The imitation faculty in monkeys: Evaluating its features, distribution and evolution

JASs Invited Reviews
Journal of Anthropological Sciences
Vol. 85 (2007), pp. 35-62
the JASs is published by the Istituto Italiano di Antropologia www.isita-org.com
The imitation faculty in monkeys: evaluating its features,
distribution and evolution
Francys Subiaul
The George Washington University, Department of Speech & Hearing Science, Mind, Brain & Evolution
Cluster, 2115 G Street, NW # 204, Washington, DC 20052
e-mail: subiaul@gwu.edu
website: www.subiaul.com
Summary - Despite more than 100 years of research, there is no agreement among experts as to whether
or not monkeys can imitate. Part of the problem is that there is little agreement as to what constitutes
an example of ‘imitation.’ Nevertheless, recent research provides compelling evidence for both continuities
and discontinuities in the psychological faculty that mediates imitation performance in monkeys and apes,
including humans. For instance, a number of studies have shown that monkeys are capable of copying
familiar responses but not novel responses that require the use of a tool, for example. And, while these studies
have been interpreted to mean that monkeys cannot engage in ‘imitation learning’ or novel imitation, research
employing a cognitive imitation paradigm—where rhesus monkeys had to copy novel serial rules pertaining
to the order of pictures, independently of copying specific motor responses—has provided convincing evidence
of novel imitation in monkeys. Rather than suggesting that monkeys are poor imitators, these results suggest
that monkeys can learn novel cognitive rules but not novel motor rules, possibly because such skills require
derived neural specializations mediating the planning and coordination of fine and gross motor movements.
If true, such evidence represents an important discontinuity between the imitation skills of monkeys and apes
with significant implications for human cognitive evolution.
Keywords - Social learning, observation learning, imitation, cultural learning, primates, monkeys,
apes, humans.
Introduction
The answer to the question, “do monkeys
imitate?” depends very much on how “imitation”
is defined. As with all complex questions phrased
so simply, the answer inevitably depends on a
confluence of factors including how one defines
imitation, how one operationalizes this definition
and finally, what evidence counts as an exemplar
of the concept. For instance, many have come to
the conclusion that monkeys cannot imitate. This
conclusion is based on a variety of studies that
required monkeys to operate tools in a specific
manner. In all cases, researchers have argued that
any social learning evidenced by monkeys is best
explained by learning mechanisms other than
imitation learning (Adams-Curtis, 1987; Beck,
1976; Fragazy & Visalberghi, 2004; Visalberghi
& Fragaszy, 1990; 2002; Whiten & Ham, 1992).
But lest you think that monkeys are the only
primates believed to be unable to imitate, note
that doubt has been cast on the imitation skills
of virtually all primate species tested to date. This
doubt is perhaps best captured in the titles of well-
known publications, which have asked, ‘Do apes
ape?’ (Whiten A et al., 2004), ‘Do monkeys ape?’
(Visalberghi & Fragaszy, 1990; 2002), and even,
‘Do humans ape or do apes human?’ (Horowitz,
2003). It’s a curious (albeit somewhat depressing)
fact that despite more than a century of research,
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36 The imitation faculty in monkeys
answers to these questions remain controversial
and disagreement abounds among imitation
experts. The problem stems, in part, from the
question itself, which rests on a tangled and
complicated concept, imitation.
Various efforts have tried to ameliorate this
problem by proving a historical context for the
term ‘imitation’ (e.g., Galef, 1988; Mitchell,
1987). Others have offered detailed definitions
of common terms used in the social learning
literature (e.g., Zentall, 1996; 2007). Still others
have proposed new concepts and alternate social
learningmechanismstoassuagetheproblem(Byrne
& Russon, 1998; Subiaul et al. 2004; Tomasello,
1990; Whiten & Custance, 1996; Whiten et
al., 2004). Others have provided a theoretical
framework explaining the mechanisms underlying
social learning (e.g., Bargh & Chartrand, 1999;
Byrne & Russon, 1998; Carpenter & Call, 2002;
Heyes, 2004; Metlzoff & Moore, 1977; 1988;
Mitchell, 1987; Tomasello & Call, 1997; Whiten
& Ham, 1992). Yet, the success of these efforts
is uncertain. Arguably, many of these ideas and
terms have led to greater confusion and disparate
usage of what should be standard terminology.
But, this by no means is a new complaint. At the
beginning of the 20th
century, Morgan (1900)
expressed the same frustration. He wrote, “In
the face of such apparently diverse usage it is
necessary to show within what limits and with
what qualifications the word [imitation] may
profitably here be used to individuate a factor in
social evolution” (p 180). Given these problems, it
seems reasonable to begin a review of the primate
faculty for imitation with a conceptual framework
that narrowly defines this faculty’s features in
order to provide the reader with the tools to judge
for themselves the imitation skills of monkeys and
other animals.
Dissecting the imitation faculty
First and foremost it is a mistake to think
of imitation as one unitary concept or skill that
organisms either have or lack entirely. Rather, the
imitation faculty is like other vertical cognitive
faculties (Fodor, 1983), such as language,
that are modular, specialized and consist of
multiple components with discrete functions.
In this conceptualization, the imitation faculty
represents a specialized psychological mechanism
involved in the copying of responses in specific
domains in a flexible and adaptive fashion. Like
other faculties, the imitation faculty can be
divided by its various functions. These functions
are best captured by super-ordinate and sub-
ordinate imitation mechanisms associated with
the processing of specific types of stimuli (e.g.,
novel, familiar, auditory, motor, social, etc.). The
super-ordinate imitation mechanisms include,
(a) ‘familiar imitation,’ or the copying of familiar
rules or responses and (b) ‘novel imitation,’ or
the copying of novel rules or responses; often
referred to as ‘imitation learning,’ which is
distinguished from ‘familiar imitation’ in that
it requires observation learning; That is, the
ability to learn through vicarious (rather than
direct) reinforcement (Bandura, 1977). Various
researchers have made similar class distinctions,
recognizing that different mechanisms likely
mediate the learning and copying of a novel
behavior(s) and the copying of behaviors that
already exists in an individual’s repertoire (Byrne
& Russon, 1998; Heyes, 2001; Visalberghi &
Fragaszy, 2002). However, these investigators
have tended to argue that these skills are not
related and consequently have tended to give
these skills different names, which imply that
they exist outside of a dedicated cognitive
faculty for imitation. In this framework, both
skills are brought together and form part of the
same cognitive faculty that mediates the ability
to flexibly copy rules or responses, imitation.
Moreover, subsumed within those two broad
functional concepts are domain-specific sub-
ordinate mechanisms of imitation involved in the
copying of different types of information (motor,
auditory, cognitive) with unique computational
demands. These different subordiante imitation
mechanisms grant individuals the ability
to flexibly copy specific rules or responses.
That is, the behavioral rule that is copied is
both deliberate and replicable; not a result of

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37F. Subiaul
happenstance or trial and error learning. In this
definition, the term ‘rule’ is broadly defined as
a response involving more than two steps (with
a clear beginning, middle and end) that are
hierarchically organized and structured to achieve
a matching response. The requirement that any
type of imitation be rule-governed and flexible is
necessary in order to differentiate imitation from
either perceptual or motivational mechanism that
in association with rapid trial-and-error learning
may represent an ancestral learning mechanism
that predates (and may have co-evolved) with
the imitation faculty. The same is true of narrow
species-specific skills such as copying mate
preferences that while impressive, learning does
not extend beyond a very narrow and specific
context (i.e., mating) and stimuli (i.e., females).
Like all psychological faculties, the imitation
faculty is adapted to solve a number of problems
common among social animals. Some of these
problems include: (a) the problem of learning
dominance relationships, where individuals can
minimize injury by inferring from observational
learning who is likely to be dominant/submissive,
(b) the diet problem; learning what is edible and
what is not or what to eat when, (c) the problem
of alliances and cooperation, where individuals
can minimize the risks of bad alliances by
inferring from observation who is a reliable/
unreliable partner, (d) the problem of extractive
foraging, where individuals can learn from
others how to process or acquire protected food
products, (e) the problem of social conventions,
where individuals use others’ behaviors to guide
where and when they should display species-
typical behaviors. And there are certainly others.
In each instance, specialized mechanisms in
the imitation faculty in coordination with
other cognitive faculties grant individuals the
flexibility to make rapid inferences about the
dispositions of others or the causal structure of
actions, bypassing the costs associated with trial
and error learning, which in some instances may
be lethal (e.g., the diet problem). Some of these
instances require ‘imitation learning’ or novel
imitation (when knowledge is first acquired and
reproduce), but others only require the copying
of species-typical behaviors—familiar imitation
(e.g., social conventions)—where previously
learned behaviors (either by imitation or trial and
error) are appropriately and adaptively displayed.
Below is an outline of the characteristics of this
most significant of mental faculties.
Super-ordinate mechanisms of imitation:
Novel imitation
Part of the confusion in the imitation
literature is that ‘imitation’ has been largely
conceptualized as ‘novel imitation’ or the
imitation of novel behaviors. For example, in
1898, Thorndike defined imitation as “learning
to do an act from seeing it done” (p. 79). Nearly
a half-century later, Thorpe defined imitation
more narrowly and in purely behavioral terms:
“copying a novel or otherwise improbable act”
(p. 122). These definitions are often viewed as
synonymous, but they are quite different. One
core difference between these two definitions is
the requirement that individuals copy another’s
behavior. Copying is, arguably, the essence
of imitation. After all, what is imitation if it
is not copying something? Yet, Thorndike’s
definition does not mention or imply
copying but rather observational learning. The
distinction between observational learning and
imitation is an important one. It is possible to
learn something from another, yet not overtly
express the acquired knowledge; for example,
learning what not to do. In such instances, one
can learn from a model without imitating the
model. Thorpe’s definition, unlike Thorndike’s,
stresses both (observational) learning and
copying. Learning is implied in the criteria
because what is copied is ‘novel’ rather than
something that already exists in the observer’s
behavioral or cognitive repertoire. Despite a
number of qualifications and revisions (e.g.,
Galef, 1988; Tomasello & Call, 1997; Whiten
& Ham, 1992), Thorndike (1898; 1911) and
Thorpe’s (1956) definition of imitation remain
influential because of their simplicity and
the ease with which they lend themselves to
experimentation.Nevertheless,thesedefinitions,
which conceptualize imitation as the copying

