mandara, showed the
opposite pattern, selecting the yellow cup 7 times in the
Hidden conditions and 12 times in the Visible conditions
although no significant difference was detected in her use
of the escape option among the four testing conditions
(P = 0.061, Cochran’s Q test).
Further analyses revealed that Mandara had a strong
side bias and she used the escape option when she had seen
the location of the two grapes was opposite to her bias.
Table 2 shows Mandara’s performance as a function of the
position of the two grapes. The conditions were collapsed
into two according to the baiting visibility in the current
analyses. In the Hidden conditions, Mandara was signifi-
cantly more likely to select the baited blue cup when it was
on her left side than when on her right side (P 0.001,
Fisher’s exact test). This result indicates that she strongly
preferred the left cup over the right when she had not seen
the location of the two grapes, and trials were more dif-
ficult when the reward was placed under the right cup. She
also showed the same bias in the Visible conditions, per-
forming better in the memory test when the two grapes
were placed under the left cup than under the other cup,
although the difference was only marginally significant
(P = 0.051, Fisher’s exact test). We further compared
Mandara’s memory performance against chance. When the
two grapes were placed under the left cup, Mandara’s
memory performance was significantly above chance both
in the Visible and in the Hidden conditions (P 0.001,
70 Anim Cogn (2013) 16:65–84
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7. binomial test). In contrast, her success rate was signifi-
cantly below chance when the preferable reward was
placed under the right cup behind the barrier (P = 0.017,
binomial test), an indication of her bias for the left side.
Mandara’s memory performance was marginally above
chance when she had seen the experimenter placing the
two grapes under the right blue cup (P = 0.057, binomial
test). As for the use of the escape response, Mandara was
significantly more likely to choose the yellow cup when
she had seen the experimenter placing the two grapes
under the right blue cup (difficult trials) than when she had
seen the two grapes being placed under the left blue cup
(easy trials) (P = 0.017, Fisher’s exact test). Such a dif-
ferential use of the escape option was not found for the
Hidden conditions, presumably because she could not tell
where the experimenter placed the two grapes behind the
barrier (P = 0.42, Fisher’s exact test). Altogether, these
findings imply that Mandara selectively used the safe
choice when she had seen that the position of the prefer-
able reward conflicted with her side bias and she was
consequently able to improve her memory performance by
avoiding trials that were specifically difficult for her.
Therefore, Mandara met the first criterion of animal
metacognition by demonstrating the increased use of
escape option as a function of task difficulty.
Learning effect
Finally, we examined whether the gorillas changed their
escape strategies during the course of the experiment by
dividing the 8 sessions into 2 phases and comparing the
subjects’ performance between the first and second phases.
Specifically, we conducted Fisher’s exact test on a 2 9 2
table (Take test vs. Escape, 1st phase vs. 2nd phase) for
each condition type per subject. The four conditions were
collapsed into two according to the visibility of the baiting.
None of the subjects except for Kwame showed significant
differences in their use of escape response between the first
and second phases both for the Visible and for the Hidden
conditions (P [ 0.096). Only Kwame showed some evi-
dence of learning, using the escape response significantly
less often in the second phase than in the first phases of the
Visible conditions (P = 0.022). His use of the escape
response did not differ between the first and second phases
of the Hidden conditions (P = 0.32). Considering that
Mandara’s escape strategy was affected by the reward
Table 1 Subjects’ performance in the four testing conditions of Experiment 1
Subject Frequency Success (%) Escape (%) Preference (%)
Correct Wrong Escape
Baraka 91.7**
Visible (?) 22 2 0 91.7** 0.0 –
Hidden (?) 10 11 3 47.6 12.5 –
Visible (–) 21 3 0 87.5** 0.0 –
Hidden (-) 8 16 0 33.3 0.0 –
Kojo 97.9**
Visible (?) 16 2 6 88.9** 25.0 –
Hidden (?) 7 9 8 43.8 33.3 –
Visible (-) 21 3 0 87.5** 0.0 –
Hidden (-) 12 10 2 54.5 8.3 –
Kwame 100**
Visible (?) 17 3 4 85.0** 16.7 –
Hidden (?) 8 10 6 44.4 25.0 –
Visible (-) 19 3 2 86.4** 8.3 –
Hidden (-) 8 10 6 44.4 25.0 –
Mandara 87.5**
Visible (?) 14 2 8 87.5** 33.3 –
Hidden (?) 10 8 6 55.6 25.0 –
Visible (-) 19 1 4 95.0** 16.7 –
Hidden (-) 13 10 1 56.5 4.2 –
Success (%) = a percentage of trials in which the subjects selected the baited blue cup when they selected one of the two blue cups; escape
(%) = a percentage of trials in which the subjects selected the yellow cup; preference (%) = a percentage of trials in which the subjects selected
two grapes instead of one in the preference trials
** P 0.01, above chance (Binomial tests)
Anim Cogn (2013) 16:65–84 71
123

8. location when she could see the baiting, we also conducted
Fisher’s exact test on her 2 9 2 table (Take test vs. Escape,
1st phase vs. 2nd phase) for each reward location of the
Visible conditions in order to examine whether she learned
her functional use of the escape response during the testing.
There was little evidence of her learning across the first and
second phases for each reward location (P = 1.00), for
both right and left. Therefore, Mandara’s escape strategy
remained unchanged during testing. She selectively
declined difficult trials in which she had seen the reward
location conflicted with her side bias from the beginning of
testing.
Discussion
In general, the gorillas were very skillful at remembering
the location of the preferable reward when they could see
the experimenter placing it under one of the blue cups,
whereas their accuracy in the memory test did not differ
from chance when the baiting took place out of their sight.
