Dominance Hypothesis: Baboons & Evolutionary Diversity

Building on the We Are Primates chapter in Through the Lens of Anthropology for Intro to Anthro 2022, we read Barbara Smuts, “What Are Friends For?” Smuts’s work with baboons invalidated the ideas of the dominance hypothesis as the origin of family life.

Next up: What is a theory?

Transcript: Dominance Hypothesis

Today we read about baboons in an article by Barbara Smuts called “What Are Friends For?” Over the years I’ve been able to condense one of the messages I have about this article to a series of alliterative Bs to watch out for: Beware being bamboozled by baboons, which I think happened to some of you. Something happens when we come into an anthropology class and we think, “aha, now we’re studying some non-human primates, that must be because they’re just like us, and here they are having friendships and stuff. That must mean they’re like us.” I want to caution you about this because baboons are not very much like us and when Barbara Smuts went off to do this research, she was not studying them because they’re very close to us evolutionarily speaking. I did warn you that we’re going to be going back into a bit of the Muckle, González, and Camp chapter 2 on the primates to explain these evolutionary relationships.

Baboons are what we call Old World monkeys, who are closer to humans than New World monkeys. This is some terminology that Muckle, González, and Camp use, even though they said back on page 22 that they’re going to try and get away from this terminology because it’s part of a colonial mentality. The “New World” refers to the Americas. The “Old World” refers to Africa and Eurasia. We’re supposed to be decolonizing that terminology, but in terms of monkeys it’s stuck around. It’s a hard habit to get rid of. I’ll look at a map and talk about what the “Old World” is, or what the people meant by the “Old World” and what the “New World” is and where this evolutionary stuff is happening. Don’t worry, we’ll get a chance to go back to these words in a second. Here is the world, with a projection that’s pretty good, it shows us that in fact South America is nine times as big as Greenland, even though on Mercator projections you’d be tempted to think otherwise. “New World” is what we’re in, the Americas, South and North America. “Old World” refers to Africa and Eurasia. That terminology got put into place because the Europeans came sailing across and said, “aha, it’s all new.” Africa and Eurasia have been inhabited for longer, but it wasn’t exactly a “New World.” We’re trying to change this terminology. When people refer to the “New World” or New World monkeys, they’re referring to what happens in the Americas versus what happens in Africa and Eurasia.

Interestingly, Muckle, González, and Camp support the idea of the hypothesis that the way monkeys got to the Americas was probably via a huge floating vegetation raft, which broke off and floated over here about 30 million years ago, and then they started evolving here. Let’s go into the New World monkeys and their characteristics. . . . The technical name for them is Platyrrhines. They’re also known as flat-nosed primates. Those are a little flatter on all the American monkeys. We’re talking about mostly in Central America and South America. All of them are tree dwellers. They’re all arboreal, and they are usually small in terms of their body size. They all have tails, and some of them also have what we call prehensile tails: tails that wrap around and can grab things or grasp things.

There’s usually little of what we call sexual dimorphism in the New World monkeys. There’s not much difference between males and females in terms of their size and characteristics. Hard to tell them apart from looking at them. Importantly, the New World monkeys were evolving in the Americas on their own from around 30 to 40 million years ago, which is quite a long time. They’re doing their own thing in the Americas, and speciating and evolving, and not having contact with what was going on in Africa and Eurasia.

Now if you look at a picture of some examples of monkeys in the Americas, New World monkeys, here are some of them like the spider monkey. There’s that prehensile tail grasping at limbs. Or that capuchin monkey. Funny, these monkeys. If somebody goes down to Costa Rica and tells you about the monkeys and says, “I can’t see how we came from monkeys,” you can be like, “it’s true, these monkeys are goofy looking.” They screech around a lot and jump. You can see that there are some primates, they’re non-human primates, they’re in the family–got some front-facing eyes there and stuff–but in general you could be forgiven if you were like, “I’m not very close to that golden lion tamarin, that doesn’t have anything to do with me.”