of specific and novel motor responses, have
largely ignored an equally important function
of the imitation faculty, familiar imitation.
Super-ordinate mechanisms of imitation:
Familiar imitation
Familiar imitation involves the ability
to flexibly and adaptively copy common or
recognizable rules/responses that exist within an
individual’s behavioral or cognitive repertoire. In
the motor domain, everyday actions fall into two
distinct and conceptually significant categories:
transparent versus opaque. Transparent responses
are those responses that are immediately
available to the senses such as transitive actions
that involve reaching for and interacting with
objects and, as a result, may be executed via a
visual-visual match (i.e., my hand on an object
looks like your hand on an object). However,
opaque responses cannot be executed in the same
fashion, as they are not available to the senses in
the same way as transparent actions. Consider
the act of imitating someone scratching their
head. What you perceive when you see someone
scratch their head is very different from what you
perceive when you scratch your own head. The
phenomenological experiences are very different.
This problem of translating a visual experience
into a corresponding proprioceptive response
has been termed the “correspondence problem”
(Dautenhahn & Nehaniv, 2002).
While to some, the distinction between
‘novel’ and ‘familiar’ imitation may be obvious,
there is significant debate as to what should
count as a ‘novel’ response. Does ‘novel’ imply
an entirely new behavior? By the most strict of
standards this would exclude all species-typical
behaviors; a constraint that significantly limits
research questions. One way around such a
constraint is to require animals to execute a
series of familiar behaviors in arrangements that
are never (or rarely) observed. This technique—
of stringing familiar actions in an arbitrary
sequence—has been employed by a number
of animal researchers (apes: Whiten, 1998;
birds: Nguyen et al., 2005; monkeys: Caldwell
& Whiten, 2002) and represents one way of
operationalizing ‘novelty’ in imitation research.
Another technique has been to use a tool in
novel problem-solving tasks (e.g., Visalberghi
& Fragaszy, 1989, 1990, 1995). Perhaps these
studies, more than any other, represent the most
strict standards of novelty, as subjects must often
learn how to handle the tool and then learn how
use the tool in relation to another object. But
there are other ways to operationalize ‘novelty’
without using tools or specific motor responses.
Subiaul and colleagues (2004), for instance,
developed a cognitive imitation paradigm,
where subjects had to copy novel serial rules
independently of copying novel motor actions.
All of these tasks require that subjects learn
something new in order to be reinforced, and
exclude the possibility that subjects already
know how to execute the target response.
Others have tried to operationalize ‘novelty’
using single actions on objects (e.g., Bugnyar
& Huber, 1997; Voekl & Huber, 2000; 2007).
Here, the rationale is that while a behavior such
as mouthing is species-typical, mouthing an
object in order to open it is novel. The problem
is that animals often explore objects using their
mouths and certainly use their mouths on objects
associated with food. So, while a particular
behavior directed toward a specific object may be
unique, the actual behavior is not. In this regard,
it’s more likely that familiar imitation of the
familiar action (e.g., mouthing) rather than novel
imitation is the primary mechanism underlying
the behavioral response in single-action
paradigms. Such paradigms also make it difficult
to distinguish between various mechanisms of the
imitation faculty and the products of perceptual
and motivational mechanisms in which, for
example, an animal’s interaction with an object
may direct an observer’s attention to that object
(stimulus enhancement) or a part of that object
(local enhancement), motivating the observer to
interact with it (social enhancement). In such
instances, these two individual’s responses may be
very similar, yet the similarities are likely to be the
products of stimulus and social enhancement as
well as rapid trial-and-error learning, rather than
by any mechanisms of the imitation faculty.

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39F. Subiaul
Sub-ordinate mechanisms of imitation: Cognitive,
motor & vocal imitation
In addition to distinguishing between
familiar and novel imitation, it is important
to distinguish between various sub-ordinate
mechanisms that form part of the imitation
faculty. These domain-specific imitation
mechanisms mediate the copying of different
classes of stimuli, for example, auditory, motor,
and cognitive stimuli and are referred as: vocal
imitation (the imitation of vocal/auditory
responses), motor imitation (the imitation of
motor actions), and cognitive imitation (the
imitation of cognitive rules, including rules
governing serial order, social conventions
and spatial relationships, for example). The
distinction between super-ordinate mechanisms
of imitation (e.g., novel v. familiar) and sub-
ordinate mechanisms of imitation (e.g., vocal,
motor and cognitive) are important because
it allows researchers to specify what type of
imitation individual organisms are capable
of. For example, an individual may be able to
reproduce familiar vocal rules (e.g., words), but
may not be able to copy novel vocal rules (e.g.,
novel words) as is the case in autism (Williams
et al., 2004). Moreover, individuals may be able
to copy novel cognitive rules (e.g., serial order),
but not novel motor rules (e.g., specific action
sequences) as appears to be the case in monkeys
(Subiaul et al., 2004). The literature summarized
below uses this framework in an effort to
clearly identify the imitation skills of monkeys.
But, certainly, ‘monkeys’ are not a monolithic
group and are more diverse in morphology,
social organization and ecological range than
apes. However, because it is impossible to test
all monkey species in a systematic fashion,
most imitation studies with monkeys—and
those discussed here—have included only a few
species of New World monkeys (marmosets and
capuchin monkeys) as well as a few species of
Old World monkeys (baboons and macaques).
Finally, it’s worth noting that this framework
does not necessarily replace familiar terms that
have become an integral part of the social
learning literature such as emulation, where
individuals copy the outcomes or ‘affordances’
of actions or goal emulation, where individuals
copy the ‘indended’ action of others using
idiosyncratic means. Rather, it questions
the logic that terms such as emulation are
alternatives to imitation. Here, I advance the
contrarian’s view that terms such as emulation
and goal emulation describe the imitation of
different types of rules or responses; specifically,
copying rules—novel or familiar—about
environmental affordances or goals, respectively.
Early methods and experimentation
on novel imitation in monkeys
‘Observation cage method’
No individual researcher has had a greater
impact on contemporary comparative research
than Edward L. Thorndike. For better or worse,
his research and critique of imitation have been
similarly influential (Thorndike, 1898; 1911).
Thorndike (1898; 1911) was amongst the first to
stress that behavioral similarities could be achieved
by many different learning mechanisms. He was
also the first to develop an experimental paradigm
to test novel imitation. His paradigm emphasized
two measures: (a) the speed of behavior acquisition
and (b) fidelity in copying the target behavior
(Thorndike, 1898). Though Thorndike and others
noted that the second measure (fidelity of copying)
was somewhat subjective, the former was not. He
reasoned that if subjects were truly using imitation
to learn from the actions of an experienced model
(who had been over-trained in the solution to a
given problem or task) then, surely, the subject
that had been exposed to the model would
discover the solution to the problem faster than
those who had not been exposed to a model.
Studies with chickens, cats, dogs and monkeys
failed to satisfy this criterion. That is, there was
no difference between the performance of animals
with a model and those without. In Hall’s (1963)
review of imitation methodologies, he refers to
Thorndike’s (1898) paradigm as “the observation
cage-method” where a naive observer is exposed to
an experienced model or “demonstrator” solving