As predicted, three out of the four subjects selected the
escape response more often when the baiting of the pref-
erable food was hidden than when it was visible, although
only one of them, Kojo, demonstrated statistically signifi-
cant results. These findings suggest that at least one subject
seemed to be able to avoid the memory test effectively
based on the absence/presence of his memory about the
reward location. The remaining one subject, Mandara,
showed an unpredicted yet functional use of the safe
choice. She was more likely to select the escape response
when she had seen the actual location of the two grapes
conflicted with her side bias (difficult trials) than when she
had seen there was no such conflict (easy trials). These
findings are consistent with a metacognitive interpretation
by showing that gorillas like orangutans (Suda-King 2008)
are able to quit a memory test when they have not seen or
are uncertain of the location of a preferable reward. Note
also that the current study administered the small number
of trials (only 24 trials for each condition per subject) in an
attempt to minimize the opportunity of learning. In fact,
none of the gorillas except for Kwame showed significant
evidence of learning in their use of the escape response
during the course of the current experiment. However,
given that the two types of conditions (Visual vs. Hidden)
differed from each other and task difficulty corresponded
with external cues such as the presence/absence of the
occluder (i.e., choosing the baited blue cup was more dif-
ficult when the barrier was presented) or the location of the
preferable reward (i.e., choosing the blue cup on the right
was more difficult than choosing the other cup), we cannot
totally rule out the possibility that the apes used such visual
cues to decline the memory test without monitoring their
own memory or uncertainty.
In Experiment 2, we tested gorillas’ use of the escape
response in the absence of such cues that may be used as
discriminative stimuli. Task difficulty was determined only
by the subjects’ memory about the location of the preferred
reward, rather than external cues such as the presence of
the physical barrier. The escape option was presented to the
gorillas on some trials, whereas it was unavailable on other
trials. If the gorillas can base their judgments on their own
memory trace strength, they should perform better in the
primary memory task when the escape option is available
than when it is not, because they should be able to improve
their memory performance with the help of the escape
response by selectively avoiding trials in which they are
likely to err.
Experiment 2
The previous experiment had shown that two of the gorillas
were able to use the escape response effectively in the
object choice memory task in which the visibility of the
baiting was systematically manipulated. However, even
with the small number of trials, the previous experiment
could not refute the possibility of associative learning
because visual cues corresponded to the task difficulty. In
order to rule out this alternative possibility, we further
examined whether the gorillas could selectively decline
difficult trials even without obvious external cues indicating
the difficulty of task. The gorillas were tested in the same
procedure as that of Experiment 4 of Suda-King’s (2008)
study, which was designed to eliminate the association
between visual cues and task difficulty. As in Experiment 1,
the two grapes were placed under one of the blue cups.
Subjects had the option to escape trials by choosing the
yellow cup. But unlike the previous experiment, the escape
Table 2 Mandara’s performance in Experiment 1 as a function of
reward location
Reward
position
Frequency Success
(%)
Escape
(%)
Correct Wrong Escape
Visible
Right 11 3 10 78.6 41.7
Left 22 0 2 100.0** 8.3
Hidden
Right 5 17 2 22.7–
8.3
Left 18 1 5 94.7** 20.8
Success (%) = a percentage of trials in which the subject selected the
baited blue cup when she selected one of the two blue cups; escape
(%) = a percentage of trials in which the subject selected the yellow
cup; reward position = the position of two grapes from the subject’s
perspective
** P 0.01, above chance; –
P 0.05, below chance (binomial tests)
72 Anim Cogn (2013) 16:65–84
123

9. response was available only in some trials, and the sub-
jects could always see the baiting. Thus, there were no
objective differences between easy and difficult trials, and
the task difficulty was determined solely by the apes’
subjective strength of their own memory trace about the
reward location. Moreover, the apes had to decide whether
they would like to take the memory test or escape it before
the memory test proceeded to the final stage. That is,
subjects had to decide before the two blue cups were
moved to the final configuration, which means that the
gorillas had to predict their memory performance before
testing. The metacognition model predicts that the gorillas
should avoid the memory test when their memory trace
strength about the location of preferred reward is weak.
Consequently, their memory performance should be better
when the escape response is available than when it is not,
because they should decline trials in which they are more
prone to err and should subsequently improve their
memory performance with the help of the escape response:
the second criterion of the chosen-forced performance
advantage.
Methods
Subjects
The same subjects as in Experiment 1 participated in the
current experiment.
Apparatus
We used the same apparatus as in Experiment 1 (blue cups,
blue dishes, yellow cups, yellow lid, yellow dish, green
dishes, platform, wooden table, and Plexiglas panels).
Procedure
The procedure of the current experiment was identical to
that of Experiment 4 of Suda-King’s (2008) study.
Experiment 2 had the following three conditions. The (?)
and (-) in the condition names indicate whether the con-
tents of the escape option were revealed or remained
concealed before the subjects made a choice (see below).
Free (-) The experimenter stood in front of each subject
and placed the platform approximately 36 cm away from
the Plexiglas panel. She placed a pair of the blue dishes
such that they were adjacent to each other and lined up on
the center line of the platform and then covered them with
the blue cups. Both of the blue cups were perpendicularly
positioned in front of the center hole of the Plexiglas panel.
The experimenter next hid two grapes under one of the blue
cups in full view of the subject and placed the covered
yellow cup full of grapes inside in front of either the left or
right hole of the panel. She finally pushed the platform
against the Plexiglas panel and allowed the subject to insert
its finger through one of the Plexiglas holes and make a
choice. If the subject chose the yellow cup, the experi-
menter lifted the lid, picked up one grape, and handed it to
the subject, upon which the trial was terminated and the
memory test did not proceed. The yellow cup thus served
as an escape response with which the subject could
immediately gain a secure but less preferable reward.
If the subject inserted its finger through the center hole
of the Plexiglas panel instead, the following memory test
proceeded: The experimenter pulled back the platform,
removed the yellow cup, and slid the blue cups from the
center to the both ends of the platform such that each blue
cup came in front of the right and left holes of the Plexiglas
panel. There were two types of the cup movements: (1) The
front cup moved toward right with the back cup moving
toward left from the subject’s perspective, and (2) the front
cup moved toward left with the back one moving toward
right from the subject’ perspective. Finally, the experi-
menter pushed the platform against the Plexiglas panel for
the subject to select one of the blue cups. If the subject
chose the baited blue cup, it received the two grapes. If the
subject selected the empty blue cup, it was shown the
location of the two grapes but did not receive any reward.
