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Neuroscience and Biobehavioral Reviews 30 (2006) 1246–1259
www.elsevier.com/locate/neubiorev
Review
Psychopathology in great apes: Concepts, treatment options and
possible homologies to human psychiatric disorders
Martin Brünea,, Ute Brüne-Cohrsa, William C. McGrewb, Signe Preuschoftc
a
Center for Psychiatry, Psychotherapy and Psychosomatics, University of Bochum, Germany
b
Department of Biological Anthropology, University of Cambridge, UK
c
Primate Center Gänserndorf, Austria
Abstract
Many captive great apes show gross behavioral abnormalities such as stereotypies, self-mutilation, inappropriate aggression, fear or
withdrawal, which impede attempts to integrate these animals in existing or new social groups. These abnormal behaviors resemble
symptoms associated with psychiatric disorders in humans such as depression, anxiety disorders, eating disorders, and post-traumatic
stress disorder. Due to the outstanding importance of social interaction and the prolonged period of infantile and juvenile dependence,
early separation of infants from their mothers and other adverse rearing conditions, solitary housing, and sensory deprivation are among
the major albeit non-specific sources of psychopathology in apes.
In contrast to the wealth of research in monkeys, psychopathology in apes has been under-studied, and only a few studies have
examined how to alleviate abnormal behavior in captive apes. Recent studies have shown that the enrichment of living conditions and
behavioral treatment (conditioning) may ameliorate some pathological features, and careful familiarization with novel physical and
social environments can help re-socialize behaviorally disturbed animals, but usually not to the extent of successful mating and raising
offspring. The possibility of psychopharmacological treatment of the most severe disturbed animal patients has only been reluctantly
considered, but a few case reports have revealed encouraging results.
This article proposes the need to expand research into ape psychopathology, which would require an essentially interdisciplinary
approach of primatology and psychiatry, ultimately to the benefit of both.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Apes; Behavioral therapy; Evolutionary psychology; Psychopathology; Psychopharmacological treatment; Re-socialization; Separation;
Stereotypies
Contents
1.
2.
3.
4.
5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Great ape societies, life history patterns and evolved temperaments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Description of psychopathology in non-human great apes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Assessment and diagnosis of psychopathology in great apes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Etiological aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Treatment of psychopathology in apes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1. General remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2. Re-socializing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3. Behavioral treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4. Pharmacological treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corresponding author. Tel.: +49 234 5077 155; fax: +49 234 5077 235.
E-mail address: [email protected] (M. Brüne).
0149-7634/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.neubiorev.2006.09.002
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Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256
1. Introduction
The existence of psychopathology in captive non-human
primates has been known for many decades (e.g.,
Tinklepaugh, 1928). The first naturalistic descriptions
highlighted the emergence of stereotypies (oft-repeated,
apparently functionless motor patterns) and of selfmutilation in conditions of sensory deprivation or physical
restraint (Yerkes, 1943; Davenport and Menzel, 1963;
overview in Ridley and Baker, 1982). Since the 1950s,
mainly through the work of Harlow and his co-workers,
psychopathology has been systematically induced in infant
macaques by separating them from their mothers (Harlow
and Zimmermann, 1959; Harlow et al., 1965; SpencerBooth and Hinde, 1971). These experiments have revealed
that early separation of infant monkeys from their mothers
and lack of attachment cause persistent, perhaps life-long
behavioral abnormalities. For example, individuals separated from their mothers at an early developmental age
were unable later to form normal social relationships and
to care properly for their offspring (e.g., Kraemer, 1997).
These findings have had an impact on research into
attachment and bonding in humans including the study
of sensitive, imprinting-like periods during ontogeny, and
have contributed substantially to modern conceptualizations of normal and abnormal personality development,
pathological anxiety, depression and post-traumatic stress
disorder, including psychotherapeutic and pharmacological treatment options for these conditions (Graham et al.,
1999).
In contrast to the wealth of data concerning various
monkey species, the phenomenology and etiology of
psychopathology in great apes has been under-explored
(Berkson et al., 1963; Randolph and Mason, 1969; Rogers
and Davenport, 1969). We deliberately use the term
psychopathology here in light of the overwhelming
evidence for the psycho-socio-biological continuity between the great apes and humans (e.g., Fabrega, 2002). In
any event, besides a few individual case reports (e.g., Brent
et al., 1989; Fritz and Fritz, 1979; Nash et al., 1999), few
studies have systematically assessed the presence or absence
of behavioral deviations in great ape populations, and
developmental and other environmental conditions affecting the likelihood of abnormal behavior.
We propose that there is a need to extend research into
great ape psychopathology for several reasons: First, an
increasing number of great apes (almost entirely chimpanzees) no longer needed in biomedical laboratories and also
from other sources (confiscated, surplus from zoos, private
homes, circuses, etc.) show varying degrees of behavioral
abnormalities due to adverse husbandry conditions. Some
of these individuals are being retired to sanctuaries,
because the usual solution for other animals of systematic
euthanasia is deemed unethical for humankind’s nearest
relations (National Research Council, 1997; Brent et al.,
1997; Beck et al., 2001). However, introducing unfamiliar
adult great apes to group housing is risky, and may need to
be supported by behavioral or even pharmacological
treatment.
Secondly, the extant great apes are severely endangered
in the wild due to habitat destruction, disease, hunting or
direct competition with humans, e.g. crop-raiding (Beck et
al., 2001; Kormos et al., 2003). These anthropogenic
factors lead to habitat and population fragmentation. On
the other hand, more and more orphaned apes are flooding
sanctuaries in habitat countries. To retain viable population sizes, or in some cases to prevent extinction of local
population of apes, it may well be necessary to re-introduce
apes from sanctuaries into the wild. Because of the
complexity of the apes’ cognition and behavior and the
fact that their survival and reproduction are based, to a
large extent, on culturally transmitted skills, psychiatric
diagnoses and psychological treatment need to be developed further in order to enhance the chances of success of
these endeavors (e.g., Pruetz and McGrew, 2001).
Thirdly, the existing framework for recording psychopathology in apes has largely been restricted to the
documentation of stereotypic movements. Diagnostic
validity and reliability, and hence, treatment of disorders
in non-speaking apes (as in immature, mentally retarded or
otherwise speech-impaired humans) could substantially
benefit from consideration of non-verbal signals of emotion such as facial expression, gesture, body posture and
vocalization (e.g., Randolph and Mason, 1969). Systematic
comparison with catalogs of human disorders (e.g. DSMIV, American Psychiatric Association, 1994) based on
shared methods is likely to reveal further pathologies in
non-human apes.