a given problem. In this paradigm, the model is
removed from the cage, following an observation/
demostration period, and replaced with a naive
observer who is then presented with the same
problem. Mills (1899), Haggerty (1909) and
later Warden and Jackson (1935) immediately
identified the shortcomings of this design. These
authors noted that the disruption of moving
the naive observer from one cage to another
introduced a significant distraction, confounding
negative results with increased anxiety, distraction,
and task ambiguity.
‘Single cage method’
The next major effort to rigorously test
imitation in monkeys (8 capuchins, 3 rhesus
macaques) was carried out by Haggerty (1909)
who developed the “single cage method” (Hall,
1963). In this new experimental design, both
the naive observer and the model were in the
same cage and had access to the same problem.
Haggerty devised eight different problems, each
varying in degree of difficulty. All involved
manipulating various levers and pulling certain
springs that were not always in sight. In one case,
a sheet of paper covered the solution to the task.
Haggerty reported that his design controlled
for the disruption of moving the observer and
allowed for an evaluation of degrees of success.
These various degrees of imitative success were
coded as, “immediate,” “partial,” or complete
“failure.” This classification placed a greater
emphasis on the outcome of the actions and less
on the means by which the subject arrived at the
outcome. Hence any matching response made by
the monkeys in these studies may have been the
resultofrapidtrialanderrorlearningorimitation.
Zuckerman (1932), for example, pointed out that
the success of Haggerty’s monkeys was probably
due more to perseverance than to an actual ability
to copy the model. Kempf (1916), Zuckerman
(1932) and Hall (1963), among other critics of
Haggerty, pointed out that it was impossible to
measure precisely what the student learned from
the model. Zuckerman suggested that Haggerty’s
monkeys could have learned to solve the tasks
by a combination of factors akin to what is
presently called stimulus/local enhancement,
social facilitation and trial-and-error learning.
Another possibility was that monkeys arrived
at the solution to the various problems by
emulation learning or even by imitation. But,
unfortunately, there were no controls to rule
out these alternative explanations. The “single-
cage method” was consequently abandoned in
response to these criticisms.
‘Duplicate cage method’
A third imitation paradigm, which Hall
(1963) named “the duplicate cage method,”
was introduced by Warden and Jackson (1935)
and later developed by Warden and colleagues
(1940). This design involved two identical testing
chambers placed side-by-side. Each chamber was
equipped with identical machinery in the exact
same location (i.e., objects to be pulled, turned
or twisted were in a fixed position). As with
other paradigms, naive animals (‘students’) were
given the opportunity to observe an experienced
model. Following this period of observation,
students were allowed to respond without being
moved from their cage. The goal was to correct
the shortcomings of past imitation researchers
such as Haggerty (1909) and Thorndike (1898;
1911). Specifically, Warden and colleagues
were interested in what they called “intelligent”
imitation (or novel imitation). To test this
type of imitation five variables were viewed
as necessary: (1) the task must be novel and
sufficiently complex, (2) the response must
appear immediately after observing the model,
(3) practice must be excluded by the experimental
conditions, (4) the act of the imitator must be
substantially identical with that of the model’s,
and (5) a sufficient number of instances must
occur, under varied conditions, to eliminate
behavior-matching by happenstance.
Warden and Jackson’s (1935) task involved
(a) pulling down a chain to obtain a reward,
(b) opening a door, (c) operating a simple latch
and then opening a door, and (d) operating two
latches and then opening a door. These tasks
were intended to vary in degrees of difficulty,
the last two being more difficult than the first

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41F. Subiaul
two. It is also important to note that the last two
have an A B element, and while reducing the
possibility of behavior matching due to stimulus
enhancement, it does not entirely exclude the
possibilityoflocalenhancementbecausethetarget
objects were in a fixed position in both chambers
and so it’s possible that after responding to one
item correctly (perhaps with the aide of stimulus
enhancement) the subject may have interacted
with the last item by default. The procedure
used was as follows: (1) a naive monkey would
be exposed to an experienced monkey. After a
given observational period, the naive monkey
was presented with the same problem in his own
cage. Performance was coded in varying degrees
of success ranging from “immediate imitation”
to “absolute failure.” Monkeys were evaluated on
whether or not they copied the precise methods
employed by the demonstrator (mechanism) and
the speed with which they acquired this behavior
(outcome). The monkeys’ imitation performance
on the very first testing session (using Warden
& Jackson’s criteria) ranged from 29% (Task 1)
to 50% (Task 4). The failure rate ranged from
36% (Task 1) to 43% (Task 4). Because these
monkeys’ performance was not compared to
a baseline condition it’s impossible to know
whether performance was driven by social
facilitation, for example. Moreover, because the
items were in a fixed position, local enhancement
cannot be ruled out in the cases where monkeys
appeared to reproduce the motor responses of
the model. Many of these short-comings were
corrected by later researchers who introduced
novel experimental paradigms, but in doing so,
introduced new confounds that resulted in new
interpretational challenges (see below).
Experimental evidence from 1970 to
Present
Novel motor imitation
Starting in the early 1970s, Beck (1972,
1973, 1976) published various studies exploring
the imitative abilities of baboons and macaques.
All of these studies involved the use of a tool
(L-shaped and rake-like) to obtain out-of-reach
food. As macaques and baboons (and, in fact,
virtually all monkeys) do not habitually use
tools in the wild, learning how to operate a tool
to procure reinforcement represents an test of
novel motor imitation. Beck used a Single-Cage
Method (Hall, 1963) to test his subjects. This
method was hampered by many of the same
interpretational confounds present in Haggerty
(1909). For instance, whereas social facilitation
may explain the few examples of motor imitation
recorded by Beck (1976) in one study, social
inhibition likely explains the lack of motor
imitation (Beck, 1973). The failure of monkeys
to learn in this second novel motor imitation
task was replicated by Chamove (1974), who
reported similar results with rhesus macaques.
Adams-Curtis and Fragaszy (1995) developed
a new task that involved the use of a mechanical
puzzle to test the imitative abilities of capuchin
monkeys. The use of a mechanical puzzle box
represents another means of testing novel motor
imitation. These researchers allowed their subjects
to interact with a protected object after observing
an expert model interact with the same project.
Adams-Curtis and colleagues (1987; 1995)
reported that while naïve capuchins appeared
interested in the task and were attracted to
various aspects of the puzzle (i.e., stimulus
enhancement), subjects neither discovered
the solution nor performed the model’s
sequence of actions while engaged in the task.
Visalberghi and Fragaszy (1990; 1995; 1996)
have been credited for testing the full range of
capuchin’s imitative abilities (Tomasello and Call,
1997). These authors devised a tool-using/tool-
makingtask,whichtestsspecificmodesofutilizing
a given tool(s) in a given context. Perhaps their
most well-known paradigm is the ‘trap tube’ task
(Fig.1).Taskssuchasthetraptubeexcludeentirely
the possibility of any familiar imitation by virtue
of both the novelty of the problem and the fact
that in some experiments the tool, while familiar,
had to be composed (or decomposed) in a specific
and novel fashion in order to be operational.
In the course of nearly half a dozen studies,
Visalberghi and Fragaszy’s results corroborated the

conclusionsofAdam-Curtisandcolleagues(1987;
1995). That is, while subjects were motivated to
interact with the tool and the experimental task
(i.e., stimulus and social enhancement), they
were either uninterested or unable to replicate
the precise means used by a model to solve the
problem or even how to use the tool, itself.
From this, Visalberghi and Fragaszy (2002)
have concluded that monkeys likely learn from
individual trial and error and/or stimulus/local
enhancement rather than from ‘true’ imitation.
Caldwell & Whiten (2004), in another novel
motor imitation paradigm, presented common
marmosets with a version of an “artificial fruit”,
a mechanical puzzle box that was originally
developed for the purposes of testing the
imitation learning skills of chimpanzees (Whiten
et al., 1996)). The apparatus used by Caldwell
and Whiten (2004) was smaller and had only a
single defense; a handle that had to be removed
to release a lid in order to obtain the food reward
enclosed. Caldwell & Whiten (2004) report
results that are very similar to those reported for
capuchinmonkeysbyAdams-Curtis,Visalberghi,
Fragaszy and colleagues. In effect, the marmosets
that saw a full demonstration interacted with the
apparatus more and were more likely to make
contact with the relevant parts of the apparatus
(i.e., stimulus enhancement) than control
monkeys who either saw a partial demonstration
(i.e., a monkey eating next to an open
apparatus) or those that saw no demonstration.
There are, of course, a number of different
reasons why monkeys may have failed to
demonstrate learning in these novel motor
imitation paradigms. In some instances (e.g.,
Beck, 1976), learning may have been inhibited by
the presence of a dominant conspecific (Fragaszy
& Visalberghi, 1989; Visalberghi & Fragaszy,
1990), in other instances (e.g., Visalberghi and
Fragaszy, 1995; 1996), learning may have been
limited by sometimes onerous motor confounds.
The latter is a significant concern given that
monkeys do not habitually use tools in the wild,
perhaps with the single exception of capuchin
monkeys (Perry & Mason, 2004). These
limitations have lead to the development of
paradigms that attempt to assess different facets
of the imitation faculty in monkeys: (a) the two-
action paradigm (Dawson & Foss, 1965) and
(b) the cognitive imitation paradigm (Subiaul et
al., 2004; 2007a); each will be discussed in turn.
Familiar motor imitation: two-action tasks
The concern over motor confounds and a
limited motor repertoire has led to the use of a
bidirectional (Dawson & Foss, 1965) or two-
action imitation procedure (Akins & Zentall,
1996). In these paradigms, reinforcement results
from the use of two (or more) possible actions
(for example, pushing versus pulling a door
to retrieve a reward). The assumption of such
paradigms is that if individuals are sensitive to
the actions of others and are capable of copying
specific motor movements, they should be more
prone to copy the model’s technique than an
alternative technique, which is also associated
with reinforcement. In fact, many animals—
from rats to various species of birds—have been
shown to copy the model’s motor response (for a
review see Zentall, 2007). However, because the
target actions are all familiar to the observers and
present in their behavioral repertoire, there is no
motor learning per se. However, because in some
Fig. 1 - Schematic of the Trap-Tube Task. In this
task, a reward is placed in the center of the tube
either to the right or the left of a ‘trap’ (a hole
from which the reward cannot be retrieved).
In order to retrieve the reward, subjects must
insert a tool that fits in the tube and push the
reward away from the trap.