Each trial was timed by the metronome such that 6 s
elapsed from the moment when the experimenter covered
the two grapes with the blue cup until the platform touched
against the Plexiglas panel for the subject to make its first
choice. When the subject inserted its finger through the
panel’s center hole, 10 s elapsed from the moment when
the experimenter pulled back the platform until it touched
the panel again for the subject to make its second choice.
Therefore, each trial lasted for about 6 s when the subject
avoided the memory test by selecting the yellow cup,
whereas it lasted for about 16 s when the memory test
proceeded.
Free (?) The procedure was identical to that of the Free
(-) condition except that the subject was allowed to see the
contents of the yellow cup. Having hidden two grapes
under one of the blue cups in full view of the subject, the
experimenter placed a yellow dish containing one grape in
front of either the left or right holes of the Plexiglas panel.
She next placed the yellow cup on the dish, covering the
grape inside, and pushed the platform against the panel for
the subject to insert its finger through one of the holes and
make a choice. If the subject chose the yellow sup, the
experimenter lifted the yellow cup and gave its contents
(i.e., one grape) to the subject. If the subject inserted its
finger through the center hole, the memory test proceeded.
The experimenter pulled back the platform, removed the
Anim Cogn (2013) 16:65–84 73
123

10. yellow cup together with the dish underneath, and the rest
of the procedure was the same as in the Free (-) condition.
Forced The procedure was identical to that of the Free
(?) condition except that the escape option was taken away
from the subject before its first choice. Immediately after
placing the yellow dish containing one grape on the plat-
form, the experimenter removed the dish and its contents.
She then pushed the platform against the Plexiglas panel
for the subject to insert its finger through one of the holes
and make a choice. In this condition, the escape response
was not available, and the subject was forced to take the
memory test. Once the subject inserted its finger through
the center hole, the experimenter pulled back the platform
and the memory test proceeded. The rest of the procedure
was the same as in the previous two conditions.
We conducted 72 trials for each condition per subject. In
order to assess the gorillas’ preference for two grapes and
maintain their motivation throughout the testing, we also
interspersed 72 pretest preference trials among the trials of
the three testing conditions. Each subject received 18 ses-
sions consisting of 16 trials each, for a total of 288 trials.
Each session was a random mixture of 12 testing trials
(four trials per condition) and four preference trials. The
subjects also received three low-value food association
trials prior to each session as a warm-up and a reminder of
the contingency between the choosing the lidded yellow
cup and receiving one grape. The beginning and final
positions of the two grapes were counterbalanced and
randomized with a restriction that the preferred reward did
not appear in the same position in more than three con-
secutive trials.
Results
Table 3 shows the subjects’ performance in the three
testing conditions and preference trials. Gorillas main-
tained their preference for the two grapes over one
(P 0.001, binomial test). Only two of the subjects, Ba-
raka and Kwame, performed significantly better than
chance in the Forced condition (Baraka, P = 0.013;
Kwame, P 0.001; binomial test). Kwame’s memory
performance was also significantly above chance in the
Free (?) condition (P = 0.028, binomial test), whereas the
others’ performance in this condition did not significantly
differ from chance level. None of the subjects’ perfor-
mance significantly exceeded chance in the Free (-) con-
dition (P [ 0.086, binomial test).
There was no statistically significant difference between
the two Free conditions in the subjects’ memory perfor-
mance (P [ 0.16, for all the subjects, Fisher’s exact test),
so for subsequent analysis we combined them to reduce the
number of comparisons. In order to examine whether the
gorillas were significantly more successful in the memory
test when the escape option was available than when it was
not, we conducted Fisher’s exact test on each subject’s
2 9 2 table (Free vs. Forced, Correct vs. Wrong), with the
two Free conditions combined. Trials in which the subjects
chose the yellow cup were excluded from the current
analysis. Contrary to the prediction, the gorillas’ memory
performance was not affected by the availability of the
escape option (Baraka, P = 0.79; Kojo, P = 0.75; Kwame,
P = 0.18; Mandara, P = 0.81). That is, there was little
evidence that the gorillas were able to improve their
memory performance by selectively avoiding difficult
trials.
Discussion
Contrary to the second metacognitive prediction, the
gorillas failed to avoid the memory test selectively when
their memory trace strength about the location of the
preferable reward was weak. None of the gorillas were
significantly more successful when they had an option of
escaping the memory test as compared to when they were
forced to take the test. One possible explanation for the
current results is that the subjects did not use the escape
response carefully because they had a substantial proba-
bility of gaining the two grapes (i.e., 50 %) even when they
could only guess the location of the high-value reward.
Consequently, in the next experiment, we reduced the
probability of receiving the two grapes to encourage the
gorillas to use the escape response.
Experiment 3
The current experiment was newly designed to encourage
the effective use of the escape response in the gorillas.
Three blue cups, instead of two, were used as potential
hiding locations for the two grapes, which reduced the
chance of obtaining the high-value food from 50 to 33 %.
As in Experiment 2, the subjects were given an option of
declining the memory test before the memory test pro-
ceeded on two-thirds of trials, whereas they were forced to
take the test on the remaining one-third of trials. Unlike in
Experiment 2, the blue cups were temporarily covered with
a box when the subjects were deciding whether or not to
take the memory test on half of trials. Therefore, the sub-
jects had to predict their future memory performance
without seeing the final configuration of the memory test.
Moreover, on some trials, they also had to make a pro-
spective judgment without the direct visual access to the
test stimuli. If the gorillas can make optimal choices based
on their own memory trace strength about the location of
the preferred reward, they should perform better in the
74 Anim Cogn (2013) 16:65–84
123

11. memory test when they choose to take the test than when
forced to do so. That is, they should selectively avoid trials
in which they forget the correct reward location (i.e., the
second criterion of the chosen-forced performance advan-
tage). Moreover, it is expected that the memory test is more
difficult when the blue cups are temporarily occluded as
compared to when they remain visible. Therefore, the
gorillas should use the escape response more often in the
former trials than in the latter (i.e., the first criterion of the
increase in the escape use as a function of the task
difficulty).
Methods
Subjects
The same subjects as in the previous experiments, except
for Baraka, participated in the current experiment. He lost
his interest during the early stage of the current experiment
and refused to participate further.
