Male dominance isn’t the default in primate societies, new study shows

A Müller’s gibbon. All five species of gibbons that were evaluated were classified as non-male-dominant. Image by JJ Harrison via Wikimedia Commons (CC BY-SA 3.0).

By Tina Deines via Mongabay

It’s a man’s world when it comes to ape societies — at least, that’s been the prevailing assumption for years. But some recent research is shaking up how we think about this group of primates to which humans belong.

Rebecca Lewis, a professor of anthropology at the University of Texas at Austin, is the lead author of the study, which was published in the journal Animals in November. She and her research team reviewed previous literature on 79 primate species, including nine ape species, and grouped them into one of three categories: male-dominant, female-dominant, and co-dominant.

The conclusion?

“Non-male biased power is not uncommon,” Lewis says.

Of the species surveyed, 58% were found to have male-biased power structures, but female- or co-dominant structures were found within every major primate group, including gibbons (lesser apes) and great apes.

All five species of gibbons that were evaluated — lar (Hylobates lar), Müller’s (H. muelleri), agile (H. agilis) and pileated gibbons (H. pileatus), and siamangs (Symphalangus syndactylus) — were classified as non-male-dominant. Among the great apes, bonobos (Pan paniscus), which are known for their matriarchal societies, are the only species to live in non-male-dominant groups.

According to Lewis, a good indicator for a female- or co-dominant species is similar size in body and canine teeth between the sexes. The smaller the difference, or sexual dimorphism, the more level the playing field between males and females.

For instance, siamangs only have a slight degree of sexual dimorphism, with males weighing just a few kilos more than females. Compare that with gorillas (Gorilla gorilla and G. beringei), where adult males grow to about twice the size of females; in gorilla troops, it’s the males that dominate.

“When you reduce dimorphism, there’s freedom for other kinds of power to arise or to manifest itself,” Lewis says. “But when males are just so much larger than females, it constrains what can happen.”

Non-male power may also arise when the supply of available female mating partners is lower than male demand, as this gives the ladies more social leverage.

Siamangs only have a slight degree of sexual dimorphism. The smaller the difference, or sexual dimorphism, the more level the playing field between males and females. Image by cuatrok77 via Flickr (CC BY-SA 2.0).

Not a shocker

For Sara Skiba, who has studied captive and wild bonobos for years, the paper’s determination of the species as non-male-biased wasn’t a shocker.

“I think that [for] anyone that has worked around bonobos or had the opportunity to be able to see them, it’s not surprising to hear that they’re a female-dominated society,” says Skiba, the director of communications at the Ape Initiative in Iowa and a postdoctoral researcher at Kennesaw State University in Georgia specializing in great ape social communication. Skiba wasn’t involved in the recent study.

For bonobos, she says, “Females are the ones that are in charge of the group.”

So how does this power structure manifest in bonobos?

“It’s not a single female that’s dominant over everybody else, but it’s rather that there are a number of females that bond very closely together and will use each other in times of conflict or to manipulate resources or in response to some event,” Skiba says.

They might do this by “whacking” a male on the head for undesirable behavior, or employing displacement strategies, where they’ll use their body to remove another individual from a desired nesting or feeding area, she says.

A social gathering of bonobos. For bonobos, says a bonobo researcher, “Females are the ones that are in charge of the group.” Image by Magnus Manske via Wikimedia Commons (CC BY-SA 2.0).

Such diversity in power structures among apes, and more broadly in primates, may come as a jolt for some. For years, lemurs have been considered something of an outlier among primates for their female-led societies. In fact, any deviations from the assumed male dominance structure within the order (including bonobos) were considered anomalies that had to be explained, Lewis says.

“I remember when I was a student, there were all these publications in the literature saying that this is an evolutionary puzzle,” she says, speaking of her early research on lemurs. “‘Why would males give up their power to these females? They’re the same size. Why give up their power?’ And so the assumption was that males had the power in the first place.”

However, these long-held beliefs of male power as the default may have developed partly due to chance. Lewis points out that some of the first primates that were studied — chimpanzees (Pan troglodytes), baboons (Papio spp.) and macaques (Macaca spp.) — live in male-dominated groups. This could have set the early tone for beliefs about social structures across all primates.

Plus, it might have “a lot to do with who’s been doing the research and publishing,” Skiba says.

“When you looked at the specific research questions or the way in which they were explaining their results, maybe the lens that they were using was a male-dominated lens,” she adds.

Some species may simply have gone under the radar too, she says. Take bonobos, for instance, which weren’t recognized as a separate species from chimpanzees until 1929.

Lewis says the new research demonstrates the importance of studying apes within the whole order of primates rather than just looking at one species or family in a vacuum. For instance, bonobos appear to be an outlier when compared to other great apes, but when you look at the broader picture across all primates, a new story emerges.

“We really see a lot about the diversity of primates and you can find some really useful explanations about evolutionary drivers of certain behavioral patterns that might not be apparent if we just look within the apes,” she says.

The long-held beliefs of male power as the default may have developed partly due to chance as some of the first primates that were studied, including baboons, live in male-dominated groups. Image by gmacfadyen via Flickr (CC BY-NC-ND 2.0).

Human influence on primate society

Conservation is key to ensuring that research like this can continue into the future; without healthy populations of apes in the wild, we can’t study them.

Bonobos are endangered, with population estimates ranging from 10,000-50,000 individuals. Meanwhile, 19 of the 20 species of gibbons are listed as either endangered or critically endangered. Habitat loss and hunting are the leading drivers in the decline of bonobo and gibbon populations.

As humans encroach on apes’ territories, it could also have another unintended effect.

“As we modify the environment and reduce the size of the habitats that these primates live in, we are changing the distribution of resources in the space, and we change the demography,” Lewis says. “We change who can live with whom and how many individuals are squished into a particular area or how much competition there is for resources, which makes power more or less important in social relationships.”

One of the changes that can occur is to the sex ratio, which directly impacts power dynamics between males and females. Gibbons, for instance, normally live in pairs or small family groups, but a shrinkage in habitat could force them to live in larger groups, which favor male power.

“We may be creating a situation that makes it look like there there’s more male-biased power out there than there would be if we hadn’t disturbed their habitat,” Lewis says.

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