On the other hand, we have the Catarrhines, or what are called the Old World monkeys. Catarrhines, it means the sharp-nosed primates, the downward-facing nosed primates. They are primarily in Africa, but also in parts of Eurasia. Many of them are up in the trees, arboreal, but some of them are also terrestrial–they’re ground dwellers.

They have tails, but there are no prehensile tails among the Old World monkeys. A lot of them have larger body sizes.

This grouping, the Catarrhines, includes Old World monkeys, apes, and humans. All of these share a dental formula of two incisors, one canine, two premolars, and three molars. We all share that across Old World monkeys, the apes, and the humans. In this group there is sometimes a more pronounced sexual dimorphism. Importantly, this group is evolving for the most part in Africa. As we talk about human evolution, all the cool stuff happens in Africa, and to an extent parts of Eurasia where the large apes even Gigantopithecus, one that evolved in primarily in what is now China. The Old World monkeys are bigger, they’re more serious-looking except for that funny nose.

They’re often sitting on the ground or sitting on their tails instead of swinging from them. Larger, ground-dwelling, sharper-faced noses. Evolutionarily speaking, there are some people who say that the Old World monkeys are closer to apes and to humans than they are to the New World monkeys, who split off fairly long ago.

Old World monkeys are closer to humans than New World monkeys, but not nearly as close as the apes. We have an Old-World/New-World monkey divergence about 30 to 40 million years ago, maybe with that floating vegetation raft. Then we have a divergence between the species that would lead to apes. The species that would keep on being ever more beautiful monkeys around 20 to 30 million years ago.

By the way, what’s the quickest way to spot an ape versus a monkey? How do we know an ape?

Apes don’t have tails. Monkeys all have tails, but apes don’t have tails.

Then we have the last common ancestor of chimps, bonobos, and humans around 6-8 million years ago. You have monkeys splitting off between American monkeys and Eurasian-African monkeys about 30-35 million years ago, then you have another divergence between apes and monkeys. The split of the last common ancestor of chimps, bonobos, and humans is probably 6-8, 8-10 million years ago. When it comes to the baboons, Smuts is definitely not studying them because they’re just like us. They’re pretty distant from us, another 20 million years ago distance. Of course, they’ve been evolving ever since on their own tracks, and doing their own things. I thought I’d put up a couple charts of how these divergences work. We have among the primates, these primates that we don’t really talk about, lemurs, they’re pretty distant. Here’s our split between New World monkeys and Old World monkeys: they put it back about 45 million years ago. Some people think it’s more like 30-40. Millions of years, it’s difficult sometimes to get, because these are not necessarily clean splits. It’s not like one day you wake up and you’re off in a different species. These often take place over long periods of time.

Then we have the divergence between Old World monkeys. The apes: gibbons, orangutans, gorillas. Then this divergence, or if we focus on the orangutans, they probably diverged off about 12-15 million years ago. Gorillas, 10-12 million years ago. Then there’s the last common ancestor of humans, chimps, and bonobos. There was a speciation that happened after the last common ancestor, at 1-2 million years ago, bonobos and chimps form their own species. This gives you a sense of some of the evolutionary relationships.

We now need to go back to why was Barbara Smuts studying these baboons, if she’s not doing it because she thinks they’re just like us. Why would you study these baboons? . . . She’s looking at something that is called the “dominance hypothesis.”

It’s also an idea that we get from television shows, popular media, it’s just in the ether, in our environment: [a popular explanation] for why we have the social structure that we do. I put the words “dominance hypothesis” in quotes, because in my opinion this is not a hypothesis. It’s just a popular view of how things happen. The idea is: females are simply the passive objects for male competition. Males are going out and fighting each other to get dominance. They can have the female that they want. The role of the female is just to sit back watch the fight, and then have offspring with whoever gets to be the dominant male. In this scenario the males become the protective ones. They’re aggressive. They’ve learned that from all this competition they had to do. They’re the providers. They’re off going out and hunting, and coming back with the stuff, in our parlance, “bringing home the bacon.” They bring home the meat to the female who has been sitting around with the little monkeys at home.