Apparatus
We used the same apparatus as in the previous experiments
(blue cups, blue dishes, yellow cups, yellow lid, yellow
dish, green dishes, platform, wooden table, and Plexiglas
panels). In addition, we used a four-sided cardboard box
(11.5 cm 9 30.5 cm 9 12 cm) on some trials to cover the
blue cups when the subject was making a decision on
whether or not to take the memory test.
Procedure
Experiment 3 consisted of two sets of tests: (1) Cover and
(2) No Cover. The two tests were identical to one another
except that the blue cups were temporarily covered with a
cardboard box in the Cover test whereas they remained
visible in the No Cover test.
(1) Cover test
The first test had the following three conditions (Fig. 1).
The (?) and (-) in the condition names indicate whether
the contents of the escape option were revealed or
remained concealed before the subjects were allowed to
make a choice (see below).
Free (-) The experimenter stood in front of the subject
and placed the platform approximately 36 cm away from
the Plexiglas panel. She placed three blue dishes such that
they were adjacent to each other and lined up perpendic-
ularly in front of the center hole of the Plexiglas panel, and
covered them with three blue cups. In order to ensure that
the subject could see the baiting of the two grapes, the blue
cups were positioned slightly angled with the front cup
slightly off centered to right, the middle cup on the center
line of the platform, and the back cup slightly off centered
to left, from the subject’s perspective. The experimenter
next hid two grapes underneath one of the blue cups in full
view of the subject and covered the blue cups with the
cardboard box. She then placed the covered yellow cup
containing grapes in front of either the left or right hole of
the panel. She finally pushed the platform against the
Table 3 Subjects’ performance
in the three testing conditions of
Experiment 2
Success (%) = a percentage of
trials in which the subjects
selected the baited blue cup
when they selected one of the
two blue cups; escape (%) = a
percentage of trials in which the
subjects selected the yellow
cup; preference (%) = a
percentage of trials in which the
subjects selected two grapes
instead of one in the preference
trials
* P 0.05, ** P 0.01, above
chance (binomial tests)
Subject Frequency Success (%) Escape (%) Preference (%)
Correct Wrong Escape
Baraka 97.2**
Free (?) 2 4 66 33.3 91.7 –
Free (-) 10 4 58 71.4 80.6 –
Forced 47 25 – 65.3* – –
Kojo 97.2**
Free (?) 1 1 70 50.0 97.2 –
Free (-) 4 5 63 44.4 87.5 –
Forced 38 34 – 52.8 – –
Kwame 100**
Free (?) 28 13 31 68.3* 43.1 –
Free (-) 19 9 44 67.9 61.1 –
Forced 57 15 – 79.2** – –
Mandara 90.3**
Free (?) 5 6 61 45.5 84.7 –
Free (-) 8 4 60 66.7 83.3 –
Forced 37 35 – 51.4 – –
Anim Cogn (2013) 16:65–84 75
123

12. Plexiglas panel for the subject to make its first choice. If
the subject chose the yellow cup, the experimenter lifted
the lid and gave one of the grapes to the ape. Thus, this
choice served as an escape response with which the subject
could obtain the secure but less preferable reward without
taking the memory test. If the subject selected the card-
board box instead, the memory test proceeded. The
experimenter pulled back the platform and simultaneously
removed the box and the yellow cup. She then slid two of
the blue cups (with the dishes underneath) further apart so
that each one of them came in front of either the right or
left hole of the Plexiglas panel and adjusted the position of
the remaining blue cup so that it was in front of the center
hole. There were six types of cup movements: (1) The front
cup moved to right with the middle cup remaining at the
center and the back cup moving to left, (2) the front cup
moved to right with the middle cup moving to left and the
back cup remaining at the center, (3) the front cup moved
to left, with the middle cup moving to right and the back
cup remaining at the center, (4) the front cup moved to left
with the middle cup remaining at the center and the back
cup moving to right, (5) the front cup remained at the
center with the middle cup moving to left and the back cup
moving to right, and (6) the front cup remained at the
center with the middle cup moving to right and the back
cup moving to left. Finally, the experimenter pushed the
platform against the Plexiglas panel for the subject to select
one of the blue cups. If the subject selected the baited blue
cup, they received two grapes. If the subject chose an
empty blue cup, it was shown the correct location of the
two grapes but did not receive any reward. Each trial was
timed by metronome such that 8 s elapsed from the
moment when the experimenter covered the two grapes
with one of the blue cups until the platform touched against
the Plexiglas panel for the subject’s first choice. When the
subject chose to take the memory test, 12 s elapsed from
the moment when the experimenter pulled back the plat-
form until it was pushed back against the panel for the
subject’s second choice. Hence, each trial lasted for about
8 s when the subject declined the memory test, whereas it
lasted for about 20 s when the ape decided to take the
memory test.
Cover test
ForcedFree (-) Free (+)
1 grape
(No test) Memory test
1 grape
(No test) Memory test Memory test
2 grapes No reward 2 grapes No reward 2 grapes No reward
Fig. 1 Examples of trials from
the cover test of Experiment 3.
Each subject received 36 trials
per condition
76 Anim Cogn (2013) 16:65–84
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13. Free (?) The procedure was identical to that of the Free
(-) condition except that the subject was allowed to see the
contents of the yellow cup. Having hidden two grapes
under one of the blue cups in full view of the subject and
placed the cardboard box over the blue cups, the experi-
menter put a yellow dish containing one grape in front of
either the left or right holes of the Plexiglas panel. She next
placed the yellow cup on the dish, covering the grape
inside, and pushed the platform against the panel for the
subject to make its first choice. If the subject chose the
yellow cup, the experimenter lifted the yellow cup and
gave its contents (i.e., one grape) to the subject. If the
subject pointed at the cardboard box, the memory test
proceeded. The experimenter pulled back the platform and
simultaneously removed the box and the yellow cup
together with the dish underneath, and the rest of the
procedure was the same as in the Free (-) condition.