Fourthly, the close phylogenetic relatedness of the great
apes to humans, the extraordinarily long period of parental
care and tutelage, and similarities in cognitive and
emotional functioning strongly suggest that many homologous psychopathological conditions occur in non-human
and human apes. Here, we argue that the adoption of a
comparative perspective including assimilation of the
knowledge about the evolved psychology of non-human
great apes will lead to novel insights into the etiology and
therapy of human psychopathologies. It is also to be
expected that psychopathologies of great apes arise from
dysfunctions of homologous brain structures, at the level of
cells and neurotransmitters (Davenport and Menzel, 1963;
Berkson and Mason, 1964; Wildman et al., 2003). Detailed
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analyses at the functional and the mechanistic levels of
similarities and differences between humans and other apes
in manifestation, course and outcome of psychopathological syndromes, including response to treatment are needed
(Brüne et al., 2004). These should substantially further our
understanding of mental well-being, therapy and prevention of psychopathologies.
Here, we seek to review the existing scientific literature
on psychopathology in great apes, and to refine existing
conceptualization of psychopathological conditions including treatment options in apes.
2. Great ape societies, life history patterns and evolved
temperaments
Much knowledge of the psychosocial lives of great apes,
both wild and captive, has accumulated since the 1960s
(overview in McGrew et al., 1996). This knowledge has
brought about a growing understanding of primate
behavioral evolution and its social, emotional, and
cognitive underpinnings. At the same time evidence from
molecular genetics has revolutionized primate phylogeny
by showing that the closest living relative of the chimpanzees (genus Pan) is another ape called Homo sapiens, and
that the three species of Pan troglodytes, Pan paniscus, and
H. sapiens together are more remotely related to gorilla
(Gorilla gorilla) and still more distantly related to
orangutan (Pongo pygmaeus) (Kaessmann and Pääbo,
2002). Here, we briefly present thumbnail sketches of the
major behavioral commonalities and differences between
the great apes including humans.
All apes are obligatorily social: There is a deeply rooted
need for social interactions, for forming social relationships, both supportive and antagonistic, even for politicking (de Waal, 1982; van Schaik et al., 2004). The social lives
of primates are so complex that it has been hypothesized
that their remarkable intelligence has evolved primarily in
response to social challenges (Jolly, 1966; Humphrey, 1976;
Byrne and Whiten, 1988; Dunbar, 2003). Each of the four
extant species of great apes lives in a different type of
society (reviewed in van Schaik et al., 2004). Chimpanzees
live in multi-male, multi-female societies, characterized by
male bonding, domination of females by males, territoriality and hostility towards strangers. Dominance relationships among males are pronounced, but are compensated
for by coalitions. Fatal aggression occurs between communities in territoriality, and within communities, especially in infanticide. Mothers are less social, but wild
females without dependent offspring or captive females can
exhibit social maneuvering similar to males. Although
males show aggressive competition over and monopolization of sexually active females, chimpanzees also use sociosexual behavioral patterns in a variety of contexts from
greeting to reconciliation.
Bonobos also live in multi-male, multi-female societies,
but their society is marked by female dominance over
males, less-pronounced dominance relations, and lower
levels of aggression, with apparently deliberate killing of
conspecifics absent (Hohmann et al., 1999). At the same
time sexual and erotic interactions abound, which are
shown in contexts as diverse as relationship formation,
food sharing and reconciliation. While chimpanzees live in
‘‘fission–fusion’’ societies, in which members of a community range in ever-varying subgroups, bonobo communities
appear to be more cohesive.
Data from hunter-gatherer societies suggest that in
ancestral conditions humans tended to live in fission–
fusion societies of monogamous or mildly polygynous
family groups nested in larger communities and linked by
male–male bonding. Generally, males dominated females,
however, dominance relationships between males or
between family units are only weakly expressed in extant
hunter-gather societies. Sexual interactions between reproductive members of different families may lead to escalating aggression, but may also be tolerated. Cooperation
within the community is explicitly encouraged, whereas
relationships to strangers are characterized by distrust and
between-group hostility. Cultural evolution has subsequently led to a vast diversity of societal organizations
and it is certainly outside the scope of this review to cover
all facets of human cultural diversity. In any event, we
would like to emphasize the view that culture does not
develop independently of human evolved psychology
(Cronk, 1999); rather, culture reflects complex interactions
between highly flexible evolved psychological mechanisms
shared by all members of the species and an extraordinarily
broad spectrum of environments human societies have
occupied in the last 100,000 years.
Gorillas usually live in one-male groups, lead by a
dominant silverback male and a number of unrelated
females and their offspring (Watts, 1996). These groups are
essentially multi-female families, sometimes including
maturing males. Relationships between harem females are
loose and undifferentiated, perhaps because of frequent
interventions of the dominant male into female interactions. Sexual behavior is rare, males are highly intolerant of
same-sex competitors, and infanticide linked to male
takeover of a group may occur.
The social structure of orangutans has been enigmatic,
because they live largely solitary lives and the quality of
their individualized relationships becomes apparent only
after years of observation (van Schaik and van Hooff,
1996). Where food is abundant females and immatures
tend to associate. Males come in two types, with and
without secondary sexual characteristics, e.g., fatty cheek
phlanges. Females prefer fully mature males with prominent secondary sexual features both for mating and for
protection from other males. Males lacking secondary
sexual features regularly harass and coerce sex from
attractive females.
Beyond these differences, for mammals of their body size
all great ape species grow slowly, mature late, have long
adult lifespans, and long intervals between births of usually
single offspring (Galdikas and Wood, 1990; Charnov and
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Berrigan, 1993), with humans lying at the extreme among
the hominids except for birth intervals, which are
considerably shorter than in apes due to cooperative
breeding and grandmothering (overview in Blaffer Hrdy,
2000). Apes and humans share a prolonged period of
infantile and juvenile dependence on the mother as the
main caregiver and model for social and technical skills
(e.g., Lonsdorf et al., 2004). In general, life unfolds in a
network of individualized social relationships. Observational learning and socially transmitted skills play an
important role in the acquisition of subsistence techniques
as well as social tactics (e.g., Whiten et al., 2003).
Cooperation with unrelated conspecifics seems to be highly
developed, as a consequence of natal dispersal strategies
and slow life histories. Social exchange networks are well
documented for both Homo and Pan.