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43F. Subiaul
instances individuals are learning how to apply a
familiar rule in a novel context, such behaviors
may be achieved by the coordinated activities of a
novel cognitive imitation mechanism (see below)
and a familiar motor imitation mechanism.
Two-action or bidirectional procedures
markedly differs from those described above
where individuals must learn a novel operational
rule. Rather, these procedures tests subjects’
ability to copy a specific, but familiar motor
response in a purposeful and replicable fashion.
As a result, these two-action and bidirectional
procedures represent a measure of familiar motor
imitation, not novel motor imitation.
Bugnyar & Huber (1997) were among the
first to use this paradigm in a monkey species, the
common marmoset. In this paradigm, monkeys
observed a model use one of two familiar actions
to open a door—pulling or pushing—and
retrieve food hidden behind it. Although there
were two possible means of opening the door,
only one method was demonstrated (‘pulling’).
The control group did not see any type of
interaction with the box, hence there was no
‘enhancement’ control. Given these limitations,
Bugnyar & Huber reported that during the
first phase of the study, two of the five monkeys
showed a preference for the modeled ‘pull’
technique. But, in subsequent testing sessions
these monkeys settled upon the alternate ‘push’
technique, perhaps because it was less motorically
complex. In a follow-up experiment by Voelkl
& Huber (2000) subjects had to open a film
canister using either the mouth or the hand.
There were three groups: (a) full-demonstration
(mouth or hand opening), (b) no demonstration
and (c) olfactory control (no demonstration, but
presented with the canisters opened by mouth).
Voelkl & Huber (2000) report that, of the six
subjects that observed a conspecific using the
mouth opening technique, four used the same
technique, whereas, none of the subjects that
saw the hand opening technique used the mouth
technique. These results are intriguing, but
complicated by the fact that the first session of
testing included 15 trials (first trial performance
is not reported) and, in those 15 trials, only two
monkeys (WI and SQ) used the mouth opening
technique consistently. The other monkeys were
either as likely (MO) or more likely to use the
hand opening technique. This contrasts with
monkeys that saw the hand technique who used
the same technique as the model 100% of the
time. This pattern of performance is inconsistent
with familiar motor imitation as defined above. If
monkeys are capable of familiar motor imitation,
then performance in the mouth opening group
shouldresembleperformanceinthehandopening
group and should not be restricted to just one type
of action. The fact that it does not, suggests that
these monkeys are biased to use a particular type
of motor response or that individual responses
are mediated by motivational and attentional
mechanism that lie outside the imitation faculty.
In a third experiment, Voelkl and Huber
(2007) focused on the mouth opening technique
and, using detailed motion analysis, evaluated
whether observers and non-observers copied not
only the global action, but the specific movement
patterns associated with the mouth opening
technique.Usingadiscriminantfunctionanalysis,
the authors report that the overall movement
patterns of observers were more like those of the
demonstrators than the movement patterns of
the non-observers. They argue that because the
movement path to successful opening is rather
broad, any similarities cannot be explained by
functional constraints alone. However, Voelkl
and Huber’s (2007) results would be more
compelling if individuals could reliably copy
movement patterns other than a single species-
typical action (i.e., mouthing).
Nevertheless, there is at least one other
study that suggests that monkeys are capable of
familiar motor imitation. Research by Fragaszy
and colleagues (2001, 2002 as cited by Fragaszy
& Visalberghi, 2004) is consistent with the
conclusion of Voelkl and Huber (2007). In a
series of studies, Fragaszy and colleagues devised a
two-action task where juvenile capuchin monkeys
living in two captive groups could obtain juice in
one of two ways: (a) by putting a finger into an
opening and turning a wheel that provided juice
or (b) by pushing down a lever in order to get a

burst of juice. Both of these actions are familiar
to animals, as such they represent examples of
familiar motor imitation. Fragaszy et al. (2001;
2002 as cited by Fragaszy & Visalberghi, 2004)
report that none of the juveniles solved the
problemorlearnedaspecifictechnique.However,
when a different group of immature capuchin
monkeys were allowed to watch adult capuchin
models, there was some evidence of familiar
imitation. Importantly, the authors note that
observers tended to adopt the solution style of
the adults in their groups. However, because these
studies are unpublished it is difficult to evaluate
the exact methods used. One potential problem
is that the actions used by the models may have
been opaque (i.e., inserting a finger in an opening
may have occluded the view of an observer)
and thus limited motor imitation performance.
Nevertheless, the results reported by Voelkl
and colleagues and those reported by Fragaszy,
Visalberghi and their associates are perhaps the
best evidence that monkeys despite their poor
performance in novel motor imitation tasks (such
as those that require the use of tool), are sensitive
to specific movement patterns and are capable
of copying familiar motor actions. But certainly,
more research is needed before we can confidently
state that monkeys are capable of familiar motor
imitationorareincapableofnovelmotorimitation.
Familiar motor imitation: oral-facial imitation
Marmosets’ ability to match opaque facial and
head movements (i.e., Voelkl & Huber, 2007)
parallels reports on human infants copying oral-
facial expressions such as mouth openings and
tongueprotrusions(Meltzoff&Moore,1977;1983;
1989). It has long been believed that the ability to
match oral-facial expression represented a human
cognitive specialization (Meltzoff, 1988; 1999)
associated with higher-order cognitive abilities
(Meltzoff & Moore, 1977; Meltzoff, 1988; 2002).
However, the uniqueness of this ability has been
put in doubt by reports showing that chimpanzees
(Myowa-Yamakoshi & Matzusawa, 2004) and
rhesus monkeys (Ferrari et al., 2006) are also
sensitive to such stimuli and copy these expressions
in a pattern similar to that present in humans.
However, researchers have cast doubt on the
notion that matching oral-facial responses is
best characterized as imitation (as defined here
or elsewhere). First, an extensive review of the
literature revealed that only tongue protrusions
are matched by human infants (Anisfeld, 1991;
1996; Anisfeld et al., 2001). Second, and perhaps
most surprisingly, a number of studies have
demonstrated that a moving pen (Jacobson,
1979), blinking light(s) (Jones, 1996) and music
(Jones, 2006) are all as likely to elicit tongue
protrusions in human neonates, as is watching
a model display the same behavior. However,
the study by Ferrari and colleagues on neonatal
imitation in macaques is unique in that the
experimentaldesignincludedanon-socialcontrol
(a spinning disk) in addition to the typical social
stimuli in such experiments (i.e., mouth opening,
tongue protrusions, etc.). Ferrari and colleagues
reportedthatlipsmackingandtongueprotrusions
occurred significantly more often in response
to displays of those same actions. However,
lipsmacking occurred the most often in response
to different types of stimuli, much like tongue
protrusions in human infants (Jones, 1996).
Ferrari et al. (2006), noting the amount of inter-
individual variation and the sensitivity to specific
oral-facial movements (e.g., mouth openings and
tongue protrusions) in both human and monkey
neonatal imitation, pointedly caution that “the
capacity to respond to the model may not reflect
a general imitative skill but rather a sensorimotor
sensitivity turned to specific facial gestures” (p.
1506). At this point it’s impossible to say with
any certainty whether these results are mediated
by a mechanism independent of the imitation
faculty or whether they simply reflect the output
an imitation faculty that is not yet mature.
Taken together, the research reviewed
above suggests that the motor imitation skills
of monkeys are significantly limited. The
preliminary conclusion is certainly more true
for novel motor imitation than for familiar
motor imitation; at least as measured by the
two-action procedure(s). This begs the question
of whether or not monkeys suffer from a novel
imitation deficit in general (Ferrari et al., 2006;

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45F. Subiaul
Visalberghi & Fragaszy, 2004) or whether
monkeys specifically lack the mechanism for
novel motor imitation. In order to answer
that question, however, a test must isolate the
copying of novel motor rules from the copying
of non-motor (cognitive) rules.
Novel cognitive imitation: copying serial rules
Although much has been written about
motor imitation as well as vocal imitation in
mammals (for reviews see Janik & Slater, 2000;
Zentall, 2006), little has been written about
the imitation of non-motor, non-vocal rules.
Subiaul (2004) and colleagues (2004; 2007;
2007a, 2007b) were the first to demonstrate that
the copying of cognitive—serial—rules can be
isolated from the copying of motor rules. The
studies conducted on novel cognitive imitation
were analogous to learning someone’s password
at an automated teller machine (ATM) after
looking over that person’s shoulder and later
entering that password on the key pad. Because
the observer already knows how to enter numbers
on the keypad, no motor learning is necessary. (In
some respects, the actual paradigm that was used
was more difficult because unlike the numbers
in a key pad, the items on the screen changed
spatial position from trial to trial; see Fig. 2).
Nevertheless, the ATM example illustrates the
two different rules that individuals might learn
from such an event. For instance, when copying
someone’s password, observers may copy a
spatial/motor rule (e.g., up, down, left, right);
ignoring the sequence of numbers being pressed.
Conversely, someone might copy the actual
numbers pressed (e.g., 2, 8, 4, 6), disregarding
the specific motor responses corresponding with
each number’s location on the touch pad. In
both instances the observer is copying a rule; the
principal difference is the type of rule: spatial/
motor versus cognitive/representational that is
learned and subsequently copied by the observer.
In one experiment (Subiaul et al., 2004),
two rhesus macaques were given the opportunity
to execute serial chains (Fig. 2) involving novel
lists of pictures in one of two ways: by trial and
error (baseline) or by observing an “expert”
macaque execute the same list in an adjacent
Fig. 2 - Simultaneous Chaining Task. (Top panel) Arbitrary pictures comprise list items, which are
displayed concurrently throughout each trial on a touch-sensitive video monitor. (Bottom pannel)
Each item’s position is varied randomly from trial to trial. The subject’s task is to respond to each
item in a particular order, regardless of its spatial position. Variation of spatial position prevents
subjects from performing the required sequence as a fixed–motor pattern or as a discrete set of
responses to specific external spatial cues, such as the choice points of a maze.