Forced The procedure was identical to that of the Free
(?) condition except that the escape option was taken
away from the subject before its first choice. Immediately
after placing the yellow dish containing one grape on the
platform, the experimenter removed the dish and its
contents. She then pushed the platform against the
Plexigas panel for the subject to insert its finger through
one of the holes and make a choice. In this condition, the
escape response was unavailable, and the subject was
forced to select the cardboard box. Once the subject
inserted its finger through the center hole, the experi-
menter pulled back the platform and the memory test
proceeded. The rest of the procedure was the same as in
the previous two conditions.
We conducted 36 trials for each condition per subject. In
order to assess the gorillas’ preference for two grapes and
maintain their motivation throughout the testing, we also
interspersed 24 pretest preference trials among the trials of
the three testing conditions. Each subject received six
sessions consisting of 22 trials each, for a total of 132 trials.
Each session was a random mixture of 18 testing trials (six
trials per condition) and four preference trials. The subjects
also received three low-value food association trials prior
to each session as a warm-up and a reminder of the con-
tingency between choosing the covered yellow cup and
receiving one grape. The beginning and final positions of
the two grapes, as well as the movement of the blue cups,
were counterbalanced and randomized with a restriction
that the preferred reward did not appear in the same posi-
tion in more than three consecutive trials.
(2) No Cover test
After the completion of the previous test, the subjects
proceeded to the second test. The No Cover test had three
conditions, which were identical to those of the Cover test
except that the cardboard box was not presented and the
blue cups remained visible throughout the trials (Fig. 2).
Free (-) The procedure was identical to that of Free (-)
of the Cover test except that the cardboard box did not
occlude the blue cups. After the experimenter hid two
grapes under one of the three blue cups, which were lined
up in front of the center hole of the Plexiglas panel, the
experimenter placed the covered yellow cup containing
grapes in front of either the right or left hole of the panel.
She then pushed the platform toward the subject allowing
them to make a choice. If the subject selected the yellow
cup, the experimenter lifted the lid and gave one grape to
the subject. If the subject inserted its finger through the
center hole of the panel instead, the memory test pro-
ceeded. The experimenter pulled back the platform and
removed the lidded yellow cup, and the rest of the proce-
dure was the same as in Free (-) of the Cover test.
Free (?) The procedure was identical to that of the Free
(-) condition in the Cover test described above except that
the cardboard box was not used and the blue cups remained
visible. Having hidden two grapes under one of the blue
cups in full view of the subject, the experimenter placed a
yellow dish containing one grape in front of either the left
or right holes of the Plexiglas panel. She next placed the
yellow cup on the dish, covering the grape inside, and
pushed the platform against the panel for the subject to
make its first choice. If the subject chose the yellow cup,
the experimenter lifted the yellow cup and gave its contents
(i.e., one grape) to the subject. If the subject inserted its
finger through the middle hole of the platform, the memory
test proceeded. The experimenter removed the yellow cup
(with the dish and the grape underneath), and the rest of the
procedure was the same as in Free (?) condition in the
Cover test.
Forced The procedure was identical to that of Forced
condition in the Cover test except that the cardboard box
was not used. After hiding two grapes under one of the
three blue cups, the experimenter placed the yellow dish
containing one grape in front of either the right or left hole
of the panel and immediately removed it. She then pushed
the platform for the subject to make a choice. Once the
subject inserted its finger through the center hole of the
Plexiglas panel, the experimenter pulled back the platform
and moved the blue cups in a previously determined con-
figuration. The rest of the procedure was the same as in the
Forced condition of the Cover test.
Each trial lasted about 8 s when the subject chose the
escape option, while it lasted about 20 s when the ape
chose to take the memory test. We conducted 36 trials for
each condition per subject. In order to assess the gorillas’
preference for two grapes and maintain their motivation
Anim Cogn (2013) 16:65–84 77
123

14. throughout the testing, we also interspersed 24 pretest
preference trials among the trials of the three testing con-
ditions. Each subject received six sessions consisting of 22
trials each, for a total of 132 trials. Each session was a
random mixture of 18 testing trials (six trials per condition)
and four preference trials. The subjects also received three
low-value food association trials prior to each session as a
warm-up and a reminder of the contingency between
choosing the covered yellow cup and receiving one grape.
The beginning and final positions of two grapes, as well as
the movement of the blue cups, were counterbalanced and
randomized with a restriction that the preferred reward did
not appear in the same position in more than three con-
secutive trials.
Results
(1) Cover test
Table 4 shows the subjects’ performance in the three
testing conditions and preference trials for the Cover test.
The gorillas maintained their preference for the larger
amount of reward (P 0.003, binomial test). Despite the
seemingly complicated movements of the cups as well as
the temporary visual occlusion of the test stimuli, one of
the gorillas, Kwame, performed significantly above chance
in all of the conditions (Free (-): P = 0.002; Free (?):
P = 0.018; Forced: P = 0.008, binomial test). Kojo’s
result approached the significance level in the Forced
condition only (P = 0.051, binomial test), while Mand-
ara’s performance failed to differ from chance level in any
of the conditions (P [ 0.20, binomial test). Consistent with
these individual differences in memory performance, Kojo
and Mandara declined the memory test in a large propor-
tion of Free trials ([90 %), whereas Kwame used the
escape response in less than 40 % of Free trials.
There was no statistically significant difference between
the two Free conditions in the subjects’ memory perfor-
mance (P [ 0.55, for all the subjects, Fisher’s exact test),
so for subsequent analysis we combined them to reduce the
number of comparisons. In order to examine whether the
gorillas were significantly more successful in the memory
test when the escape option was available than when it was
not, we conducted Fisher’s exact test on each subject’s
No cover test
ForcedFree (-) Free (+)
1 grape
(No test) Memory test
1 grape
(No test) Memory test Memory test
2 grapes No reward 2 grapes No reward 2 grapes No reward
Fig. 2 Examples of trials from
the No Cover test of Experiment
3. Each subject received 36
trials per condition
78 Anim Cogn (2013) 16:65–84
123

15. 2 9 2 table (Free vs. Forced, Correct vs. Wrong). Trials in
which the subjects chose the yellow cup were excluded
from the current analyses. None of the gorillas as an
individual showed a significant difference in the memory
performance between the two types of conditions (Kwame,
P = 0.51; Kojo, P = 0.24; Mandara, P = 0.33). However,
all of the subjects showed the predicted behavioral pattern,
demonstrating higher memory accuracies in the Free con-
ditions than did in the Forced condition (see Table 4).