With respect to individual differences within species,
research into human personality psychology indicates a
hereditary basis for temperament, character and personality (Cloninger et al., 1993; Bouchard, 1994). Such
individual differences concern warmth, gregariousness,
assertiveness, activity, excitement-seeking, and positive
emotions (Costa and McCrae, 1995). In addition, important differences arise between individuals because their
motives for attachment, achievement, and power differ in
strength (Emmons, 1989). These features correlate strongly
with emotional expression and social relationships. In
monkeys, personality dimensions include timidity, assertiveness, sociability, impulsivity and curiosity. Moreover,
growing evidence indicates systematic differences between
species of macaques in temperament related to speciestypical differences in the nature and distribution of power
(Capitanio, 2004; Preuschoft, 2004a, b; Preuschoft and van
Schaik, 2000). In apes, several studies have described
considerable inter-individual variation of personality
(Buirski et al., 1978; Buirski and Plutchik, 1991; Clarke
and Boinski, 1995). King et al. (2005) have recently
demonstrated that in chimpanzees six personality factors
can reliably be identified, of which five strikingly resemble
the ‘‘Big Five’’ personality factors found in humans, i.e.
neuroticism, extraversion, openness, agreeableness, and
conscientiousness (McCrae and Costa, 1987), with a sixth
factor specific for chimpanzees being ‘‘dominance’’ (King
et al., 2005). Interestingly, in an earlier study ‘‘conscientiousness’’ was found to be unique to humans and
chimpanzees (Gosling, 2001), and in both chimpanzees
and humans extraversion, emotionality and openess seem
to decrease with age (King et al., 2005). In addition,
Lilienfeld et al. (1999) developed a Chimpanzee Psychopathy Measure. This measure not only exhibited moderate
inter-rater reliability, it also correlated with ‘‘extraversion’’
and ‘‘agreeableness’’ of the five factor model in humans,
suggesting that psychopathy can also be determined as a
personality variant in chimpanzees (Gosling et al., 2003).
Cloninger (1986, cited in Capitanio, 2004) has suggested
hereditary components in exploratory behavior, harm
avoidance and reward dependence, which are related to
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the level of activity of biogenic amines such as dopamine,
serotonin and noradrenalin in the brain. In terms of
psychopathology in humans it has recently been demonstrated that allelic variation in the activity of monoamine
oxidase (MAO-A), an enzyme that is critically involved in
the metabolism of dopamine, serotonin and norepinephrine, may contribute to the development of personality
disorders and panic disorder (Deckert et al., 1999; Jacob et
al., 2005). To the best of our knowledge, such allelic
variations have not been studied in apes. However, as
summarized above, there is fairly good empirical evidence
for homologous personality traits in apes and humans
inherited from a common ancestor as fitness-related
characteristics of these species (Bouchard and Loehlin,
2001), and naturally occurring variation may include
extremes of personalities at the borderline to personality
‘‘disorders’’ (see Section 5).
3. Description of psychopathology in non-human great apes
In contrast to the assessment of human psychopathology
the absence of verbal reports of subjective distress
constrains the examination of psychopathology in apes to
a purely descriptive level. This task foremost requires a
thorough familiarity with the scope of behavioral elements
and tactics seen in normal apes, as well as information on
activity budgets (resting, foraging, social and reproductive
behavior) in the wild (Kortlandt, 1961; Pruetz and
McGrew, 2001). This information is only gradually
accumulating, with most being known about common
chimpanzees. At the same time, exploration of psychological processes like attention and emotional self-management in apes has only barely begun (Tomasello and Call,
1997; Preuschoft and de Waal, 2002).
Most reports of pathological behavior in apes come from
captivity, and of these, relatively gross behavioral abnormalities such as stereotypes figure most prominently.
Inappropriate living conditions—broadly defined—lead
to an inability to copulate, incompetent maternal behavior,
inappropriate emotional reactions—mainly fear and aggression—to companions, lack of species-typical communicatory signals, and generalized learned helplessness
(Brent, 2001; Seligman, 1991; Rogers and Davenport,
1969; Brent et al., 1997). Walsh et al. (1982) have listed
over 20 behavioral patterns in captive chimpanzees deemed
(statistically) abnormal for their unusual frequency,
severity or gross anomaly. The list includes bizarre
postures, hand clapping, coprophagy, eye poking, spreading of feces, patting of own genitals, hair pulling, head
banging, head shaking, head wiping, flipping of the lower
lip, ‘‘raspberry’’ vocalization (lip pursing and spitting air),
rocking, self-clasping, self-mutilation, self-slapping, sticking out the tongue, sucking of objects, such as own body,
skin, tongue or penis, urine drinking, wetting of the head
with water, and regurgitation and reingestion of food (e.g.,
Baker, 1996; Gould and Bres, 1986; Morgan et al., 1993).
Many of these patterns of behavior have never been seen in
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wild populations, and none is habitual or customary for
any group in nature. Although most such behavioral
irregularities have been observed in apes under conditions
of captivity, it would be an oversimplification to equate
‘‘wild’’ with ‘‘normality’’ and ‘‘captive’’ with ‘‘pathology’’
(for a more detailed discussion, see sections below).
However, one subgroup of stereotypic behavioral
patterns strikes an observer as particularly bizarre and
disquieting. These are fixed and apparently ritualized
movements in which body parts seem to acquire a life of
their own (pers. obs. SP), as briefly illustrated here by two
single case reports. For instance, an individual’s hands may
start to move about the head like objects floating on the
air, while the chimpanzee watches them and even tries to
fend them off. In another case a chimpanzee’s left foot
occasionally acquires the status of an interactive partner.
This can happen in the middle of a play bout with a human
through cage mesh: exuberant laughter is suddenly directed
at the foot, which is caught, gnawed and then wrestled free.
In other situations the same individual may attack the foot
and furiously bite it, creating bleeding wounds. The
chimpanzee then screams with pain and rage, hits and
shakes the foot apparently as if to fend off and to retaliate
against an aggressor. This may happen so regularly that the
skin develops a callus at the frequently injured site. Autoaggression also occurs in humans and in primates other
than chimpanzees. Human patients report a compulsion
for self-injurious behavior that is accompanied by a
desperate longing to ‘‘feel something’’ (Levenkron, 1998).
This concurs with the idea that auto-aggression is often
found in primates suffering from severe forms of social
isolation.
