Female Monkeys More Dominant In Groups With Relatively More Males

Female monkeys are more dominant when they live in groups with a higher
percentage of males. This is caused by self-organisation. This surprising
discovery was made by researchers at the University of Groningen. What
makes the study particularly interesting is that the researchers used a
computer model which can simulate interaction between monkeys. Their
findings will be published on July 16 in the open-access journal PLoS ONE.


Many animals living in groups have a social hierarchy, a so-called 'pecking
order'. Monkeys, too, have a social hierarchy. Highest in the pecking
order is the most dominant monkey, who consistently wins aggressive
interactions (such as biting) with other group members. At the bottom of the
hierarchy is the lowest-ranking monkey, who consistently loses interactions
with other members of the group. Monkeys have to fight for their place in
this hierarchy every day.


Simulation


The position of females in the hierarchy varies among different monkey
species. In most species females are ranking below the males. This is no
wonder, because they are usually much smaller than males. However, in the
case of the Lemur species of Madagascar the females are dominant, in
bonobos, males and females roughly equal each other in dominance, and among
a lot of other species (macaques and the grivet, for instance) females are
weakly dominant.


"This means that the most dominant females rank above approximately a third
of the males," says Charlotte Hemelrijk, theoretical biologist at the
University of Groningen and the first author of the article (which she
wrote together with her former PhD student, Dr. Jan Wantia and a Swiss
anthropologist, Dr. Karin Isler).


Until now, it was unknown how this female dominance develops. Researchers
in Groningen therefore created a virtual world, Domworld, with which they
could simulate the interactions between monkeys.


Large amount of literature


Surprisingly, the computer model predicted females to be more dominant in a
group with a relatively large number of males. To verify this prediction,
the researchers analyzed data of aggression of a large amount of literature
in which primate behaviour is described in order to calculate for the
first time female-dominance among many different groups and monkey species.
Their analysis showed the predictions of the computer model to be
accurate. "This is an interesting way of conducting research," says
Hemelrijk. "You discover something unexpected in the virtual world and then
you test your findings in the real world."


Inborn or self-organisation?


So why are females more dominant in groups with a higher percentage of
males? Two competing theories about the development of dominance exist,
explains Hemelrijk. "According to the first theory, dominance is inborn. A
monkey with good genes is bigger and will therefore win aggressive
interactions more easily. The second theory states that dominance develops
through self-organisation. An individual monkey wins an aggressive
interaction by chance. As a consequence, the monkey's self-confidence grows
and it also wins other aggressive interactions. It's a
self-reinforcing effect," says Hemelrijk.















More complex than thought


If the first theory were correct, one would expect dominant females to be
relatively bigger in size compared to male members of their species than
less dominant females of other species. The researchers found this not to
be the case. Instead, the second theory turns out to perfectly explain
female dominance, as the relation between female dominance and the
percentage of males can only be found among monkey species living in groups
with
aggressive behaviour that is sufficiently intense and frequent.


"Male aggression is more intense than that of females. In groups with more
males, males are more often defeated by other males. Consequently,
high-ranking females may be victorious over these losers. Furthermore, the
presence of more males in the group leads to more interactions between
males and females, causing more chance winnings by females. Through a
self-reinforcing effect, these females will go on to win more frequently in
later interactions and grow more dominant," says Hemelrijk.


According to the researcher, the study casts new light on monkeys. "The
assumption was always that the degree of female dominance over males - or
male dominance over females- was static, but it turns out to be much more
dynamic and complex than expected." Dominance also plays a factor of
importance in human interaction, says Hemelrijk. "It would not surprise me
if self-organisation would prove to play a role in the development of
dominance between the sexes among human beings too."


Female Dominance over Males in Primates: Self-Organisation and Sexual
Dimorphism
Hemelrijk CK, Wantia J, Isler K (2008)

PLoS ONE 3(7): e2678.doi:10.1371/journal.pone.0002678

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