(2) No Cover test
Table 4 shows the subjects’ performance in the three
testing conditions and preference trials for the No Cover
test. The gorillas continued to prefer the larger amount of
reward over the smaller one (P 0.001, Binomial test).
One of the gorillas, Kwame, who was the most successful in
the Cover test, performed significantly above chance in all
of the conditions (P 0.001, binomial test). Kojo’s per-
formance significantly exceeded chance level in the Forced
condition only (P = 0.004, binomial test). Mandara’s
performance failed to differ from chance level in any of the
conditions (P [ 0.21, binomial test). Again, consistent with
these individual differences in memory performance, Kojo
and Mandara declined the memory test in a large proportion
of Free trials ([88 %), whereas Kwame used the escape
response in less than 17 % of Free trials.
There was no statistically significant difference between
the two Free conditions in the subjects’ memory perfor-
mance (P [ 0.48, for all the subjects, Fisher’s exact test), so
for subsequent analysis we combined them to reduce the
number of comparisons. In order to examine whether the
gorillas were significantly more successful in the memory
test when the escape option was available than when it was
not, we conducted Fisher’s exact test on each subject’s 2 9 2
table (Free vs. Forced, Correct vs. Wrong). Trials in which
the subjects chose the yellow cup were excluded from the
current analyses. The gorillas’ memory performance did not
significantly differ by condition type (Kwame, P = 0.099;
Kojo, P = 1.00; Mandara, P = 0.41), although Kwame
showed the predicted performance pattern.
Table 4 Subjects’ performance
in the cover and No Cover tests
of Experiment 3
Success (%) = a percentage of
trials in which the subjects
selected the baited blue cup
when they selected one of the
three blue cups; escape (%) = a
percentage of trials in which the
subjects selected the yellow
cup; preference (%) = a
percentage of trials in which the
subjects selected two grapes
instead of one in the preference
trials
* P 0.05; ** P 0.01, above
chance (Binomial tests)
Subject Frequency Success (%) Escape (%) Preference (%)
Correct Wrong Escape
Kojo
Cover 100**
Free (?) 2 0 34 100 94.4 –
Free (-) 1 0 35 100 97.2 –
Forced 18 18 – 50.0 – –
No Cover 100**
Free (?) 1 2 33 33.3 91.7 –
Free (-) 1 0 35 100 97.2 –
Forced 21 15 – 58.3** – –
Kwame
Cover 95.8**
Free (?) 14 10 12 58.3* 33.3 –
Free (-) 15 7 14 68.2** 38.9 –
Forced 20 16 – 55.6** – –
No Cover 91.7**
Free (?) 29 1 6 96.7** 16.7 –
Free (-) 29 1 6 96.7** 16.7 –
Forced 31 5 – 86.1** – –
Mandara
Cover 83.3**
Free (?) 1 0 35 100 97.2 –
Free (-) 2 1 33 66.7 91.7 –
Forced 16 20 – 44.4 – –
No Cover 91.7**
Free (?) 1 2 33 33.3 91.7 –
Free (-) 3 1 32 75.0 88.9 –
Forced 13 23 – 36.1 – –
Anim Cogn (2013) 16:65–84 79
123

16. Overall analyses
The previous analyses of the two tests in Experiment 3
revealed that one of the three tested gorillas, Kwame,
generally showed the predicted pattern, performing better
in the memory test when he had the option of declining the
memory test than when he was forced to take the test. The
same subject was also the only animal whose escape fre-
quency was at a middling level, whereas the remaining two
subjects, Kojo and Mandara, almost always avoided the
memory tests, yielding a very small number of data points
for memory performance in the Free conditions. We
therefore focused on Kwame’s data and combined
Kwame’s data across the two tests and conducted Fisher’s
exact test on his 2 9 2 table (Free vs. Forced, Correct vs.
Wrong). Kwame demonstrated the pattern of performing
better in the memory test in the Free conditions than in the
Forced condition, although the result failed to reach sig-
nificance (P = 0.10).
We further compared Kwame’s memory performance
between the Free conditions and the Forced condition
across the sessions. Table 5 shows the subjects’ success
rates in the memory test for the Free conditions and the
Forced condition across 12 sessions (6 sessions 9 2 tests).
In order to examine whether the predicted behavioral pat-
tern was detected for Kwame, we conducted a sign test on
his memory performance across all the sessions. Kwame
showed the predicted pattern in 8 sessions, whereas he did
the opposite only in 1 session and his memory performance
was the same between the two types of conditions in 3
sessions (see Table 5). A sign test confirmed that he was
significantly more successful at selecting the baited blue
cup in the Free conditions than in the Forced condition
(P = 0.039). These findings suggest that at least one sub-
ject showed the statistically significant evidence of the
chosen-forced performance advantage (i.e., the second
criterion of animal metacognition) by declining the mem-
ory test when his memory trace strength about the location
of the preferred reward was weak. However, the results
should be interpreted with caution because if Mandara and
Kojo were included in the analyses and we had to correct
Kwame’s P value for multiple comparisons, the corrected
P value was not significant (P = 0.11, Sˇida´k correction).
The discrepancy in the results between the two tests
might have partly come from the fact that Kwame’s
memory performance in the Forced trials reached 100 %
correct in the last two sessions of the No Cover test (see
Table 5). Once his baseline performance reached 100 %
correct in the Forced trials, there should not be any chosen-
forced performance advantage, and this was exactly what
we found. Kwame scored 100 % correct both in the Free
and in the Forced trials in the last two sessions of the No
Cover test. When we conducted Fisher’s exact test, the data
from these sessions were included, which might have
reduced the likelihood of obtaining statistically significant
results. On the other hand, the sign test naturally excluded
ties from the analyses (which seems more appropriate in
the current context because of the ceiling effect in the last
two sessions), which might have led to the significant
results.
We also compared the subjects’ performance across the
two tests. First, we examined whether trials were more dif-
ficult when the blue cups were temporarily covered with the
box than when they remained visible by conducting Fisher’s
exact test on each subject’s 2 9 2 table (Correct vs. Wrong,
Cover vs. No Cover) for the Forced trials. Only one subject,
Kwame, was significantly more successful at selecting the
baited blue cup in the No Cover test than in the Cover test.