Now if you’re the male, going off and doing all this providing, bringing back the stuff for the female monkey at home, what do you want to make sure has not been happening while you’re away? What do you want to ensure has not been going on back home? You’ve been off working. So, they demand sexual exclusivity because they would not want to go off and do all this work, then have all this monkey stuff going on at home. The idea is that if you do that, then you ensure genetic paternity, because they’re your offspring. So, you go from male dominance out to the stable nuclear families that we have today. We’ve gone from male competition all the way up to “Leave it to Beaver.” . . . The old shows in the 1950s, shows about mom, dad, and 2.1 children, and a little dog. Like I said, calling this a “hypothesis” is elevating it to a level of science that it probably doesn’t belong at, but it’s so ingrained in our society that we do have to test it out. We have to see.

Baboons are potentially a good test case for this hypothesis. They are not in stable nuclear families. The baboons are promiscuous. They do not place priority on long-term mating. They might have the same mates from season to season, but they can also switch it up. They are promiscuous, but that doesn’t mean random. That doesn’t mean you’re just mating whoever. They do have patterns. They seem to be making choices. This helps us out, because we can see who’s mating with whom, and especially among the baboons it’s good because they’re not having non-reproductive sex.

So, bonobos, for example, in the wild maybe don’t have as much non-reproductive sex as they do in zoos, but they do have a wide sexual repertoire. Just because they’re having sex does not mean they’re going to have any new little bonobos. Whereas the baboons, like some species, have very specific times–sometimes this is called “being in heat,” but that sounds yucky. We’ll call it estrus: if they’re having sex, they’re probably going to be fertile. If you’re a good baboon researcher, you can track who’s mating with whom, and who’s likely to have paternity in the new baboon.

They’re also extremely aggressive, strong, and the males have teeth that are sharper than lions. They’re not afraid, or they don’t shy back from using them. Smuts tells us that on average there’s a slash wound about one time a year on the females.

Also, in terms of body size, the males are about two times as large as the females.

To be clear, although we never want to discount human aggression, this is not the same as humans. I hope we’re not walking into our classrooms with a slash wound every year on average. We hope this is different.

Now, before we get into what happens with the baboons, I want to talk about that sexual dimorphism a little bit. Among the baboons, males are about two times as large. I thought it might be good to go over some of the primate sexual dimorphism by body size, because it’s something that Muckle, González, and Camp talk about. In the group of the Old World monkeys, the apes, and the humans, there is a range of variation of sexual dimorphism. As we saw among the monkeys in the Americas, there’s not much. But that doesn’t mean that apes are all sexually dimorphic. The gibbons for example, one of the apes, they’re about the same size, males and females. Then there are several species in which the males are a little bit larger than females about 8-10-15% larger

Anybody know any primate species where the males are on average a little bit larger than the females? Are we familiar with any primate species like that?

There are differences all the way around, but on average this is true among humans, and it’s also true among chimpanzees, and bonobos, which is not extremely surprising because of our most recent common ancestor. There are some species in which the males are about 50 percent larger than females, like the gorillas. Although, there are different groups of gorillas. There are mountain gorillas and lowland gorillas and some of them have more sexual dimorphism by their group. We just saw the baboons, that’s going into the monkeys, but that’s a pretty dramatic difference.

What I want you to note is the variability across these groups. It’s something that has been important to us when we’re talking about non-human primates and the human primate, is how much diversity there is and variation within these groups. Some of it is variable by species, other is variable by groups within the species. Of course, there’s also individual variation. When we say that men in the United States are on average eight percent larger than women, that means there’s going to be a lot of overlap it’s not that all the males group up at the larger size. There’s a significant overlap in terms of the range of variability. Also in human beings, interestingly, this varies cross-culturally, by a society, and even historically. When I used to look this up, I used to say 10-15 percent in the United States. It’s now looking like seven or eight percent. What could change the amount of sexual dimorphism in a society like the United States over say the past hundred years?

. . .

