Female lemurs call the shots in their societies. Not only do dominant females choose their own mates; they also use prompts—such as tail and fur pulling or the occasional nip—with both males and females to dictate which other females in the group can mate. Primatologists have long categorized the world’s 108 lemur species as a female-ruled outlier group among primates, with the vast majority of other primate societies thought to be male-dominated.
But a recent study in Animals calls this assumption into question. Though male power is more common overall among primate species, it’s by no means the default social dynamic. In fact, in 42 percent of the species examined in the study, primates lived in groups in which females were either dominant or on a level playing field with males.
“The traditional old-school thinking in primatology has always been around male dominance, but this study allows us to rethink that,” says Erin Vogel, a primatologist at Rutgers University, who was not involved in the new research.
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The study’s authors used statistical modeling to examine dominance and factors that might contribute to it in 79 living primate species. These factors included sexual dimorphism (differences between males and females in body size and other physical characteristics), the number of females in each group that went into heat simultaneously, the length of time in which those animals were in heat and the female-male ratio within each group.
The study found that dominance didn’t correspond to how close species were evolutionarily or geographically, but it did correlate with a predictable set of features—including whether males were larger than females and whether males’ canine teeth were bigger. In species where the females and males had a similar body and canine size, females either dominated or shared power equally with males.
Bonobos and chimpanzees, the two primates most genetically similar to humans, provide a clear example of how size can affect dominance, says the new study’s lead author Rebecca J. Lewis, a biological anthropologist at the University of Texas at Austin. Male chimpanzees are larger than females and are known for violently attacking females that they hope to mate with. Female bonobos, on the other hand, are similar in size to males and establish tight bonds with their female counterparts—an arrangement that shuts out males and leads to female-dominated societies. “The power dynamic shifts between chimpanzees and bonobos as a result of size,” Lewis says.
White-cheeked gibbons, medium-sized monkeys that are native to Southeast Asia, are an example of codominance. Similar size and stature mean that the sexes seem to share overall power but dominate different social areas. Males show some signs of territorial dominance, protecting their mates and offspring, and females are more likely to control food acquisition.
More subtle factors can also influence power dynamics. For example, if a group has more females, males tend to dominate because they have a wider choice of mates. When the sex ratio changes toward more females within a group of sifaka lemurs, for example, males are less likely to make submissive vocalizations “because the more females [there are], the more [those females’] power is reduced,” Lewis says. Power is also concentrated in females of species that go into heat for a relatively short time period or that don’t go into heat as a group. In both cases, males have a more limited ability to mate with females at any given time, shifting the power dynamic toward females.
The new study also peered back in time, examining fossils for any physical indications of dominance patterns in the last common ancestor of all primates. Using a method called ancestral state reconstruction, the researchers looked at canine tooth size and estimated body size based on the fossils of eight different extinct primates—distant ancestors of modern species. “While patterns of intersexual dominance don’t fossilize, indicators of dominance do,” says study co-author Christopher Kirk, an anthropologist at the University of Texas at Austin. He says the team found patterns that would correspond with a variety of intersexual power relationships, so defining the dominance status of our last common ancestor could go either way. This “challenges common assumptions about the evolutionary inevitability of ‘male dominance,’” the authors wrote in the study.
Vogel says combining data on living and extinct primate species was creative and “restructures the way we think about primate societies, showing us that females do have a lot of control, especially in choosing their mate.” She adds that we really shouldn’t be shocked that male versus female power is much more nuanced than we previously thought—but still, somehow, “we are.”