The two tests were equally difficult for the remaining two
subjects (Kwame, P = 0.009; Kojo, P = 0.64; Mandara,
P = 0.63). We further conducted Fisher’s exact test on each
subject’s 2 9 2 table (Take test vs. Escape, Cover vs. No
Cover) for the Free trials in order to examine whether the
subjects showed differential use of the escape response
across the two tests. As predicted, Kwame was significantly
more likely to use the escape response in the Cover test
(difficult trials) than in the No Cover test (easy trials)
(Kwame, P = 0.013; Kojo, P = 1.00; Mandara, P = 0.53).
His results remained significant even when the P value was
corrected for multiple comparisons (P = 0.038, Sˇida´k cor-
rection). Altogether, one subject demonstrated significant
evidence of the increase in the use of the escape response as a
function of task difficulty (i.e., the first criterion of animal
metacognition).
Discussion
Overall, one of the tested gorillas fulfilled the first behav-
ioral criterion of animal metacognition by using the escape
response significantly more often in the Cover test—
objectively difficult trials—than in the No Cover test—
objectively easy trials. The same subject’s response pattern
was also consistent with the chosen-forced performance
advantage, and he met the second criterion at the statisti-
cally significant level but only when the P value was not
corrected for multiple comparisons. It is important to note
that when we conducted the sign test to compare Kwame’s
memory performance between the Free and Forced trials,
the comparisons were made within each session. Unlike in
Experiment 1 in which the presence of the barrier was
associated with difficult trials, each session of Experiment 3
included only one type of test (Cover or No Cover), and
thus, there were no external cues that could potentially
indicate the task difficulty of trials. The difficulty within
each session was solely determined by the subject’s mem-
ory trace strength. The memory task was difficult when the
80 Anim Cogn (2013) 16:65–84
123

17. subject forgot, whereas it was easy when he remembered. In
other words, Kwame had to discriminate difficult trials (in
which he forgot) from easy ones (in which he remembered)
without discriminative or associative cues. Therefore, the
chosen-forced performance advantage demonstrated by this
subject cannot be explained by associative learning alone,
and the performance difference between the Free and
Forced trials seemed to emerge because the ape selectively
declined the memory test when he forgot.
Response competition is unlikely to explain Kwame’s
chosen-forced performance advantage, because he had to
decide whether or not to take the memory test before the
memory test actually proceeded to the end stage. When the
subjects were making a choice between taking the test and
escaping, the blue cups were still lined up in the middle of
the platform and they were blind as to the final location of
the cups. Furthermore, on the first half of the testing trials
(i.e., in the Cover test), the test stimuli were temporarily
blocked from the subjects’ view, and the apes had to make
prospective judgments on whether or not to take the
memory task. Note here that Kwame’s behavioral pattern
followed the second metacognitive prediction even in the
Cover test alone. Taken together, most parsimonious
explanation for his performance in the current testing
seems to be that he used his own memory trace about the
location of the preferable reward and effectively avoided
trials in which he would likely make an error.
General discussion
In the current research, we evaluated whether gorillas
selectively declined a spatial memory test when they did
not remember the location of a preferred reward. Results
showed that at least three gorillas showed significant
results consistent with one or both of the two criteria of
animal metacognition. In Experiment 1, one subject, Kojo,
demonstrated evidence of being more likely to decline the
memory test when the baiting of the preferred reward was
visually blocked as compared to when it remained visible,
while another subject, Mandara, selectively used the escape
option more often when she had seen that the actual
location of the two grapes conflicted with her side bias.
These findings are consistent with the first metacognitive
criterion of using the escape option more often in difficult
trials than in easy ones. In Experiment 3, one subject,
Kwame, was able to improve his memory performance by
avoiding trials in which he would perform poorly despite
the fact that he had to make a choice between taking the
test or escaping before the memory test was introduced.
That is, he was significantly more accurate when he had the
option to escape than when he did not (but only if the
P value was not corrected for multiple comparisons), thus
demonstrating evidence suggestive of fulfilling the second
criterion of the chosen-forced performance advantage. The
same subject was also significantly more likely to decline
Table 5 Subjects’ success rate in the cover and No Cover tests of Experiment 3 across sessions
Session Kojo Kwame Mandara
Free Forced ± Free Forced ± Free Forced ±
Cover
1 N 83.3 (5/6) NA 71.4 (5/7) 66.7 (4/6) ? N 16.7 (1/6) NA
2 100 (2/2) 50.0 (3/6) ? 70.0 (7/10) 66.7 (4/6) ? 100 (1/1) 16.7 (1/6) ?
3 N 50.0 (3/6) NA 62.5 (5/8) 33.3 (2/6) ? 100 (1/1) 66.7 (4/6) ?
4 N 66.7 (4/6) NA 40.0 (2/5) 83.3 (5/6) - N 50.0 (3/6) NA
5 100 (1/1) 33.3 (2/6) ? 50.0 (4/8) 50.0 (3/6) 0 N 50.0 (3/6) NA
6 N 16.7 (1/6) NA 75.0 (6/8) 33.3 (2/6) ? 50.0 (1/2) 66.7 (4/6) -
No Cover
1 100 (1/1) 50.0 (3/6) ? 100 (10/10) 66.7 (4/6) ? 66.7 (2/3) 33.3 (2/6) ?
2 0.0 (0/1) 66.7 (4/6) - 100 (10/10) 83.3 (5/6) ? 0.0 (0/1) 33.3 (2/6) -
3 100 (1/1) 50.0 (3/6) ? 90.0 (9/10) 83.3 (5/6) ? 66.7 (2/3) 50.0 (3/6) ?