4. Assessment and diagnosis of psychopathology in great
apes
Clinical psychology and psychiatry of humans is
presently a largely a theoretical enterprise. Symptoms are
assessed and categorized according to a widely accepted
diagnostic system, the DSM (American Psychiatric Association, 1994). In general, there are four ways in which
symptoms of human psychopathologies are recognized.
One is by the verbal self-reports of suffering by patients,
the second is by assessing an objective deviance in form,
context or frequency of a behavior from the statistical
norm, the third refers to the functional outcome, and the
fourth is by comparison to an ethical standard of the
psychological mechanism in question. The latter presupposes social norms and rules, which are generally accepted
by all members of the social group, where individuals are
forced by the community to obey to these standards (Fehr
and Fischbacher, 2004), and the individual’s capacity to
feel shame or embarrassment at the failure to adhere to
such a standard, which has been invoked to play a role in
depression (O’Connor et al., 2002). The existence of shame
and guilt has not been documented for any other ape but
H. sapiens. Therefore, at this point we are reluctant to
include any deviation from ethical standards as a criterion
for diagnosing psychopathology in non-human apes.
To illustrate this dilemma, we would like to briefly draw
attention to an instance reflecting a behavioral deviation
from the statistical norm, but perhaps no ‘‘pathology’’ in a
functional (evolutionary) perspective. Passion and Pom,
two female chimpanzees at Gombe, killed and cannibalized
at least six chimpanzee infants (Goodall, 1986) with such
persistence that a human psychiatrist is tempted to render
this as antisocial personality ‘‘disorder’’. Biographically,
Passion, Pom’s mother, was said to be neglectful and
emotionally distant to her daughter. Passion’s maternal
behavior and what has been outlined above concerning the
heritability of personality traits might explain the apparent
transmission of the aggressive behavior to the next
generation. However, we are cautious to acknowledge this
as evidence for a ‘‘disorder’’. Whether infanticide is a
behavioral abnormality or an adaptive reproductive
strategy has been a matter of controversy (Hrdy et al.,
1994). Only recently has the full pattern become evident, so
that it is now known that in many taxa, males, after
immigration or rising in rank, kill infants they have not
sired. Whenever the period of lactation exceeds that of
gestation infanticide ensures that females become receptive
earlier, so that a new sire can impregnate them and thus
increase his own reproductive output—at the female’s and
his predecessor’s cost (van Schaik and Janson, 2000).
Infanticidal behavior in females is uncommon, but can also
be understood as being adaptive by reducing competition
for resources with one’s own offspring (Pusey et al., 1997).
Therefore it seems problematic to assert a psychopathological condition, even though Passion and Pom would
perhaps score highly on the Chimpanzee Psychopathy
Measure (Lilienfeld et al., 1999).
Interestingly, evolutionary psychologists have recently
highlighted this problem in humans as well. From an
evolutionary perspective, human ‘‘psychopathy’’ (Hare,
1996), which embraces many aspects of antisocial behavior
and antisocial personality, can be interpreted as adaptive
‘‘cheater morph’’ variant or alternative strategy, which
could be adaptive by exploiting resources without reciprocating and be maintained at low prevalence rates in
populations through frequency-dependent selection (Mealey, 1995; Troisi, 2005). In the case of psychopathy the
suffering is usually not on the side of the psychopathic
individual but rather on the side of those being cheated, i.e.
the social partner(s). Although the concept of ‘‘psychopathy’’, in part, overlaps with antisocial personality
disorder, the term ‘‘disorder’’ clearly relates to a normative
aspect of human psychopathology, both in the sense of
ethical standards and statistical norm. For example, the use
of the phrase ‘‘normative’’ here implies some sort of
reaction of the group or community to the behavior in
question, meaning that others acknowledge the behavior as
undesirable or ‘‘deviant’’. In the case of Passion and Pom,
however, there was no observable sign of punishment or
expulsion from the community in the chimpanzees at
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Gombe, although such regulatory responses have been
reported in captivity (de Waal, 1996). This may suggest
that collective punishment is probably much more common
in humans (Fehr and Fischbacher, 2004), perhaps due to
complex adaptations supporting reciprocal altruism, including the evolution of a sophisticated ‘‘theory of mind’’
(Trivers, 1971; Brüne and Brüne-Cohrs, 2006). In any
event, the example of psychopathic behavior in apes and
humans may illustrate the difficulties in interpreting
behavior as ‘‘deviant’’ or ‘‘normal’’ if ethical aspects come
into play. Although this problem may also be evident in
evolutionary conceptualizations of disorders (both mental
and somatic) as ‘‘harmful’’ and ‘‘dysfunctional’’ (Wakefield, 1999), we would like to emphasize this point of view
worth to be considered for cross-species comparison, in
spite of the fact that the conceptualization of psychopathology as ‘‘harmful dysfunction’’ (HD) may have its
limitations by simplifying and partly obfuscating the
problem of continua between normalcy and pathology,
and how exactly pathology can be quantified. In sum,
assessing psychopathology in great apes is necessarily
constrained by the absence of verbal self-reports. As the
example above illustrates, applying moral standards is also
inadequate, such that the remaining two ways, i.e. deviance
in form, content or frequency, and functional outcome are
the only reliable measures.
Even in humans the assessment of psychopathologies
through verbal report is fraught with problems, as a person
may not be able to adequately introspect internal states, or
may have reasons to give fallacious reports. Here, nonverbal behavior (a neglected area of research in contemporary psychiatry) can play an important role. Patterning
of movement such as direction, speed, rhythm, acceleration
or deceleration is understood by virtually all animals from
early on. They are, for instance, used in the habituation–
dishabituation paradigm with which cognition in human
neonates is investigated. We may therefore regard this as
an ‘‘inter-species body language’’. Moreover, many facial
displays and acoustic utterances are homologous between
human and non-human primates (e.g., Marler, 1976;
Preuschoft, 2000; Provine, 1996). Even the basic drives
and emotions seem homologous: hunger, thirst, lust, care,
playfulness, fear, rage, sadness, attachment, and affection.
