Females are dominant sex, primate study suggests ( 2003-11-26 17:12) (Agencies)
Feminists might be surprised to hear it, but females are the dominant sex in
most primate communities. Far from being passive bystanders in a world governed
by machismo, a new study suggests females may determine social evolution in
primates.
Researchers believe that "girl power" may not be a new phenomena. A recent
study shows that females led the way in the evolution of social relationships
among primates.
Researchers based in the U.S. and Sweden who analyzed evolutionary change in
groups of primates found the numbers of males lags behind females. The number of
females in a group tends to be larger than the number of males; the more females
there are the more males there will be, but only after a period, when the males
have had time to catch up to the changing population.
As mixed-sex, multi-male groups are common in more advanced primate societies
(including humans), scientists say the study highlights the importance of
females in understanding social evolution.
The time lag between numbers of females and males was revealed using a family
tree (or phylogenetic tree), with various branches showing relatedness between
species.
"A simple example is the relationship between us and chimps which looks like
a V," said Patrik Lindenfors, a zoologist at the University of Virginia,
Charlottesville. "Chimps are on one tip of the V, with us on the other, and our
common ancestor at the bottom. The branches are the lines that connect the
three."
Lindenfors and his team also used information on the approximate dates
species diverged to work out group composition among their ancestors. "For
example, the common ancestor of chimps and humans most probably was group-living
because both chimps and humans are group-living," he added.
Such a reconstruction of ancestral group sizes would show one of the
following patterns: no relationship between male and female numbers and social
evolution, change being driven by either males or females, or the sociality of
both sexes evolving in unison.
Lindenfors added: "If one sex drives social evolution then when a change
happens to the group composition of this sex, for example the average number of
females in a group increases, the other group would change as well, but with a
time lag."
This is exactly the scenario the researchers found, with changes in the
number of males consistently lagging behind females. Their findings are now
published online in the scientific journal Biology Letters.
Baboons and Chimps
Almost 40 primate species were covered by the study. They included baboons,
chimpanzees, and macaques—all known to live in large, mixed-sex groups.
So why should it be females that first seek to live in larger societies?
Scientists believe communal living is particularly beneficial to females
because a ready food supply is crucial for successful reproduction. A primate
"sisterhood"' would be better equipped to locate and defend food resources than
individual animals. Similarly, the risk of predation is reduced if others are
keeping a watchful eye.
But for males access to females is considered the major factor influencing
reproductive success. Unlike females, which must gestate then rear their
offspring, males can breed any time, and the more matings the better. So
operating as an unattached "free agent" may be the best approach.
Lindenfors said: "The number of females that they can impregnate is what
matters most for reproductive success." But, he adds, "the males should go where
the females are."
This last quote refers to the work of behavioral ecologist Jeanne Altmann,
who coined the expression. While a single dominant male can monopolize more than
one female, Altmann suggested this could be disadvantageous to females because
of increased female breeding competition and the danger of outside males killing
young they know not to be their own.
Altmann and others suggest females that manage to attract more males to their
group would increase mate choice and reduce levels of infanticide. Studies also
indicate males are better at detecting and defending against predators.
To pull in additional males there would have to be more females in a group
than the alpha male could manage. As numbers grow, his chances of hanging on to
his harem lessen. Scientists believe females develop sexual strategies to make
this scenario more likely.
More Males
Peter Kappeler, president of the European Federation of Primatology, provides
an example, saying, "Females can synchronize their receptive periods. If all
females of a group become receptive within a short period of time, it becomes
increasingly difficult for a particular male to monopolize matings. As a result
most females are able to mate with several different males."
However, researchers say synchronized estrus and other adaptations geared
towards multi-male, mixed-sex group living would take time to evolve, so male
numbers would lag behind. Males would also have to learn to live together while
in female company. Then there's the problem of sexually-transmitted diseases,
says co-author Charlie Nunn, an evolution and ecology researcher at the
University of California, Davis.
Nunn said: "As the number of females in a group increases, there tends to be
more males, and with this social system promiscuous mating commonly occurs. This
may favor the transmission of STDs, along with many other directly transmitted
pathogens." He says this would slow the evolution of larger, multi-male groups.
If females drive social evolution in primates, what about humans? To what
extent have female ancestors shaped human society, and if ancient man had it his
way how differently would we be living today?
Scientists say these are difficult questions, but Kappeler adds, "This and
similar studies are relevant to understanding human social evolution in that
they identify general principals which should have affected the evolution of
human social behavior."
So when it comes to our own origins, perhaps "girl power" isn't a
20th-century invention after all.