chamber (social-learning condition). This
paradigm corresponds with Warden & Jackson’s
(1935) duplicate cage method. Figure 3 shows
two monkeys during the ‘Observation Phase’
of the social-learning condition. When the
monkeys’ performances in the baseline and in
the social-learning conditions were compared,
results revealed that naïve “student” macaques
who observed an “expert” executing a new list
during the social-learning condition, learned
significantly faster than in a baseline condition
where they had to learn new lists entirely by trial
and error.
In a second experiment (Subiaul et al., 2004),
student macaques were given the opportunity
to observe an expert execute a list (e.g., list A).
At the end of 20 trials, the student was tested
on a different list (e.g., list B). Students in this
social-facilitation condition could not learn from
the expert because both students and experts
executed different lists of arbitrary pictures. As
in the social-learning condition, performance in
the social-facilitation condition was compared
to baseline where subjects had to learn new lists
entirely by trial and error. In this experiment, any
difference between a student’s rate of learning in
the social-facilitation and the baseline condition
would be the result of social facilitation (Zajonc,
1976) rather than novel imitation. Yet, the rate
of learning in the social-facilitation and the
baseline condition did not statistically differ.
In all three conditions, computer feedback
was available to students. However, in the
social-learning condition (Experiment 1) the
student could have learned the ordinal position
of individual picture items from computer
feedback alone, rather than from the actions of
the model. To test whether performance in the
social-learning condition could be replicated
by providing naïve students with computer
feedback only, in Experiment 3 all features of
the social-learning condition were maintained,
except that during the computer feedback
condition no monkey was present in the adjacent
chamber and the computer automatically
highlighted the target items in the correct serial
order. This control condition is often referred
to in the literature as the ‘Ghost Control.’ After
20 trials, the student was tested on the same
list. As was done in the previous experiments,
Fig. 3 - Cognitive Imitation Paradigm. During an ‘Observation’ phase, a ‘student’ monkey (right)
was given the opportunity to learn the order of novel picture items from he ‘expert’ monkey (left)
immediately before being tested on the order of those same items.

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47F. Subiaul
students’ performance in this ‘Ghost Control’
was compared with performance in the baseline
condition. Results demonstrated that monkeys
did not benefit from computer feedback alone,
as evidenced by the fact that the rate of learning
in the Ghost Condition did not differ from
the rate of learning in the baseline condition.
However, a similar test given to human children
and individuals with autism showed that
children learned in the ghost control (Subiaul et
al., 2007b); a result that has been replicated in
numerous motor imitation studies with children
(Huang & Charman, 2005; Thompson & Russel,
2004) but not other primates, including apes,
who have failed to learn in this control condition
(Hopper et al., 2007; Tennie et al., 2006).
What might account for the differences
between monkeys and other animals and
human participants’ performance in the Ghost
Condition? One hypothesis is that the difference
may rest on the propensity of human subjects
(but not non-human animals) to generate
cognitive rules about agency, goal-directedness
and/or intentionality to aid imitation in the
ghost control. This potentially unique human
ability has been reported in human infants,
who attribute intentionality and/or goal-
directedness to a ball that jumps over a barrier
and navigates around obstacles (Csibra, 2003).
Yet, no comparable evidence exists for monkeys
(Cheney & Seyfarth, 1990; but see Hauser, 1998
for one possible exception) or other animals.
In sum, the fact that monkeys are capable of
flexibly copying novel cognitive rules suggests
that monkeys lack a mechanism for novel motor
imitation, specifically, rather than a general novel
imitation mechanism. In other words, the fact
that monkeys can copy novel cognitive rules from
a model demonstrates that monkeys are capable
of certain types of novel imitation. Additionally,
given that some evidence exists for novel motor
imitation in apes but similar evidence is absent in
monkeys indicates that the motor planning and
execution systems of apes might be more derived
than those of monkeys and other animals,
perhaps as a result of apes’ long history using tools
(Mercader et al., 2002; Mercader et al., 2007).
Novel cognitive imitation: copying food preferences
Given the results of Subiaul and colleagues,
what might be the function of cognitive
imitation in more ecologically valid settings?
One possibility is that novel cognitive imitation
is critical for learning and copying social rules
that provide individuals with the tools to manage
dominance hierarchies, kin relationships and
socio-political relationships or alliances at low
costs; costs which are too high if not impossible
to manage without a social learning mechanism
such as familiar imitation or novel cognitive
imitation. But novel cognitive imitation may
also be critical in foraging problems, particularly
those that involve learning what foods are
palatable. After all, Reader and Laland (2002)
note that anecdotal reports of innovation and
social learning are most common in foraging.
In a number of studies, Visalberghi and
colleagues have explored this very question using
a captive population of capuchin monkeys.
Capuchin monkeys are in many regards an
ideal species with which to study the cognitive
imitation of novel food preferences because while
they are moderately neophobic of new foods,
captive capuchins sit near each other during
feeding and closely attend to what others are
eating (de Waal, 1997; Visalberghi & Fragaszy,
2004). Capuchins are also very tolerant; allowing
conspecifics to take small bits of food they have
dropped (Visalberghi & Fragaszy, 2004). Given
these characteristics, it seems that their behavior
and motivation are optimal for novel cognitive
imitation. Visalberghi & Fragaszy (2004) reason
that there are at least three mechanisms by which
individuals could learn and copy novel food
preferences. These mechanisms range from (i) a
general (arousal/motivational) mechanism that
increases feeding without regard to particular
food items, to (ii) a more subtle mechanism where
subjects are attracted to novel items in general
(e.g., neophilia), to (iii) cognitive imitation,
where individuals acquire a dietary rule(s)
pertaining to the palatability of particular foods.
Visalberghi & Fragaszy (2004) cite a number
of studies that suggest that capuchin monkeys use
a general (motivational or arousal) mechanism

thatincreasesfeeding,particularlywhenpresented
with novel foods (i.e., neophilia). For Visalberghi
& Fragaszy, this precludes any evidence for
novel cognitive imitation. These conclusions
are buttressed by a number of studies showing
that monkeys are more likely to eat when in the
presence of others than when alone (Galloway,
1998 as cited by Fragaszy & Visalberghi, 2004;
Visalberghi & Adessi, 2000). For example, Adessi
& Visalberghi (2001) presented capuchins with
novel food items (consisting of different food
products that were mashed and differentially
colored) in three different conditions: (i)
alone, (ii) in the presence of non-eating group
members and (iii) in the presence of eating
group members. They reported that as group
size increased, so did the consumption of the
novel food product by the observing monkey.
Moreover, the sight of a conspecific eating a
novel colored food was sufficient to increase food
consumption (independently of condition) for
two of the three different types of novel food
products presented. While these results provide
important insights into the feeding behavior
of capuchin monkeys, they are not designed
as social learning experiments per se as subjects
are not provided with a choice, for example,
between a ‘palatable’ and a ‘non-palatable’
food item. However, Visalberghi & Fragaszy
(2004) cite unpublished data (i.e., Adessi &
Visalberghi, 2002) that suggests that even when
provided with a such choice, capuchin monkeys
do not show a preference for the ‘palatable’
food. Moreover, in studies that changed the
palatability of a familiar food item, there was
no difference between a social condition (with
a model eating the now unpalatable food) and
an individual learning condition, where subjects
discovered the palatability of the food item by
happenstance (Visalberghi & Adessi, 2001).
In contrast to capuchins, cotton-top tamarins
living in a family group avoided palatable food
(tuna) that was experimentally manipulated to be
unpalatableafterobservingaconspecificrejectthe
food (Snowdon & Boe, 2003). In three groups of
tamarins, the aversion for the unpalatable food
was long-lasting. The avoidance and disgust
reactions toward tuna was still present after 10
months. These results provide some evidence of
vicarious learning. But, given the tamarins were
presented with either a palatable or unpalatable
food choice, it’s possible that mechanisms that
lie outside of the imitation faculty could have
mediated the avoidance response. An ideal study
on cognitive imitation for novel food preferences
would have a ‘student’ see a model eat different
types of novel food items that are entirely
unknown to the student and whose palatability
vary along a continuum. The dependent variable
would then be whether the student’s response(s)
(i.e., latency to approach and expressions of
disgust/avoidance) prior to tasting the foods in a
forcedchoicetest,forexample,areconsistentwith
the responses of the model. To my knowledge
no such study has been published, but such
studies would clarify whether the mechanism(s)
mediating food preferences are consistent with
the operations of the imitation faculty or some
other independent mechanism associated with
approach/avoidance responses.
Imitation in the brain: mirror neurons
and resonance mechanisms
Given the performance of monkeys in motor
imitation tasks, it may come as a surprise to
many that studies on the macaque brain have led
to important insights into the neurobiology of
motor imitation. In a now famous series of studies
that initially sought to understand the neural
mechanisms of goal-directed actions (Rizzolatti et
al., 1988), Rizzolatti and colleagues (di Pellegrino
et al., 1992; Gallese et al., 1996; Rizzolatti et
al., 1996) stumbled upon a class of neurons
in the inferior frontal lobe (F5) that became
active (fired) both when the monkey executed
a specific action such as grasping a peanut and
when they observed someone else execute that
same action (i.e., grasping a peanut), hence, the
name: “mirror neuron.” But mirror neurons do
not act in a vacuum. Research has demonstrated
that F5 is part of a network with a number of
neural regions that appear to be important for

