Sex differences - not as universal as previously
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In capuchin monkeys, as in many species, males are larger than
females. Are males also more bold, more explorative and less anxious
than females? Far from it. A new study revealed that capuchin
monkeys show hardly any sex differences in their individual
behaviours. These findings shed new light on an age-old question.
(© Photos: ISTC-CNR, Rome & Jana Uher, Primate
Personality Net & Free University Berlin)
Capuchins are known for their remarkable cognitive abilities. Now
an international research team led by Jana Uher (Primate Personality
Net & Free University Berlin), Elsa Addessi and Elisabetta
CNR Laboratory of Cognitive Primatology, Rome, Italy)
comprehensively explored individual differences in the capuchins'
behaviours for the first time.
In the Centro Primati in Rome, 26 adult tufted capuchin monkeys
were studied using 15 different behavioural tests in which, for
example, the monkey individuals could explore novel objects,
manipulate various apparati, or interact with humans or
conspecifics. In addition, the monkeys were observed prior to their
main feeding and during their daily activities in their groups.
The behavioural tests were captured on video and coded later
using a special coding software; the observations were recorded
using computerised methods. Overall, 146 behavioural variables were
obtained. This meticulous and comprehensive recording of the
monkeys' individual behaviours in various situations enabled
detailed and illuminative analyses.
(© Photos: Jana Uher, Primate Personality Net & Free University
Importantly, all behavioural tests and observations were conducted
repeatedly and in two nonoverlapping periods of 10 days each.
Overall, the behaviours of a single monkey were recorded for 31.2
hours. These comprehensive data sets allowed the researchers to
analyse whether individual differences occurred only by chance -
after all, like us, monkeys can have a good day or a bad one - or if
the monkeys, in fact, show stable individual differences; that is,
behavioural patterns that are specific to them as individuals. This
proof of stability over some time is essential because only
individual-specific patterns are referred to as "personality".
So far, "personality" differences have largely been studied via the
use of human's everyday language-the majority of research on human
individual differences is based on the person-descriptive words that
are catalogued in our lexica and, in particular, in "personality"
questionnaires. These methods can explore what people think about
themselves and how they describe other human individuals or
individuals of other species - but these methods cannot explore how
individuals actually behave.
The capuchin monkey study was therefore based on a new research
paradigm developed by Jana Uher to explore and categorise
"personality" differences independent of human's everyday language.
The paradigm also comprises novel methodologies and approaches that
are needed to systematically explore and categorise
individual-specific behaviours not only in humans, but in nonhuman
species as well. The study explains and demonstrates the application
of this novel paradigm and the behavioural research methodologies.
The results were intriguing. They showed that capuchin monkeys
indeed show stable individual differences across a broad range of
behaviours. The monkeys differed from one another not only in their
overall behavioural tendencies but also in the particular situations
in which they showed a particular behaviour in particularly
pronounced ways. There were some individuals who, of their own
initiative, greeted their human observers in their capuchin-specific
manners and tried to contact these humans, whereas other capuchin
individuals did so only if the human observers themselves tried to
establish contact with them.
Other capuchin monkeys, in turn, approached their human observers
only when they were given food. This situation- specificity of
individual behaviour is well documented in humans and also in great
apes. It constitutes an important component of the enormous
diversity in which individuality becomes apparent.
(© Photos: Jana Uher, Primate Personality Net & Free University
Stunning and unexpected was the finding that, although males are
larger and heavier than females, as is the case in many other
species, capuchin monkeys did not exhibit sex differences in their
behaviours except in aggressiveness and dominance. Instead, there
were pronounced individual differences in both sexes. Amongst both
the males and the females, there were individuals who closely
inspected a large bed sheet that was about 20 times larger than
these monkeys and hanging between two horizontal poles, one much
higher than the other.
