"Personality" differences but no sex differences
in the individual behaviour of four monkey species from three
Press release in PDF-Version
In their appearance, the sexes differ from one another in many
animal species. Males are often bigger and physically stronger than
females. But sex differences in body morphology need not go along with sex
differences in behaviour as a recent study on monkeys showed. In each of
four species, stable individual behavioural differences-thus,
"personality" differences-occurred but sex differences were largely
absent. These findings shed new light on many evolutionary psychologists'
assumption that sex differences in human behaviour inevitably result from
the bodily differences between men and women and thus constitute an
evolutionary heritage of humans.
(© Photos: Dr Jana Uher, primate-personality.net. London
School of Economics & Free University Berlin)
"Men are from Mars and women from Venus"-our everyday life is full of
stories about sex and gender differences. It is also a hot topic in
scientific psychology. Over the last decades, psychologists tried to
explain behavioural differences between men and women not only with
biological differences between the sexes but also increasingly using
The central problem of evolutionary psychology, however, is the fact that
our human ancestors no longer exist. Scientists can rely only on
archaeological findings and the behaviour of modern humans to make
conclusions on our ancestors' behaviour. But these conclusions are often
very speculative-and ultimately cannot be tested.
In evolutionary psychology, the assumption is widespread that pronounced
behavioural sex differences have already occurred among our ancestors: The
physically stronger men went on hunting, whereas the physically weaker
women gathered fruits and roots and focused on child rearing and the
social community. For this reason-so the assumption-men are more bold and
aggressive, whereas women are more socially oriented and anxious.
Because fossil bones do not reveal much about the social behaviour of
their former carriers, scientists often seek to find evidence for their
evolutionary theories from research on modern humans. Most findings on
gender differences in "personality" derive from research using
questionnaires. Questionnaires can capture what humans think of themselves
and of others, but they cannot capture the peculiarities and differences
that can actually be observed in individuals' behaviours. Therefore,
questionnaire answers cannot be used to unravel whether gender differences
in human individual behaviour are culturally influenced or whether they
are, in fact, biologically determined or even evolutionarily derived as
often assumed. Ultimately, making conclusions from modern humans'
behaviour to that of their evolutionary ancestors and using these
conclusions, in turn, to explain modern humans' behaviour remains
circular. Possible mistakes cannot be detected.
Comparative psychologists therefore use a different approach and study
today's living species, in particular, human's closest living
relatives-the nonhuman primates. Today's species' behaviours, social
systems and ecological adaptations can be directly explored and their
degree of evolutionary relatedness can be genetically analysed. The
knowledge and data base on their communalities and differences that can
thus be secured can be used to make conclusions about possible behaviours
of their ancestral species. Such comparative research opens up new avenues
also for testing evolutionary theories on humans.
In a recent study in the Berlin Zoo and the Animal Shelter of Berlin,
Germany, Jana Uher and her research team have studied individual
behaviours in four monkey species. These species are endemic to three
different continents: Weeper capuchins originate from South America,
Mandrills from Africa, and toque macaques and rhesus macaques from Asia.
The study is based on a novel research paradigm that the personality
expert has developed to explore "personality" differences independently
from everyday language and thus also in nonhuman species (see the Science
Blog "A new scientific paradigm for research on individuals").
In this new research, Jana Uher adapted and further developed
approaches from cross-cultural "personality" psychology for the purposes
of systematic cross-species comparisons of "personality" differences. To
test these novel methodologies, her team observed individual monkeys of
each of the four species over 4-5 weeks, each individual for in total
60-80 hours. The researchers recorded grooming, body contact and proximity
to conspecifics as well as aggressive and dominant behaviours. In all
these behaviours, pronounced and temporally stable individual differences
occurred that are commonly called "personality" differences (see the
Science Blog ""Personality" differences compared between four monkey
species: A novel methodology to unravel communalities and differences").
Surprisingly, sex differences were largely absent. No sex differences
occurred in aggressive and dominant behaviour-in none of the four species.
In the social contact behaviours, only two sex differences emerged: In the
toque macaques, females spent their time with greater numbers of
conspecifics than did the males and, in the rhesus macaques, females
groomed others more often than males. But these differences occurred
neither in the weeper capuchins nor in the mandrills.