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49F. Subiaul
motor imitation. For example, Perrett, Harries,
Bevan and colleagues (1989) have demonstrated
that neurons in the superior temporal sulcus
(STS), which encodes biological motion and
goal-directed actions, project to mirror neurons
in F5. STS and F5 neurons project to area PF
in the rostral part of the inferior parietal lobule
that has been associated with action observation.
A fraction of these neurons respond to the
observation of actions executed by others and
another group of neurons in this region have
mirror properties, much like mirror neurons in
F5. According to Rizzolatti (2005) the STS-F5-
PF circuit in the macaque brain provides multiple
descriptions of specific actions with identical
meaning (e.g., grasping), ‘concentrating’ the
different visual descriptions of the same action
on a select group of neurons on the one hand and
‘labeling’ them with motor meaning on the other.
Rizzolatti (2005) and colleagues believe that
through this mechanism, actions are understood
and by extension, provide the means by which
they can be imitated. Based on a number of
studies, Rizzolatti claims that “the mirror neuron
system plays a central role in the imitation of
actions that are already in the motor repertoire of
the individual” (71); that is, familiar imitation.
It is hypothesized that this feat is achieved by
matchinganobservedactionwithmotorresponses
stored in the premotor cortex, resulting in a fast
and efficient response to the observed action.
The presence of such a mirror neuron system
in the monkey brain, is consistent with the
conclusions of Voelkl and Huber (2007) who
argue that monkeys are capable of matching
familiar motor actions (i.e., familiar motor
imitation). Perhaps most importantly, research
into the neurobiology of imitation has also
provided some insights into why monkeys may
be capable of familiar motor imitation but not
novel motor imitation. Rizzolatti (2005) argues
that the motor and sensory computations that
the monkey mirror neuron circuit can make are
constrained relative to that of the human mirror
neuron circuit. In particular, Rizzolatti (2005)
notes that the monkey mirror neuron circuit
appears to be insensitive to intransitive actions
(i.e., actions executed in the absence of objects
that are real or imagined, as in pantomime) and
so may be unable to generate sensory codes for
them. This feature of the monkey mirror neuron
system contrasts with this same system’s well-
documented ability to encode transitive actions
(i.e., actions directed toward specific objects),
where the meaning of an action is defined in
relation to an interaction with a specific object.
No such meaning can be derived from intransitive
actions as they are entirely abstract and ‘objects’
or ‘goals’ must be imagined as they are not visible.
The relative poverty of the monkey mirror
neuron system contrasts with research with
human subjects using transcranial magnetic
stimulation (TMS), which suggests that the
human mirror neuron system is capable of
storing rich representations of intransitive
actions. From this, Rizzolatti (2005) concludes
that, “Without the storage of intransitive actions
to complement basic object-related actions
and precise copies of actions, the capacity of
the monkey system to imitate the behaviors
of others should be rather limited” (74).
Research into the neurobiology of imitation
has shed new light on an old question and
furthered our understanding of what comprises
the imitation faculty and how this faculty
may be represented in the brain. The work by
Rizzolatti and colleagues has also provided us
with important clues as to what characterizes the
monkey imitation faculty and what distinguishes
it from the human imitation faculty. In
particular, the work by Rizzolatti and colleagues
is consistent with the behavioral literature which
shows that when monkeys must copy familiar
actions directed to objects (c.f., Voelkel & Huber,
2007) or when copying familiar facial expressions
like lip-smacking (Ferrari et al., 2006), they
are more likely to succeed than when they are
tasked with copying novel actions governed by
rules pertaining to the relation between actions
and objects (i.e., intransitive actions); a feature
of novel motor imitation paradigms (e.g.,
Visalberghi and colleagues, 1990; 1995). The
work by Subiaul and colleagues (2004; 2007a)
is consistent with this conclusion, demonstrating

that the limiting factor for monkeys is motor in
nature, because despite the inability to copy novel
motor responses, macaques appear capable of
copying novel cognitive rules. However, note that
in Subiaul et al’s paradigm, the actions executed
by the monkeys were familiar.
Do monkeys have ‘traditions’?
Theexistenceofcomplex,population-specific
behavioraltraditionsinthewildcertainlysuggests
social learning, if not specific functions of an
imitation faculty. But, unfortunately, there are
many limitations to studying imitation learning
in the wild. The most obvious limitation is that in
field settings when an animal copies the actions
of another, it’s impossible to distinguish whether
such copying represents an instance of familiar
or novel imitation as it’s impossible to know with
any certainty whether the behavior was acquired
by trial and error and is now being copied by
familiar imitation or whether the behavior
was copied de novo, which would correspond
with an instance of novel imitation. Another
problem is that in field settings it’s impossible
to evaluate whether imitative responses (familiar
or novel) are flexible and adaptive or whether
they are mediated by attentional or motivational
mechanisms (i.e., stimulus/local enhancement).
Nevertheless, extensive behavioral traditions
that are characterized by population-specific
and complex behaviors that seem difficult if
not impossible for multiple individuals to
discover through individual innovation or
happenstance may represent a unique piece of
evidence in support of an imitation faculty in
primates in general and monkeys in particular.
Below, two potential examples of ‘traditions’
in monkeys are reviewed and critiqued.
‘Proto-culture’ in Japanese macaques
Kawai (1965) and colleagues were the first
to suggest that monkeys could create and sustain
traditions, which they called ‘proto-culture.’ The
behavior in question was sweet potato washing
(SPW) which spread within a population of
Japanese macaques. Sweet potato washing is a
behavior that involves taking a piece of food, in
this case a piece of sweet potato,to a body of water
and washing off the sand by placing the food
object in the water. Kawai described monkeys
dipping the potato in water by holding it in one
hand and then removing the sand by brushing the
potato with the other hand (Kawai, 1965). Sweet
potato washing originated with a 1.5 year old
macaque name Imo. According to Kawai (1965),
the “behavior spread to others gradually, and by
1956 eleven monkeys acquired it” (3). Learning
wasinitiallybiasedinfavoroffemalesandyounger
members of the troop. However, by 1963 males
and females were equally represented among the
sweet-potato-washers (20 females, 17 males).
Kawai(1965)andcolleaguesalsodocumented
the spread of another behavior, wheat washing
(WW) in Japanese macaques. WW resulted
from researchers dropping wheat grains on the
beach sand where monkeys would scoop-up a
handful of sand mixed with wheat grains and
then drop it in a nearby pool of water where
the wheat would float while the sand dropped
to the bottom of the pool. This allowed the
monkeys to easily grab the floating grains of
wheat. As with SPW, more females than males
were represented in the WW sample. Like SPW,
WW was characterized by an age-specific trend.
Specifically, younger individuals were, again,
more likely to be WW than older individuals.
In fact, no one past the age of 12 was seen
engaged in WW. Kawai (1965) does point out
that whereas SPW was a characteristic feature of
individuals who were 1 and 2 years old, WW was
predominantly found in individuals that were 4
and 5 years old. This suggested to him that SPW
might be a less complex task to master than WW.
Critiques of Japanese Monkey ‘Proto-Culture’
Critiques of Kuwai’s (1965) work have
centered on the nature of the learning and the
rate of transmission. With regard to the nature of
learning, Tomasello and Call (1997) first note that
washingfoodismuchmorecommoninnonhuman
primates than was previously thought. They point
out that potato washing has been witnessed

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51F. Subiaul
independently in four other troops of human-
provisioned macaques. This suggests that various
individuals had learned the behavior on their own
(i.e., via individual learning). These critics go on
to argue that experimental evidence has shown
that when other primate species in captivity are
provided with sandy food, they learn quite rapidly
to process the food in bowls of water (Visalberghi
and Fragaszy, 1990). Yet, interestingly, this was not
the case in Koshima. Even though all animals were
provisioned on the same beach, the behavior did
not independently erupt in multiple individuals.
Galef (1992) has critiqued the work on the
basis of the speed of transmission. According to
Galef (1992), the mean and median time it took
for the behavior to spread to most group members
was approximately two years. Tomasello and Call
(1997)likeGalef(1992)arguethat,“thiswouldnot
seem consistent with a process of imitation, which
is typically thought of as a rapid process” (277).
Moreover, they point out that as the number of
SPW increased, the rate of transmission remained
constant. In sum, Tomasello and Call (1997)
explain SPW (and other cases of social learning
among Japanese macaques) as follows:
“There is an individual ‘inventor’ who did not
learn by any form of social learning. Other
individuals were then exposed to the appropriate
learning conditions at different times, with
those more closely related to the inventor being
exposed first. They then learned individually as
well, likely facilitated by processes of stimulus
enhancement in which their engagement with the
wheat or rocks was stimulated by the engagement
of the inventor. As in the other cases, individuals
in closest proximity to the inventor and other
practitioners would be in a better position to see
and be attracted to the objects with which they
were interacting” (278).
Anthropologists have similarly been critical
of the notion of animal culture (i.e., Holloway,
1969; Kitahara-Frisch, 1991). Whereas,
Holloway (1969; 1981) pointed to the symbolic
aspects of culture, Kitahara-Frisch (1991) focused
on the characteristics of cultural transmission.
In a critique of the Japanese monkey culture,
Kitahara-Frisch (1991), for example, notes
that four characteristics are generally present
in definitions of “culture.” These include: (1)
the notion of culture as a comprehensive and
organized whole, (2) a mode of social behaviors,
(3) behavior that is non-genetically transmitted
across generations and (4) an expression of a body
of common understandings, such as are manifest
in consciously transmitted rules. Kitahara-Frisch
(1991) acknowledges that some of these features
are present (for example 2 and 3) but not all.
Some of these criticism and definitions of culture,
however, may be regarded as too anthropocentric
and invalid for making between-species
comparisons. A similar criticism had been leveled
towardTyler’s(1871)famousdefinitionofculture,
which he famously stated was “…that complex
whole which includes knowledge, belief, art, law,
morals, custom, and any other capabilities and
habits acquired by man as a member of society.”
Among the most important (but rarely cited)
features of culture were articulated by Kroeber
(1928) nearly a century ago. Kroeber identified
eight features: (i) innovation, (ii) dissemination,
(iii) standardization, (iv) durability, (v)
diffusion, (vi) tradition, (g) species-valid, and
(h) transcendent. Other features have since been
added. Tomasello (1999), for example, has added
the feature of ‘accumulation,’ where particular
behavioral patterns evolve and build upon pre-
existing behavioral patterns. Galef (1992) has
added ‘imitation,’ arguing that culture consists
of traditions that are transmitted by novel motor
imitation, specifically. And, Whiten and van
Schaik (2007) have added ‘variability,’ arguing
that culture is characterized by multiple traditions
(at least two or more) in multiple domains (e.g.,
social, ecological). Table 1 defines these different
components of ‘culture’ and their distribution in
humans, non-human (NH) apes and monkeys
(specifically, capuchin monkeys). Using this
criteria it’s clear that Japanese macaques lack
many of the critical features of ‘culture.’ Perhaps
most critically, Japanese macaque ‘proto-culture’
fails to meet the criteria of ‘species-valid’ and
‘transdence’ (Refer to Table 1).