I’m also thinking about how parents would treat their offspring differently. In the old days we used to say that the boys and the men would get the protein, get the biggest piece of chicken. The girls could eat cakes and carbs, and that might produce a difference between how people develop. There were also limitations about what sports and what things you were supposed to be able to do, and how much you could move around. Those [limitations] have hopefully been reducing a little bit, and so has the dimorphism, biologically speaking. What we do culturally can have important effects on our developing bodies, our biology.

So, baboon findings: what do we find out? . . . Just because you have male dominance and male competition doesn’t mean that the females don’t have any role in their choice of mates and who they would be hang out with. Barbara Smuts begins to talk about what she calls “friendships.” How does she know that the baboons are friends?

We have to be careful here. When she says “friendship” that doesn’t mean she can talk to them and ask them about how they define a friend or look up their social media accounts to see who’s in their circles. She’s doing a measure of if they’re grooming each other and if they’re close to each other over time. She discovers something that’s interesting. It’s not the most dominant males that have the most friends, but the older ones.

What do the females get out of being friends with males?

They get protection. We’ve just learned about how we have these aggressive and sometimes quite violent, vicious males. If they have a friend, they get protection, and they get access to those resources and feeding spots. They also get care for offspring, infant care. Here again, Smuts has been tracking this, and this has been confirmed now. This article was written before we could do genetic testing. This has been confirmed by genetic testing now, that the male friends might care for infants that are not necessarily their own, that are not genetically related. They will care for their own, but statistically speaking you’re more likely to get care from a male friend than you are from a paternity relationship.

Now you might ask yourselves what is the male friend getting from this? Smuts speculates that this may increase their mating chances later. There’s not an immediate benefit, but over time this might work out for them. Again, this article was written before we had we could do genetic testing, but this is another finding that has been at least corroborated or reinforced by the genetic testing. There is this benefit later for the males.

We now return to our hypothesis, the “male dominance hypothesis.” Do these ideas about the origins of family life from male dominance, are they supported? The answer, of course, is the big “NO.” What do we see instead? We see that you can have long-term social bonds, these friendships and relationships over time, but the baboons show us that they can do this without having a division of labor between one sex going out and getting the food or bringing it back and sharing food. There’s not what we call a sexual division of labor amongst the baboons, and yet they still have these long-term social bonds. We also see that you can have friendship and not necessarily have sexual exclusivity demands. You can have these relationships, but they are not demanding sexual rights over the other. Finally, we have males performing parental or caring roles with infants without necessarily needing to have biological or genetic paternity.

So the baboons help us to falsify this “male dominance hypothesis” for the origins of family life. This is a good example because we’ll be talking about hypotheses and theories in science. The way that science works, which can be frustrating sometimes to people, is by falsifying hypotheses. You come up with an idea, a hypothesis to test, and you go out and see if you can falsify it. In this case it was falsified. Does that prove something else? We often talk about, “proving the hypothesis.” No. Hypotheses can be falsified, but probably never completely proven. This makes science sometimes frustrating because you’re always trying to in some ways, invalidate ideas. It always looks like they’re changing their minds, but it’s all a process of self-correcting hypothesis testing. In this case we cannot necessarily say what the origins of family life are. We haven’t proven where it comes from, we have disproven this idea that it comes from the male dominance. The other super-important point is that people look to the non-human primates, and they say they want to look for evidence to support what they see in human beings. If they want to support long-term monogamous relationships they’ll try and find a primate species that does it, and say, “aha, there you go.” Or if they believe that humans are naturally aggressive or naturally peaceful, some people will go to the chimpanzees and say, “aha, there you go look at them fighting each other,” or they’ll go to the bonobos and say, “aha, look at all that sex and peace they’re having, look at those dominant females.” Just be careful. People often look to primates to support whatever ideas they have about humans. But, in fact, when we look at the non-human primates, we see exactly what we see among humans: a wide variation or diversity in terms of our behaviors, ideas, and societies.


In Intro to Anthro 2022 after discussing We Are Primates, we read Barbara Smuts, “What Are Friends For?” Smuts’s work with baboons invalidated the ideas of the dominance hypothesis as the origin of family life.

Next up: What is a theory?

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