Their neurological underpinnings are an ancient vertebrate
heritage (e.g., Panksepp, 1998). The physiology of emotional responses, including stress, is basically identical in all
mammals, and the same applies to outward symptoms of
these processes such as breathing rates, immobility, and
perhaps blushing (e.g., Sapolsky, 1998). Not only are these
responses immediate and hard to fake; they are also almost
inadvertently monitored by social interaction partners
(Preuschoft and Preuschoft, 1994). Thus, there is a wealth
of information on which an observer can draw in assessing
the psychological state of a non-speaking ape, including
indirect measures of distress such as stress hormone levels
in blood samples or feces (Bloomsmith and Else, 2005;
Reimers et al., submitted), and facial expressions of
1251
emotion. Therefore, O’Connor et al. (2001) have plausibly
outlined that the concepts of major depression, anxiety
disorders, antisocial, borderline and schizoid personality
‘‘disorders’’ may—to some degree—well be applied to apes,
because scores of ‘‘subjective’’ well-being obtained from
ethological observation suggest homologous pathological
phenotypes in humans and other apes.
The comparative view of ethologists is not only
descriptive, but nowadays also firmly rooted in a theoretical framework of evolutionary, and thus functional
thinking at both the proximate level (mechanism and
ontogeny) and the ultimate level (adaptive value and
phylogeny; Tinbergen, 1963). This enables an ethologically
inspired psychopathology to incorporate not only assessment methods for symptoms, but also potentially valuable
hypotheses about the etiology of disorders, and translate
these into testable therapeutic interventions based on the
functional thinking of modern sociobiology. In a similar
vein, Troisi (2003) has proposed to take into account as
criterion for psychopathology in human and non-human
primates the maladaptive consequences of behaviors that
significantly interfere with an individual’s pursuit of
important biosocial goals such as mating, coalition
formation, social bonding, etc.
Current research into social evolution is based on the
investigation of wild populations, as well as on the
physiological, social, behavioral, and psychological heritage of primates. Knowledge about this heritage is highly
relevant for the understanding of psychopathologies in
apes, because it informs about innate needs, that is,
adaptive dispositions and propensities. Making use of
these innate propensities in therapy is likely to improve the
efficacy of practices.
5. Etiological aspects
Perhaps the foremost characteristic of primates is their
sociality. Early in life this begins with the period of
neonatal dependence not only for suckling, as in all
mammals, but also for being carried around at all times,
supporting infantile thermoregulation and providing protection from various hazards. Later on, the relationship
with the mother remains a special one, but siblings and
other relatives gain in importance, as do age-mates and
other group members. How crucial these early experiences
of socialization are becomes evident when infants are
separated from their mothers. Early rearing circumstances
and opportunity for attachment have great impact on the
development of stereotypic movements (Maki et al., 1993).
Early maternal separation leads to a variety of grave
psychological disturbances, including stereotypies, autoaggression, and incompetent social and reproductive
behavior (Davenport and Menzel, 1963; Rogers and
Davenport, 1969; Fritz et al., 1992a–c). These pathological
behaviors may persist into adulthood despite attempts at
amelioration, such as enrichment of enclosures or resocialization. However, the development of body-rocking
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in young chimpanzees may be prevented in laboratory
conditions if infants are left with their mothers and in their
social group. Likewise, the presence of peers may prevent
later onset of body-rocking in juveniles (Spijkerman et al.,
1994).
In addition to the problem of early separation, captive
housing conditions later in life also influence the development of pathological behavioral patterns. Being housed
alone increases stereotyped behavior and decreases exploratory behavior in chimpanzees (Brent et al., 1989).
Singly housed subjects and chimpanzees housed in groups
of two to three individuals exhibit a broad range of
behavioral abnormalities, including intense and unprovoked aggression (Baker, 1996). Besides social deprivation,
sensory and motor deprivation in barren and uncontrollable environments constitutes a major stressor, leading to
physiological and behavioral disorders (van Holst, 1994;
Berkson et al., 1963). Along with remarkable intelligence,
primates are endowed with acquisitive brains and a strong
need for stimulation and investigation, so that boredom
creates bizarre and rigid behavior as well as apathy.
While social incompetence, impaired impulse control,
and stereotypies are generally considered pathological
(maladaptive), at times stereotypies in apes may serve as
a coping strategy and self-stimulation, behaviors that may
also be observed in mentally retarded or sensorily deprived
humans (Rojahn et al., 1997). Moreover, stereotypies are
not inevitably detrimental to social functioning or successful reproduction in captive apes. For example, Fritz et al.
(1992a–c) found no differences in body-rocking between
copulating and non-copulating captive chimpanzees. Many
of Walsh et al.’s abnormal patterns appear well-suited in
unusual living conditions in which normal ways to fulfill
normal needs are barred. Such basic needs are the desire
for stimulation, which is pronounced in the highly
intelligent and investigative apes, and the desire for
security, which apes usually find in their ability to control
situations, or with bonded companions (Berlyne, 1960;
Mason, 1965). Patting, clasping, being rocked when carried
by a mother, sexual stimulation, and grooming are all
normal social behaviors, and turn into abnormalities only
when they are shown excessively or become self-directed as
a way to compensate for the absence of social partners who
would normally fulfill such needs. Pazol and Bloomsmith
(1993) reported a decline of body-rocking in infant
chimpanzees over time that was apparently independent
of the amount of human contact, but is understandable if
the need to be rocked diminishes with the growing
locomotor independence of the infant irrespective of its
social circumstances. Chimpanzees use a variety of
behavioral patterns to catch a companion’s attention:
raspberry sounds, spitting, displaying, clapping hands, etc.
As ‘‘vacuum’’ behaviors they come to seem bizarre,
because the target for such behavior is not available. Other
bizarre behavior may develop because if an individual
undergoes some kind of inadvertent conditioning leading
to ‘‘superstitious’’ behavior (Skinner, 1948), although the
positive reinforcer usually remains unknown. A variety of
stereotypies may develop to counter boredom by providing
some kind of controllable stimulation in an otherwise
barren and uncontrollable environment (e.g. Walsh et al.,
1982). Such stereotypies appear to become rewarding in
themselves, and continue to be executed even after the
situation in which they arose has lessened or disappeared.
Yet this inherently rewarding quality may be what keeps
the afflicted ape in some kind of emotional balance.
There is also evidence that pathological behavior can be
culturally transmitted from mother to offspring, or
between group members (Kalcher pers. obs.). Nash et al.
(1999) described coprophagy and self-plucking of hair to
occur more often in mother-reared captive chimpanzees
than in hand-reared individuals, but the conditions in
which the mothers of the affected individuals had grown up
were not reported.