‘Traditions’ in Capuchins
A number of field studies with wild capuchin
monkeys have identified patterned behaviors
that appear to be common to certain groups
but not others. Panger and colleagues (2002) for
instance have identified various food-processing
techniques that varied across 3 different sites. The
behaviors they describe include: (a) rubbing food
products against hard substrates or (b) pounding
them (using rocks or sticks) to break them open,
(c) using a fulcrum to apply pressure on a food
product, (d) taping an object with fingertips,
usually in a rhythmic fashion, and (e) wrapping
an object in a leaf and then rubbing the wrapped
product against a substrate (‘left wrapping’).
Panger et al. (2002) report that of the 61 food
species that were compared, 20 were processed
using a different technique. Additionally the
techniques used were not homogenous across
sites. There was geographic variation in the
technique used to process identical species of
food. Specifically, Panger and colleagues highlight
three behaviors: fulcrum use, leaf-wrapping and
a third behavior ‘army anting,’ which involves
monkeys following a column of foraging ants and
capturing insect or animal prey that are flushed
out by the swarming ants. These behaviors, they
note, are present in some sites but not others
and their distribution has no obvious ecological
correlate. However, leaf-wraping, may represent
an example of a clumped ‘idiosyncratic behavior’
that resembles a specific group ‘tradition’ but is
not. In one site, Loma Barbudal, the individuals
that were observed displaying this behavior
were peripheral and had very few opportunities
to learn tool-use behaviors from other group
members. Thus, it’s likely, this behavior may have
been ‘discovered’ through trail and error learning.
Another compelling piece of evidence
indicativeof‘traditions’hasbeenreportedbyPerry
Tab. 1. Components of ‘Culture.’ Below is a list of the characteristics of culture proposed by different
authors and their distribution in humans, non-human (NH) apes and monkeys (specifically, capuchin
monkeys).
COMPONETS ON CULTURE HUMANS NH APES MONKEY
INNOVATION: New behavioral pattern is invented* + + +
DISSEMINATION: Transmitted from individual to individual* + + +
DURABILITY: Pattern persists beyond demonstrator’s presence* + + -
DIFFUSION:Pattern spreads across groups* + + +
TRADITION:Pattern endures across generations* + + -
STANDARDIZATION: Pattern is consisted and stylized* + + ~
SPECIESVALID: Not an artifact of human influence* + + +
TRANSCENDENT: Not determined by biophysical enviroment* + + +
ACCUMULATION: Traditions build over time** + - -
IMITATION: Ability to copy novel motor responses‡ + + -
VARIABILITY: Two or more patterned behaviours in more than one domain§ + + +
CONFORMITY: Preference for groups’ standard(s)•
+ + -
* Criteria from Krober (1928), ** Tommasello & Call (1997), ‡ Galef (1992),
§ Whiten & van Schailk (2007), •
Heinrich & McElreath (2007).
Key: (+) present; (-) absent; (~) unknown or debatable

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53F. Subiaul
and colleagues (2003). Perry and her associates
describe five different social conventions in four
large study sites (Santa Rosa, Lomas Barbudal,
Palo Verde, Curu). By their very nature, such
conventions appear, on the surface, to be
entirely arbitrary and independent of ecological
variables. Thus avoiding many of the criticism
leveled against the Kawai (1963) observations of
potato and wheat washing behaviors of Japanese
Macaques. The conventions in question include:
(a) hand sniffing, (b) sucking, (c) finger game,
(d) hair game, (e) toy game. As can be seen in
Table 2, one group—Abby’s Group—frequently
exhibits all of the conventions in question,
while a second group “Cuajiniquil” frequently
exhibits two of the five behaviors. According to
Whiten and van Schaik (2007) the distribution
of these social conventions meet their criteria
for ‘tradition’ whereas those of the Japanese
macaques do not because it meets their criteria of
variability (seeTable 1). However, note that these
behaviors lack a number of features present in
both chimpanzee and human cultures, including:
durability, tradition and imitation (see Table 1).
Durability and tradition are absent because some
of the described conventions among capuchin
monkeys disappeared when the innovator
disappeared. These features of monkey ‘culture’
may explain why such patterned behaviors are
extremely rare in monkeys and, where present,
lack the number and variation that has been
documented in apes.
BEHAVIOURS SANTA
ROSA
(Cuajiniquil)
LOMAS
BARBUDAL
(Abby’s Group)
PALO VERDE
(Station Group)
CURU
(Better’s
Group)
HAND SNIFFING
Monkeys insert their finger in another
monkey’s nose or cups their hands over
their mouth or nose
+ + + -
SUCKING
Sucking another monkey’s fingers, toes,
ears or tails for a significant amount of
time
+ + ~ ~
FINGER GAME
A monkey inserts their finger in another
monkey’s mouth who clamps down
on their finger, preventing them from
removing their finger
? + - -
HAIR GAME
Monkeys take turn bitting tufts of hair
from each other’s bodies in a playful
fashion
? + - -
TOY GAME
Monkeys take turn taking things out of
each others mouths (sticks, leaves, etc.)
- + - +
Key: (+) behaviour is common; (~) behaviour is extremely rare; (-) behaviour not seen after more than 250 hours; (?)
insufficient data or inadequate data
Tab. 2. Five different ‘traditions’ described by Perry et al. (2003) in four different wild capuchin
groups. Although there might be some behavioral overlap in some of these traditions (e.g., ‘games’)
not that the distribution of each is unique.

The characteristics of traditions in the
wild as well as their respective function point
to selection pressures acting on individuals as
well as on networks of individuals (particularly
kin) creating a baldiwinian effect between the
need for certain traditions and the cognitive
mechanism(s) that maintain them. Although
field studies make it impossible to distinguish
between novel and familiar imitation and
in many instances it’s impossible to separate
attentional and motivational mechanisms, it’s
likely that at least in the case described by Perry
and colleagues (2003), such conventions likely
represent examples of familiar imitation. The
same may be true for the examples described by
Panger and her associates (2002).
The evolution of the
imitation faculty
Any contemplation of the evolution of the
imitation faculty must begin with the question
‘What is imitation for?’ How might having
a simple imitation faculty consisting of only
familiar imitation, for example, be adaptive?
How might it increase fitness? First and foremost,
familiar imitation solves the problem of where
and when to execute species-typical behaviors
(e.g., social conventions in capuchin monkeys).
In contrast, novel imitation solves the problem
of copying new and, perhaps, rare behavioral
rules or responses at a low cost. However, in
each both cases, imitation (familiar and novel)
reduces the costs (e.g., time, energy) associated
with trial-and-error. When viewed this way,
familiar imitation minimizes the need to learn
where or when to execute familiar responses,
in the case of novel imitation, it minimizes
learning how and, perhaps why to do a novel
response. As many have noted, these problems
are particularly acute in environments that are
constantly changing. That environment may be
social, it may be physical or it may be both. The
more flux, the greater the need to quickly adapt
to the new situation and the greater the selection
pressures favoring an imitation faculty. This view
has been supported by various mathematical
models which have, in effect, demonstrated that
the evolution of the imitation faculty is linked
to life in ever-changing environments (Boyd &
Richardson, 1986; Henrich & McElreath, 2003).
An evaluation of animals such as birds and
primates who live in variable social and physical
environments, suggests that these animals possess
social learning skills consistent with at least a
basic imitation faculty (Reader & Laland, 2002;
Lefebvre et al., 1998). Interestingly, Reader
and Laland (2002) have reported that among
primates, brain size correlates most significantly
with social learning, but also with innovation
and tool-use. But innovation may represent an
important problem-solving adaptation that co-
evolved with the imitation faculty. Consistent
with this view is the fact that in their analysis,
social learning, innovation and tool use are all
strongly inter-correlated (Reader & Laland,
2002). Similar data exists for birds (Lefebvre,
et al. 1996; Lefebvre, et al., 1998), providing
evidence of convergent evolutionary processes.
The research summarized above shows that
new and old world monkeys, like apes and
humans, possess a faculty of imitation. That is,
a psychological faculty that translates incoming
sensory information into a matching response.
However, macaques, capuchins and marmosets
appear capable of only familiar motor imitation
andrhesusmonkeyshaveevidencednovelcognitive
imitation. But these same monkey species appear
incapable of novel motor imitation. This feature of
the monkey imitation faculty contrasts with that
of chimpanzees, orangutans and gorillas (as well as
humans) who are capable of at least certain types
of novel motor imitation (Byrne 2005; Whiten
et al., 2004). From this it follows that novel
motor imitation is likely to be a derived feature
and a characteristic of the hominoid imitation
faculty; one that is perhaps intricately linked with
tool-use. In this view, the more dependent an
organism is on technology or motor learning for
subsistence, the more sophisticated and versatile
the imitation faculty. The main reason being
that the use of technology—tools—requires
specialized sensorimotor, inferential and, perhaps,

