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.
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.
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