There is no doubt that adverse early rearing conditions
and social and sensory deprivation constitute risk factors
for developing psychopathology. However, similar to
etiological factors of psychopathology in humans (overview in Rutter, 2005) and their measurable consequences
for the ability to cope with stress (Van Voorhees and
Scarpa, 2004), the diversity of possible manifestations in
apes suggests that these risk factors are non-specific. In
other words, it will probably turn out to be extremely
difficult to link-up a certain risk factor, say, separation
from the mother at 6 months of age, with a distinct type of
psychopathology, say, automutilation.
The question arises, would apes also show psychopathologies had humans not distorted their lives? Insights may
come from psychopathology in wild populations. In spite
of the paucity of systematic evaluation, field studies (e.g.,
Goodall, 1986) suggest the existence of genuine psychopathological conditions in wild chimpanzees, particularly
depression in orphaned chimpanzees. The affected individuals cease eating, lose weight, show malaise, and decline
to play or explore. Sometimes the severity of the condition
leads to death from starvation or infectious diseases, even
in individuals who are old enough to forage for themselves.
The loss of the primary attachment figure (the mother)
produces an emotional reaction in young chimpanzees that
greatly damages their psychological well-being. In humans,
this condition would be diagnosed as clinical depression
(Bowlby, 1980).
Several studies have buttressed this assumption showing
that in adult individual monkeys separated from their
mothers at an early developmental age, persistent changes
at the neurochemical level made them vulnerable to
depression and anxiety disorders, and these changes might
be similar in all primate species (Graham et al., 1999;
Lyons et al., 2000; Troisi, 2003). Troisi (2003) also
mentions traumatic change in social status as a likely
candidate to induce depression in apes living under natural
conditions.
Goodall (1986) described an adult female chimpanzee,
Olly, who was very shy and often separate from the group.
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Her social withdrawal emerged after experiencing several
adverse events during childhood and adolescence, which
could have impaired her social competence. From a
psychopathological perspective, Olly’s behavior is reminiscent of avoidant or anxious personality ‘‘disorder’’ in
humans, but this assumption is clearly tentative and could
only be supported by measures of pathophysiological
correlates such as stress hormones or the activity of
biogenic amines in the brain.
Another potential source of behavioral abnormalities in
captive apes is pathological ageing processes (perhaps this
issue cannot be examined in wild populations because
selection may readily eliminate aged individuals). Aged
chimpanzees show neuropathological changes in brain
tissue similar to those alterations found in Alzheimer’s
disease in humans (Finch and Sapolsky, 1999; Finch and
Stanford, 2004). However, behavioral evidence of cognitive
decline has not been found in chimpanzees and gorillas
aged over 35 years (Bloomsmith et al., 2001). Careful
observation of old to very old individuals in captivity could
be an important avenue for the understanding of normal
and pathological ageing in apes and humans.
6. Treatment of psychopathology in apes
6.1. General remarks
Treatment of psychopathological conditions in great
apes aims at two major goals: (1) to carefully reintroduce
behaviorally abnormal individuals to novel or existing
social groups, based on the assumption that social contact
and interaction constitutes a basic need for individuals of
species adapted to a highly developed social environment;
(2) to reduce ‘‘suffering’’ of the affected individuals, based
on phenomenological similarities with human psychopathological conditions (from which we know that
subjective distress is a major component of most disorders),
behavioral observation of social withdrawal, and indirect
measures of distress via the hypothalamic–pituitary–axis
(HPA). However, deviations from the statistical norm in
captive compared with wild populations do not necessarily
warrant treatment. For example, Bard and co-workers
(Bard, 1994; Bard et al., 2005) have found that mutual gaze
between chimpanzee mothers and their infants is low in the
wild and in captivity, whereas cradling contact is high.
Surprisingly, the opposite has been found in a different
group of captive chimpanzees without any noticeable
thriving problems in the young, suggesting some interchangeability of tactile and visual contact. Similar inverse
relationships between the amount of eye contact and
cradling seem to exist in human societies with remarkable
cross-cultural differences (Keller et al., 2004). Thus, rather
than frequent mutual gaze being a sign of behavioral
aberrance in captive apes, this ‘‘cultural’’ distinctiveness
suggests behavioral flexibility of apes and humans and the
ability to adopt group-specific patterns of behavior. Such
behavioral peculiarities are neither harmful, dysfunctional,
1253
nor do they thwart important biosocial goals, such that
they cannot be a primary target for treatment.
6.2. Re-socializing
When formerly isolated or socially deprived primates are
transferred to social housing conditions with conspecifics
this process is called ‘‘re-socialization’’, even when the
individuals have been isolated from birth. The term thus
pays tribute to the insight that primates have an inherent
need and propensity for social life. In general, resocialization programs are extremely time-consuming and
demanding in terms of human resources. In profoundly
disturbed individuals it may take years to make them
tolerate the presence and proximity of con-specifics. Resocialization can be risky, because escalated aggression
may occur in limited space between socially incompetent
individuals unable to make proper risk assessments.
Introduction of formerly isolated primates into existing
groups of socially competent conspecifics is particularly
dangerous. Formation of groups from inexperienced
individuals bears other risks, but few institutions have
experienced, friendly and tolerant ‘‘therapeutic’’ primates
at hand that can ease the way into sociality for former
isolates (Fritz and Menkhus Howell, 1993; Fritz and
Howell, 2001). Risk factors for the introduction to new
social partners are again separation from the mother (the
earlier the separation, the worse the effect) and nursery
rearing without individually responsive care, which produces more debilitation than with responsive humans or
with peers. Longer periods of solitary housing lead to more
serious debilitation, as do prior experience with only few
social partners, and impoverished housing conditions.
Primates with high levels of stereotypic behaviors are hard
to re-socialize. Chimpanzees who tend to respond primarily
fearfully or aggressively to others (people or conspecifics)
have more difficulties adjusting to new social partners than
those who are friendly or calm (Fritz and Fritz, 1979; Fritz,
1989; Brent, 2001). In many cases several risk factors are
combined in the life history of one problematic individual,
while in others the record of risk factors may be incomplete
or simply unknown.
However, social housing free of agonistic conflicts is not
the same as successful re-socialization, because conflicts
simply may be evaded by apathy, spatial avoidance of
group members or complete lack of social interaction.
On the other hand, long periods of the latter may be
useful for latent learning, and more active coping along
with more competent social behavior may arise with
considerable delay (Kalcher, unpublished). It is important
to challenge social novices on a level at which they have a
chance to succeed, e.g., solitary rather than group feeding
may be preferable at the outset (Peterhans, 2006).