These findings are remarkable because, in all four species, males are
larger than females. This difference is particularly pronounced in
mandrills: Males weigh twice as much as females-this is one of the largest
sex differences of all primate species. Mandrills are also the worlds'
largest monkey species-and the most striking one: The males show exotic
looking blue, white and red colouration of their faces and hind quarters.
"The main aim of the study was to present the new methodologies for
cross-species comparisons of 'personality' differences-among them sex
differences-and to demonstrate their application in the four monkey
species. Therefore, we have studied only one social group per species. But
statistical estimations showed that the results will be similar in studies
on larger samples of individuals", says Jana Uher.
In a previous study conducted with her research collaborators in Rome,
Italy, the personality expert has investigated four groups of tufted
capuchins over longer periods of time. These South-American monkeys also
showed pronounced "personality" differences in their behaviours but hardly
any sex differences. "Meanwhile, we have data from eight social groups of
monkeys from five different species in which we found stable individual
differences but hardly any sex differences in behaviour", she says (see
the Science Blog "Sex differences, not as universal as previously
Sex differences did however occur in crab-eating macaques, a monkey
species originating from South-East Asia. Together with her Dutch
colleagues from Utrecht University, Jana Uher explored over three years
not only the behaviour of 104 monkeys, but also how 99 human observers
assessed these monkey individuals on "personality" questionnaires after
having conducted systematic observations of the monkeys' behaviours in
their social groups.
In agreement with the behavioural data, the observers judged males and
females as equally sociable and males as more curious, more playful and
less aggressive than females. The observers also judged males as less
anxious than females, although they had recorded more anxiety behaviours
for males than for females. Did widespread stereotypes about the bravery
of male individuals and the anxiousness of females ones bias these
Male crab-eating macaques also showed more impulsive behaviours than
females but, surprisingly, this sex difference was not reflected in the
"personality" assessments. "It may be possible that the raters have
already considered that the males behaved generally more impulsively than
females. When generating their assessments, the observers may therefore
have compared the males not with all individuals but instead only with
other males", Jana Uher assumes (see the Science Blog "Human's
'personality glasses' - Why we form impressions of individuals. New
insights into a uniquely human ability").
"There is no doubt that questionnaire assessments are biased by
stereotypical beliefs. But in which kinds of judgements these biases occur
and in what ways they influence the assessment outcomes, seems to be quite
different. This complicates the current picture of findings. We urgently
have to explore, how people generate their answers to standardised
questionnaires. Otherwise, all questionnaire data are meaningless", Jana
In humans, explorations of behavioural gender differences are particularly
complicated because pertinent cultural beliefs influence not only the
interpretation and assessment of observable behaviours-these beliefs can
also change individuals' behaviour in and of itself. Many studies have
shown that it is only during childhood that individuals acquire
gender-related beliefs and also learn to behave accordingly. But
sometimes, beliefs about gender-differences are maintained although such
differences cannot be observed in individual behaviour. Anthropological
studies from Great Britain could show, for example, that - contrary to
widespread beliefs - adult men gossip as much as women. Thus obviously,
gender differences in behaviour are much less biologically influenced than
The new findings from the four monkey species support this idea. Jana Uher
emphasises, however: "These findings from other primate species do not
mean that gender differences in individual behaviour were largely absent
also in our human ancestors. But these new results show that differences
in body morphology do not necessarily go along with differences in
behaviour, as often assumed in evolutionary psychology."
This research is part of a project funded by the German Research
Foundation (DFG, UH249/1-1).
Uher, J. (2015e). Comparing individuals within and across
situations, groups and species: Metatheoretical and methodological
foundations demonstrated in primate behaviour. In D. Emmans & A.
Laihinen (Eds.). Comparative neuropsychology and brain imaging
(Vol. 2), Series Neuropsychology: An interdisciplinary approach.
(chapter 14, pp. 223-284). Berlin: Lit Verlag.
Last update: 14.09.2015
Keywords: Weeper capuchin (Cebus olivaceus), mandrill (Mandrillus
sphinx), toque macaque (Macaca sinica), rhesus macaque (Macaca
mulatta), macaques, sex differences, gender differences,
personality, rating, assessment, judgement, personality
questionnaire, individual differences, individual behaviour,
individual-specific behaviour, attribution biases.