The male Sandokan quickly started to explore the sheet in detail and
even used it as a slide many times. In contrast, Vispo, another male
of his group, tried all possible ways to avoid any contact with the
sheet; in fact, he started walking bipedally when he moved over the
poles! As is the case for humans, there were all possible
intermediate variations of behaviour between these two extremes in
both males and females. These new results question assumptions about
the universality of sex differences. They show that, in group-living
species led by a dominant male, typically male and typically female
tendencies in individual behaviours do not necessarily have to occur
despite pronounced sex differences in body size.
Sex differences in central "personality" characteristics play an
important role in many theories of human evolution, especially with
regard to living together and the partition of labour between men
and women in social communities. However, "personality" differences
in humans have been studied thus far almost exclusively with
assessments that have employed standardised questionnaires.
But our everyday language is shaped by sociocultural perspectives
that unintentionally influence our judgements of individual
behaviours. In everyday life, we judge the same behaviours
differently depending on whether they are displayed by a male or a
female. Therefore, questionnaire assessments are not suitable for
studying differences in individual behaviours between men and women.
Questionnaire methods are frequently applied because they are
efficient and easy to use, but they very likely constitute an
important source of error that has been greatly underestimated so
Moreover, it is well- known that cultural beliefs about typically
male and female behaviours-that is, sex and gender
stereotypes-influence and change individuals' behaviours in
socio-culturally desirable ways. Many studies have shown that
children learn to adopt the particular beliefs of their specific
sociocultural community and to behave accordingly only over the
course of their development.
It may be well possible that differences between males and females
are much less rooted in human biology than previously thought. The
new study on capuchin monkeys at least shows that pronounced sex
differences in body size need not in general go along with many
differences in males' and females' behaviours as often assumed so
Interestingly, there were hardly any age differences in the many
behaviours studied either; older capuchin monkeys behaved only a bit
less impulsively than younger ones. Although all the monkeys were
adults, their age range from 8 and 33 years is quite substantial.
The analyses also showed that older capuchin monkeys were not more
stable in their individual behaviours than younger ones.
Rather, there were pronounced individual differences. Amongst both
the younger and the older capuchins, some individuals showed very
stable individual behavioural tendencies, thus rendering their
behaviours more predictable for human observers than was possible
for the behaviours of individuals who behaved very differently from
day-to-day. Such differences in the degree of consistency in
individual behaviours are also well-known in humans and in the great
apes. They constitute a further component of the diversity in which
individuality can become apparent.
Finally, the researchers explored the impact of the capuchins' early
life experiences on their individual behaviour as adults. Monkeys
who had to be taken care of by humans in their first year of life
were less aggressive toward human observers, more distractible by
humans, and spent less time close to their conspecifics than
These results are remarkable because all monkeys were frequently
brought in contact with their conspecifics during their first years
of life and could already be successfully introduced into a group at
the age of one year. This means that all of the capuchin monkeys had
been living together with conspecifics continuously for at least 7
years, some even for 32 years. But still, their early life
experiences had a significant impact on their behaviours as adults.
These results show how long-lasting the effects of the hand rearing
of primate babies in zoological institutions can be on both the
individuals' social behaviours towards humans and towards their
The study is part of a research project funded by the
German Science Foundation DFG.
Contact person: Dr. Jana
Uher, J., Addessi, E., & Visalberghi, E. (2013).
Contextualised behavioural measurements of personality differences
obtained in behavioural tests and social observations in adult
capuchin monkeys (Cebus apella). Journal of Research in
Personality, 47, 427-444.
Uher, J. (2013). Personality psychology: Lexical approaches,
assessment methods, and trait concepts reveal only half of the
story. Why it is time for a paradigm shift. Integrative
Psychological and Behavioral Science, 47, 1-55.
Last update: 25.02.2014
Keywords: Cebus apella, sex differences, age differences, social
status differences, behavioural differences, personality, individual
differences, rating, individual behavior, individual-specific
behavior, assessment bias, judgment bias, sex stereotypes, gender
stereotypes, age stereotypes, behavioral test, behavioural
observation, primates, capuchin monkey, tufted capuchin.