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55F. Subiaul
problem-solving (or innovation) mechanisms
working in a coordinated fashion to selectively
attend to and encode certain types of information
that produces a template that serves as the basis
for a matching response. Such pressures should be
stronger among apes than monkeys because while
monkeys have specialized dentition and digestive
systems, apes have somewhat generalized dental
anatomy and, with the exception of gorillas, lack
specialized digestive systems (Ankel-Simons,
2000). These anatomical differences mean that
whereas monkeys are able to enjoy a relatively
diverse diet, apes do not have the same luxury.
In monkeys, diets range from non-ripe fruits and
mature leaves to insects, small animals and gum.
Ape anatomy, however, limits dietary options to
a narrow range of foods that consist mostly of
mature, non-fibrous fruits with high sugar and
calorie content (Maier, 1984). As a consequence
of these dietary limitations, the great apes occupy
a fairly narrow range of ecological habitats, being
largely restricted to tropical and woodland forests
(Potts,1998;2004).Contrastthenarrowecological
range of chimpanzees and orangutans to that of
macaques that have made a home in the arid lands
of Africa as well as the snowy hillside of Japan.
These ecological, morphological and dietary
pressures that, among primates, are mostly
unique to the great apes, placed a premium on
novel behavioral, cognitive, and life history
strategies that are critical to fitness (Potts, 2004)
and presumably served as a compensatory
mechanism for morphological limitations. One
such behavioral strategy used to broaden the apes
diet is the systematic pursuit of prey in groups—
or ‘hunting’—(Watts & Mitani, 2002), another
has been extractive foraging using tools (Goodall,
1986; Whiten et al., 1999). Yet another, might
have been the fission-fussion social organization
of chimpanzees and bonobos. Both of these
behavioral innovations—hunting and tool-use—
likely favored an elaboration of the imitation
faculty, in particular, the evolution of a robust
novel imitation mechanism that was functionally
integrated with other domain-specific imitation
mechanisms (e.g., motor and cognitive imitation).
Certainly, the novel motor imitation skills of apes
are less robust than those known to be present in
children as young as 2.5 years of age (Herrman
et al., 2007). These more derived novel motor
imitation skills likely date to the first members of
the genus Homo, where the need and dependence
on stone-tool technology and other methods of
subsistence including hunting and gathering
placedincreasingpressuresonvariousmechanisms
of the imitation faculty. Some of these elaborations
may have included functional connections with
other domain-specific mechanisms mediating,
theory of mind, agency-attribution, and certain
attentional and memory systems.
Nevertheless, given the ecological
circumstances of non-human great apes,
an imitation faculty capable of novel motor
imitation would immediately increase the fitness
of chimpanzees, for example, as it would have
provided individuals with the skills to effectively
steal the technical knowledge of conspecifics and
immediately use that knowledge to supplement
their diets. Given the importance of such a skill,
it should then be no surprise that apes have
elaborate tool-traditions (Whiten et al., 1999)
which afford the means to develop and maintain
these skills across generations ('tradition') and
outside the innovator's presence ('durability').
Yet, note that traditions as they exist in
chimpanzees and orangutans are mostly absent
in monkeys. And where they exist, as appears to
be the case in capuchin monkeys, they comprise
of just 2 or 3 behaviors (Boinski et al., 2003;
Panger et al., 2002; Perry et al., 2003) which lack
the diversity and complexity that characterized
chimpanzee and orangutan behavioral traditions
(Table 1). These differences may rest on the fact
at least in captivity, chimpanzee traditions are
mediated by motor imitation coupled by a strong
tendency to always use the group’s preferred
technique; 'conformity' (Whiten et al., 2005).
No comparable evidence exists for capuchin
monkeys, or any other monkey species. Again,
perhaps the discontinuity between cultures
in monkeys and apes is not surprising, given
that monkeys’, as a group, are characterized by
numerous anatomical specializations that are
specifically adapted to their niche, which in no

small measure grants them the ability to exploit a
wide range of diets and habitats without tools or
the need for sophisticated traditions.
Given the evidence that capuchin, marmoset
and rhesus monkeys as well as chimpanzees,
orangutans and gorillas share a familiar imitation
mechanism, familiar imitation is likely to be the
most basic and ancestral feature of the imitation
faculty, and the feature that is likely to be present
in all animals that possess a faculty of imitation.
The models proposed by Boyd & Richardson
(1986) and Henrich & McElreath (2003) explain
this facet of the imitation faculty best. While it’s
possible for an animal to possess an imitation
faculty that can copy only familiar responses
(familiar imitation), it’s difficult to imagine an
imitation faculty capable of novel imitation, yet
incapableoffamiliarimitation.Fromthisitfollows
that the evolution of a derived imitation faculty
that includes the ability to copy novel responses
is premised on mechanisms that mediate familiar
imitation. Some of the neurobiological evidence
reviewed above provides some insights into
how the elaboration of the STS-F5-PS circuit
in the macaque brain, for example, can make
at least novel motor imitation possible via the
representation of intransitive actions (Rizzolatti,
2005). However, logically, novel motor imitation
is premised on novel cognitive imitation. The
former seems difficult (if not impossible) without
first having the ability to copy novel cognitive
rules. But what selection pressures might have
driven the elaboration of this faculty? One
possibility is the need to develop and acquire
more effective extractive foraging techniques;
specifically,techniquesthatrequiretheuseoftools.
The data summarized above provides
compelling evidence that monkeys possess a
faculty of imitation. This imitation faculty
appears to consist of the ability to copy familiar
motor actions (i.e., familiar motor imitation) as
well as novel cognitive rules (i.e., novel cognitive
imitation). These skills, afford monkeys the
ability to appropriately copy the (familiar)
actions of their conspecifics; a skill likely to be
a specific adaptation to the pressures of group
living, such as pressures associated with managing
social hierarchies and group feeding. From this
it follows that familiar imitation should be
common in most social species where the ability
to adaptively copy the familiar behaviors of
conspecifics during synchronized activities like
foraging, feeding and territory defense would
afford important fitness benefits; reviews of social
learning in a variety of animals suggests that this
is the case (see Zentall, 2006). Novel imitation
should be common in species with generalized
anatomies and where technical (or specialized
motor) knowledge is critical for survival. Thus,
in this view, the elaboration of a critical social
cognition skill—imitation—was the product not
of social factors but physical factors associated
with knowledge of tools, motor actions and
spatial relations.
Conclusions
There is no simple answer to the question
‘Do monkeys ape’. In fact, there is no simple
answer to the question ‘Do humans ape.’ The
main reason being that imitation is not a unitary
skill. Here, imitation has been conceptualized as a
multifaceted psychological faculty whose function
is to adaptively and flexibly copy rules or responses
expressed by others. This faculty has two general
functions: the ability to copy familiar skills and
the ability to copy novel skills as well as domain-
specific functions that correspond with the the
ability to copy different types of stimuli: motor,
vocal and cognitive. Given the behavioral and
neurobiological evidence, it’s been proposed that
the ability to copy familiar responses represents
the primitive state of this faculty. The ability to
copy novel responses represents a more derived
state of the same faculty.
Monkeys appear capable of copying familiar
motor rules as evidenced by studies demonstrating
that marmosets use the same opening technique
as a model to open a sealed can, for example
(Voelkl & Huber, 2007). Macaques also possess
the ability to copy familiar facial expressions such
as lip-smacking and mouth opening. However,
monkeys have not evidenced novel motor

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57F. Subiaul
Info on the web
imitation. This result is in contrast to apes, who
have been shown to differentially copy novel
or arbitrary motor actions (Horner & Whiten,
2005; Myowa-Yamakoshi & Matsuzawa, 1999;
Whiten, 1998). Yet, despite monkeys’ inability
to copy novel motor responses, they are capable
of copying novel cognitive (non-motor) rules
(Subiaul, et al., 2004); a result which demonstrates
that while monkeys may be incapable of novel
motor imitation, they are capable of novel
cognitive imitation or imitation learning.
Insum,whilenovelmotorimitationrepresents
a highly advanced cognitive skill that may be
unique to the great apes perhaps due to dietary
pressures favoring tool-use in apes (but not in
monkeys), familiar imitation represents a more
general, yet indispensable skill for social animals
who must conform to the vagaries of sometimes
strict social hierarchies and coordinated group
activities such as feeding and territory defense.
These pressures common in social animals should
favor a basic imitation faculty whose primary skill
is to adaptively copy familiar rules and responses
expressed by conspecifics. Such a skill in response
to social pressures is not significantly different
from the folk saying among humans that compels
us to engage in familiar imitation, ‘When in
Rome, do as the Romans do.”
Acknowledgements
I would like to thank Jennifer Vonk and Chet C.
Sherwood as well as two anonymous reviewers for
reading and providing me with useful comments on
an earlier version of this manuscript.
http://www.subiaul.com
Cognitive imitation; imitation learning in children and monkeys
http://www.emory.edu/LIVING_LINKS/
Living Links (Emory University, Yerkes National Primate Research Center): Comparative psychology,
great apes and monkeys
http://www.cognitiveevolutiongroup.org/
Cognitive Evolution Group (University of Louisiana): Cognitive evolution, comparative research on great
apes and human children
http://primatelit.library.wisc.edu/
PrimateLit (University of Wisconsin, Madison): Bibliographic database of primate research
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