Although active teaching has not been seen in any other
animal than humans, some mammals create opportunities
for immatures to learn certain vitally important skills by
observation and experimentation (Caro, 1992; Boesch,
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1991). Human therapists mimic this by giving tailored
learning opportunities to primate patients. For example, a
timid individual may be exposed to a kind partner, or an
overly aggressive individual can be reigned in by confrontation with a dominating partner (Preuschoft, unpubl.
data). Careful observation of how primate patients respond
to such dosed challenges can then direct subsequent
interventions.
In general, re-socialization programs have succeeded for
over 90% of previously disturbed individual chimpanzees
(Fritz, 1989), if active social interaction with all members
of the group is the target variable. However, successful
mating or mothering is unattainable in most cases (Fritz et
al., 1992–c), suggesting that sexuality and care-giving is too
complex a task to accomplish for profoundly disturbed
individuals with perhaps irreversible damage to these
behavioral systems.
6.3. Behavioral treatment
Several studies have consistently shown that the enrichment of enclosures reduces stereotypic movements by
preventing boredom, stimulation of foraging techniques
and increasing the level of activity in several ape species
(McGrew, 1981). Movable or stationary objects and the
number of companions may be more important for the
animals’ stimulation than the size or construction of the
enclosure (Wilson, 1982; Perkins, 1992). Longitudinal
studies have revealed that even unnatural enrichment
objects such as television, balls or mirrors may still be
used by singly housed or pair-caged chimpanzees, even
after years of exposure (Brent and Stone, 1996). Fritz et al.
(1992a–c) have demonstrated that coprophagy may be
reduced by giving chimpanzees the opportunity to make
‘‘wadges’’. Wadge-making is a naturally occurring behavior in chimpanzees in which the non-digestible part of
plants, e.g. husks or leaves, are not swallowed but are
compressed in the mouth between the palate and the
tongue and then spat out (Fritz et al., 1992a–c).
Only quite recently have researchers started to apply
systematic behavior therapy using negative and positive
reinforcement to reduce pathological behaviors such as
regurgitation and reingestion in individual animals (Morgan et al., 1993). Such a treatment protocol has been found
to be effective for captive chimpanzees in combination with
dietary change. However, the abnormal behavior recurred
after several months, which underpins the necessity for
careful follow-up and recording of changes in behavior
over extended periods of time (Fritz and Menkhus Howell,
1992).
Operant conditioning is also used as a means in care and
management of wild animals. With apes mainly positive
and negative reinforcement is used to train cooperative
behaviors with caretakers. Cooperative behavior ranges
from voluntary movement between areas of the holding
facility or separation from group members, to allowing
collection of fecal or urine samples, and even to compliance
with receiving injections or donating blood. Thus, stress
during daily routines and veterinary treatment can be
vastly reduced. In addition, apes apparently appreciate the
attention and contacts with a caretaker but also the
opportunity to learn something and to achieve competence
(Laule and Whittaker, 2001; pers. obs. SP).
6.4. Pharmacological treatment
In contrast to experimental administration of psychotropic substances in monkeys to reduce stereotyped
behavior (e.g., Hugo et al., 2003), little is known of
treating apes for psychopathological (Howell et al., 1997)
or neurological conditions (Fish et al., 2004). This is
remarkable, because medical treatment of physical conditions (infections, diseases) is a standard procedure with
captive apes. Early case reports on side-effects of antipsychotic drugs in apes suggested action on the extrapyramidal motor system and behavior similar to humans
(Byrd, 1974). Only recently have modern psychopharmacological drugs been prescribed to apes for ameliorating
well-defined behavioral abnormalities, because it became
obvious that environmental enrichment alone or behavioral therapy (conditioning) might not suffice to treat
behavioral abnormalities in apes (Fritz, 1989). For
example, Howell et al. (1997) treated an adult male
chimpanzee for chronic regurgitation using sertraline, a
selective serotonin reuptake inhibitor (SSRI) in conjunction with behavioral therapy. The chimpanzee’s behavior
was observed over a 39-week period. Target behaviors
preceding regurgitation were aggression, fear, excitement
and solitary inactivity; social behavior unrelated to the
onset of regurgitation was also recorded. Overall, sertraline
significantly decreased the frequency of regurgitation in
both low-dose (up to 100 mg/day) and high-dose (up to
200 mg/day) conditions. This effect outlasted the discontinuation of psychopharmacological treatment. Observation
pertaining to the adjunct target behaviors revealed a
significant decrease of solitary inactivity, but no effect on
aggression, fear or excitement. Most interestingly, prosocial behavior increased under low-dose but not high-dose
drug treatment. Vomiting was noticed as a possible sideeffect of SSRI treatment, consistent with side-effects of
SSRI observed in humans.
Similarly, Prosen and Bell (2001) reported the successful
treatment of a male bonobo for self-mutilation, vomiting,
aggression, motor restlessness and stereotypies. Initially,
the subject was unable to eat in a social group, to relate to
adult females or males, to engage appropriately in
reconciliation behavior, or to play. At times he showed
signs of extreme anxiety. His early rearing conditions were
abnormal with close contact to his father in a small cage
and early separation from his mother, and the overall
clinical impression was that of severe retardation in terms
of social and sexual competence. The bonobo’s condition
started to improve after 2 weeks of SSRI treatment and
behavioral therapy, and the stereotypies declined substan-
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tially. Also, the bonobo’s ability to initiate contact with
adult females improved steadily, and after three years of
treatment he could tolerate living with up to seven animals
(Prosen and Bell, 2001).
These case reports, although encouraging of how
behavioral and psychopharmacological treatment can
ideally be combined to the benefit of apes suffering from
psychopathological conditions, need to be replicated in
larger samples and should not only focus on SSRI and the
serotonin system, but also include other agents such as the
modern antipsychotic substances, which lack severe extrapyramidal side-effects.
7. Discussion
In this article, we set out to argue that a substantial
number of great apes in captivity show behavioral
abnormalities and measurable signs of distress, which
together are suggestive of homologies with human psychopathological conditions. We further reasoned that the
question of how to ameliorate these conditions is underresearched, and that an interdisciplinary approach between
primatology and psychiatry to address this question is
direly needed. Some readers may find the use of the term
‘‘psychopathology’’ a little too farfetched or provocative;
we believe, however, that the similarities between human
and ape mentality are so great that it is justified to assume
the experience of subjective distress in affected apes,
although admittedly the subjective aspects of psychopathology are at best indirectly observable.