4 N 66.7 (4/6) NA 90.0 (9/10) 83.3 (5/6) ? N 33.3 (2/6) NA
5 0.0 (0/1) 33.3 (2/6) - 100 (8/8) 100 (6/6) 0 N 33.3 (2/6) NA
6 N 83.3 (5/6) NA 100 (12/12) 100 (6/6) 0 N 33.3 (2/6) NA
Trials in which the subjects chose the escape response were excluded from the analyses. Bold numbers indicate the percentages of correct choices
(i.e., choosing the baited blue cup) made by the subjects in the memory test. Numbers in parentheses indicate the frequency of correct choices
divided by that of take-test responses. Free = Free (-) and Free (?) conditions combined; Forced = Forced condition; ± = direction of
memory performance; ? = a session in which the subject’s memory performance in the Free conditions exceeded that in the Forced condition as
predicted; - = a session in which the subject’s memory performance in the Forced condition exceeded that in the Free condition; 0 = a session
in which the subject’s memory performance did not differ between the Free and Forced condition; N no data available as the subjects never chose
to take the memory test within a session; NA not applicable because no data were available for the Free conditions
Anim Cogn (2013) 16:65–84 81
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18. difficult trials (the Cover test) than easy trials (the No
Cover test) even with the corrected P value, hence meeting
the first criterion.
Several nonmetacognitive models have been previously
proposed for the effective use of the metacognitive response
in animals, and the current study was designed to rule out
such lower-level models. First, associative learning seems
not to explain the gorillas’ performance given that the apes
received relatively few trials. In fact, only one subject
showed some effect of learning in Experiment 1, whereas
all the others’ performance remained statistically unchan-
ged throughout testing. Moreover, within each session of
Experiment 3, there were no obvious objective cues avail-
able (e.g., presence/absence of the occluder) that corre-
sponded to task difficulty. As such, the present study was
designed to remove these discriminative cues and force
subjects to rely on their own memory trace strength in order
to selectively decline difficult trials appropriately.
Second, response competition is an unlikely mechanism
for Kwame’s performance in Experiment 3 because he had
to make prospective judgments on whether or not to take
the memory test before the test actually proceeded. He
could not see the final configuration of the blue cups (i.e.,
the apes had no idea into which directions cups would
move) at the time of the take-test/escape choice, and
therefore, the primary responses were not in direct com-
petition with the escape response. Additionally, in half of
the trials, he was even temporarily prevented from seeing
the blue cups at the moment when he made the prospective
judgments, which makes the response competition account
even more unlikely. Taken together, Kwame’s perfor-
mance pattern in Experiment 3 appears to imply that he
was able to make the optimal decision of avoiding difficult
trials on the basis of his own memory trace strength about
the location of the preferred reward. It should be still noted
here that Smith et al.’s response strength model can be
applied to Kwame’s chosen-forced performance advantage.
That is, Kwame might have acquired a certain level of
attractiveness toward the escape response during the
experiment and consequently might have compared the
strength of such attractiveness with his own memory trace
strength about the location of the preferred reward. The
attractiveness of the escape option might have determined
the trade-off between the delay length to reinforcement and
the amount of reinforcement.
Although the current experiments demonstrated the
effective use of the escape response in some gorillas for the
first time, we recognize that there are some limitations in
the current study. First, we tested only four subjects, and
only one subject at the individual level showed the chosen-
forced performance advantage, and that only when the
P value was not corrected for multiple comparisons. The
remaining subjects declined the memory test in a majority
of trials, yielding a very small number of data points for the
free-choice trials, which hindered statistical analyses of the
second criterion on these individuals. It is therefore nec-
essary to modify the testing procedure, so that the subjects’
frequency of the escape use can be kept to a middling level.
Second, the very important question of how much the
gorillas’ use of the escape response depends on explicit
memory exceeds the scope of the current study. Future
research should examine the degree of flexibility this spe-
cies can demonstrate in their use of the escape response.
Specifically, it would be important to investigate whether
gorillas can use different kinds of metacognitive responses
not only prospectively but also retrospectively under vari-
ous kinds of conditions, as has been demonstrated for
rhesus monkeys did.
Finally, the current findings also converge with the
recent positive results of mirror self-recognition in some
gorillas. It would be interesting to examine whether the
ability to recognize oneself is actually correlated with
metacognitive skills in this species. If those who are more
competent at using the metacognitive response also per-
form better in the mirror test, a plausible conclusion would
be that these two kinds of tests actually tap into the same
cognitive faculty. Whether the awareness of one’s own
body is integrated with that of their own cognition in great
apes is a fascinating question, but further research is
required to address such a question. Here, we want to
advance the possibility that metacognition and self-
awareness may originally have been mediated by distinct
neural-cognitive systems as evidenced by the likely dis-
sociations between metacognition and self-awareness in
monkeys who evidence metacognition but not mirror self-
recognition (Anderson and Gallup 2011). However, these
two systems may have converged in the course of hominid
evolution, where the more ancestral metacognitive system
responsible for the representation of internal mental states
converged with the system mediating the representation of
somatosensory and proprioceptive information. To test this
hypothesis will require testing more animals where these
two systems may be dissociated. Evaluating the develop-
ment of metacognition and mirror self-recognition in typ-
ically as well as atypically developing human children may
also provide important clues into the cognitive architecture
of these skills and help answer whether they are mediated
by one system or two.
In conclusion, the current research revealed the functional
use of the escape response in gorillas for the first time. Three
of the tested gorillas were able to meet the first criterion of
animal metacognition by selectively declining the test when
the task was more difficult in a relatively small number of
testing trials with little training. One of the successful sub-
jects also demonstrated evidence suggestive of the chosen-
forced performance advantage (i.e., the second criterion).
82 Anim Cogn (2013) 16:65–84
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19. The study of great apes’ introspective skills has, unfortu-
nately, lagged behind that of monkeys for which much more
evidence exists. More research focusing on the introspective
skills of our closest relatives is necessary to shed light on the
evolutionary origins of mind and self-awareness.
Acknowledgments I thank Lisa Stevens for allowing me to work
with the great apes at the Smithsonian’s National Zoological Park and
all the zookeepers of the Great Ape House for their support. I also
thank Milton Tierney for constructing the testing apparatus. This
study was supported by a research grant from the David Bohnett
Foundation, the Smithsonian Institute Fellowship to CK, and a
CAREER Grant from the National Science Foundation to FS (BCS-
0748717). All of the experiments were approved by the IACUC of the
National Zoological Park and complied with the current laws of the
country in which they were conducted.
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