Although there is perhaps no silver bullet to address
psychopathology in great apes, a thorough comparison of
symptoms found in apes with symptoms found in humans
(bottom-up) may help generate testable hypotheses regarding the underlying disturbed psychological mechanisms
involved. In a second step it may be worth identifying the
biosocial goals that are thwarted by the pathological
process, and to explore the nature of the distorted
mechanism in evolutionary perspective (Brüne, 2002).
However, the comparability of the psychology of different
species has of course limitations, simply because great apes
and humans differ in their behavioral ecology reflected in
‘‘software’’ differences, i.e. cognition, emotion, and behavior, as well as in ‘‘hardware’’ differences, i.e. brain size
and organization (e.g., Rilling and Insel, 1999: Allman et
al., 2001; Semendeferi et al., 2002). Such differences have to
be taken into account for any kind of comparative
approach.
In spite of this cautionary statement, there is no doubt
that similarities between great apes and humans exist in
terms of their vulnerability to psychosocial stress and the
development of persistent behavioral abnormalities, i.e.
psychopathology. As far as is known to date, the main
sources of psychopathology in captive apes, where the host
of data stems from chimpanzees, are early separation of
infants from their mothers, sensory-motor deprivation or
social isolation over prolonged periods of time (Maki et al.,
1255
1993; Fritz et al., 1992a–c), although these risk factors are
probably non-specific. Behavioral abnormalities include
stereotypic movements, withdrawal, extreme anxiety or
unprovoked aggression. Many of these behaviors are
problematic in that they impede socialization of the
affected individuals. Re-socialization procedures may
increase locomotion, play and grooming between conspecifics, reduce aggressive and quasi-aggressive behavior
directed at humans as well as reduce some behavioral
stereotypy. Yet, many apes continue to show a lack of
initiative and investigatory behavior, and their social
competence increases only slowly, while reproductive
incompetence (inability to copulate or to nurse an infant)
may persist lifelong, despite a capacity to engage in erotic
social interactions (Clarke et al., 1982; Brent et al., 1991;
Baker, 1997). Moreover, abnormal behaviors such as
stereotypies and maternal incompetence have a tendency
to be socially transmitted to other, formerly unafflicted
group members (Maestripieri, van Hooff pers. comm.;
pers. obs. SP).
In addition, re-socialization, integration of strangers into
existing groups, a general lack of space plus constant
confinement with the entire group present unnatural
challenges to the capacities for conflict management and
stress coping of captive apes. Severe and potentially fatal
aggression is in the normal repertoire of great apes
(Wrangham, 1999), and can become a major problem
behavior in captivity (de Waal, 1986; Alford et al., 1995;
Baker, 1996). High-density indoor housing, e.g.,
during winter time, may crowd captive chimpanzees and
increase tension and anxiety levels as indicated by elevated
rates of rough scratching and yawning (Aureli and de
Waal, 1997).
Many psychopathological conditions may persist despite
careful re-socialization, enrichment of enclosures and
behavior therapy (conditioning). The most severe cases
that have been resistant or only partially responding to
behavior therapy may justify additional psychopharmacological treatment, in the first place to improve the
individual’s responsiveness to behavioral therapy. In
contrast to psychopharmacological studies in monkeys
(e.g., Hugo et al., 2003), however, there is a paucity of
experience in treating psychopathological conditions in
apes using psychotropic drugs such as antidepressants
(Howell et al., 1997; Prosen and Bell, 2001). There is
limited evidence whether or not administration of SSRI
will reduce stereotypies in apes and increase prosociality.
Whether excessive aggression in apes can be ameliorated
using modern antipsychotic drugs is totally unknown.
Finally, most behavioral and drug treatments of psychopathology in apes have been applied to chimpanzees, and
little is known of how to deal with pathologies in gorillas
and orangutans. Moreover, in light of the vast differences
between monkeys and humans with respect to the level of
gene expression in the brain (Caceres et al., 2003), and
similar though minor differences between great apes and
humans (Carroll, 2003; Khaitovich et al., 2005), it is
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possible that apes and humans respond differently to
psychopharmacological treatment, which can be important
in terms of finding the adequate dose.
Systematic assessment of psychopathology in captive
great apes has largely been restricted to the recording of
gross abnormalities such as stereotypies, level of activity
and explorative behavior. In contrast, research into ape
psychopathology has largely disregarded non-verbal cues
such as facial expressions, vocalizations and gestures.
Taking these aspects of expressive behavior into account,
however, is essential, particularly when attempting to find
parallels in psychopathology among apes and human on
common etiological grounds. For example, symptoms
associated with pathological anxiety, depression, and
posttraumatic stress disorder may become manifest in both
apes and humans due to traumatizing experiences during
infancy and childhood, leading to persistent changes of the
hypothalamic-pituitary stress system (Bennett et al., 2002;
Coplan et al., 1996; Graham et al., 1999; Kraemer, 1997;
Lyons et al., 2000; van Voorhees and Scarpa, 2004). Such
stressors are also present in the wild, where life is less
secure, but coping mechanisms may be more effective in the
environment of evolutionary adaptation. Yet, the prevalence of psychopathological conditions in wild populations
is completely unknown.
An interdisciplinary approach of psychiatry and primatology has been preliminarily attempted in the past
(Berkson et al., 1963; Berkson et al., 1995), and these
research areas have mutually fertilized research into ape
psychopathology and human mental retardation, where the
possibility of obtaining verbal report may be limited in
severe cases.
Understanding psychopathology in the great apes
may benefit not only the afflicted individuals, but
provide insight into the nature and nurture of human
psychopathology, particularly the impact of adverse
early rearing conditions, poor attachment, social
deprivation, and perhaps in some cases regarding the
response to combined behavioral and psychopharmacological treatment. In light of scientific discussions of whether
to include chimpanzees in the genus Homo (Wildman et al.,
2003), we believe it is a logical extension of the debate
about humane and ethical treatment of our closest relatives
that we owe them proper psychiatric treatment in order to
reduce their anthropogenic distress (Brüne et al., 2004).
Research into the fascinating across-species approach to
psychopathology has barely begun; we believe that this
kind of research ought to be adapted to the needs of the
great ape patients and carried out as non-invasively as
possible.
Acknowledgments
We are grateful to Professor Horacio Fabrega and
unknown reviewers for their helpful comments to revise
our manuscript.
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