Male Coercion

From a lecture by Barb Smuts


Barb Smuts wrote the book on baboon friendships, and she edited our textbook. Now working on male coercion, she first became interested in it when doing her dissertation on friendships in baboons. Although most interactions between males and females were affilitative or neutral, there was some aggression, too. This hadn't even gotten much attention. Males weigh twice as much as females and have huge sharp canines- the biggest, sharpest canines on the african savannah-- So there is potential for males to inflict serious injury to females. She began to wonder how often aggression took place.

She collected data over 2 year period based on focal samples of females who were pregnant or lactating- in other words, not attractive to males. She found that the non-estrous female was attacked once a week. This usually didn't involve biting, but there is always that potential. A female typically got a serious wound once a year- including a slash with canines that will draw blood. A slash is generally not too awful, but it does interfere with foraging and mothering. Only once did she see a fatal attack and she doesn't think the male intended to kill the female.

So why are the males attacking the females?

Immediate context- What were they doing right before the attacks?
20% of the time, the aggression occurred in context of feeding competition; the female got in the way of the male's food. 20% of the time was when the male was involved in aggression with another male and redirected his anger at the female. The remainder was in various 5 or 6% sized contexts. However, 25% of the attacks were classified as unprovoked; Smuts couldn't figure out any reason for the attack.

She thinks that in some cases the male was punishing the female for something she did several hours or several days earlier. Why does she think this? Once she saw a female attack another female who was a friend of a male. Smuts watched the male, and when the attacking female returned, he attacked her. So many of these 'unprovoked' cases might involve earlier incidents which hadn't been seen by the observer. In other cases, she thinks that it's just the male displaying his superiority over the female and reminding her of his power over her so when he tried to mate later, she would be more likely to submit to him.

Functional terms- what can the males potentially gain in fitness?
The data we've been discussing was from a study on non-estrous females, but most species for which we have data are from estrous females because most attacks happen to estrous females. This leads us to believe that male attacks must be related to mating or to mating access.

Review on male-male competition and female mate choice
The main way males can increase their mating success is by fighting with each for mating. Fighting establishes a mating hierarchy where the higher-ranking males have more access to females when the females are likely to be fertile. When you think about it, the outcome of male-male competition doesn't guarantee mating access- one way it does is if females prefer to mate with dominant males. This is apparent in many species including savannah baboons. They might pick dominant males 'cause they have better genes or because they can protect them better, being stronger.

When the dominant male protects females, he offers them benefits, but he can also offer them other benefits like in friendships; grooming, and long-term affiliative relationships with the infants. So males are increasing their access to mates by providing females with benefits- they do something good for females, making them more likely to chose them for mates.

Another way for the males to increase their mating success is by inflicting costs onto the females if they don't cooperate- this is an alternative to providing benefits. A primary way to inflict costs is by threats or actual aggression.

Sexual coercion

Definition: Any use of force or threatened use of force by a male against a female that functions to increase that male's mating access to the female, or that functions to decrease the probability that she will mate with another male, or both.

This is a functional definition- when we see aggression by a male, we can't say it's sexual aggression unless we see that it actually had the result of increasing his mating success or decreasing other male's success.

A gamut of examples

Please remember that this is a continuum- a male might threaten a female when she looks at another male or he might force copulation when she is screaming. We emphasize that forced copulation is not common among nonhuman primates.

Rhesus macaques-
Joe Manson did a study on their mating behavior and found a high incidence of male aggression. When a female is mating with a low-ranking male, a high-ranking male is likely to come along and attack and chase the female, not the other male. Smuts spent a few weeks with this population when she was thinking about this subject, and was really struck by how much violence there was. What Manson points out is that even though females pay this cost, they continue to solicit low-ranking males. So here, female choice is coming into conflict with male mating strategies.

Jane Goodall has provided data on how often male aggression occurs in chimps. It's most common against estrous females- for instance a picture shown in lecture which shows a male who has lifted a female up and is biting her on the back of the leg.

Goodall has a detailed analysis with good data and she noted a few things- a lot of male aggression against female chimps occurred in the first few days of their estrous cycle, before swelling. A female chimp swells for a few weeks and ovulates at the end, but most aggression is at beginning of swelling- it seems that males attack females repeatedly during their first phase to condition them to sexual submission so that as she reaches ovulation, she's more likely to submit to the advances of the male who attacked her. This is mostly based on circumstantial evidence. However, females usually don't reject sexual advances. The best strategy for male to get good mating success is to convince (read: coerce) a female to go away with him and mate in peace. The alpha male doesn't have to do this because he can keep the others away, but lower guys need to, otherwise they'll get interrupted. Some females go with guys they like but others are kept there by coercion. If she tries to leave, he bites her and things like that. So this aggression occurs hours or days before the actual mating- it's not as immediate as in the rhesus.

Hamadryas baboons-
Female remain with males year after year throughout pregnancy lactation etc. This lack of association with any other males is maintained by the male hour by hour with instant reaction if the female strays too far away from the male. If a female moves too far off, the male gives the eyelid flash. If she doesn't come back, he rushes over and grabs her and gives her a ritualized bite on the back of the neck. Then she comes rushing back and presents to him (a form of ritualized submission). When she comes into estrous again, she is deeply conditioned to submit to this male after all these attacks.

As she said earlier, forced copulation is rare in nonhuman primates- the only species it's been documented in often in the wild is the orangutan. Remember that they're solitary females with ranges, and males who have larger ranges covering several females. When a female first comes into estrous, younger males who normally wouldn't have a chance later in the estrous cycle follow her around and force copulation. Observers know it's forced because the females scream and struggle.

(Chimps also force copulations infrequently, but it's just been seen in certain individuals in Gombe.)

Female Counter-strategies

Female-female coalitions
Vervet monkeys show the most widespread female counter-strategy. They form broad coalitions among female kin to defend each other against males. The size difference between males and females is 10-20% and two females can easily overwhelm a male. There is some male aggression against low-ranking females, but if a male tries anything with high-ranking female, she just turns around and whacks him one and he doesn't retaliate 'cause he knows she'll get all her relatives on him and make him feel it.

Female-female coalitions against males are not found only in primates. Lions form female bonded groups and if male lions show aggression, especially toward the infants when they've just joined a group, the females band together against him. They showed that the number of females in a pride affects the number of babies that are killed when a new male moves in.

Male-female affiliations
Another way to gain protection is by being in affilitative bond with another male who will come to your aid if you're attacked by another male. This is common in baboons 'cause female coalitions aren't too useful when dimorphism is as large as it is in baboons.

This strategy of affiliating with the male is most developed in the mountain gorilla. Females are not bonded or closely related. They don't bond with each other, but just with the male. Wrangham proposed that females do this to protect themselves against male harassment- more because of infanticide than sexual coercion but if you think about it, infanticide functions just like coercion because it brings the female into estrous sooner and increases the male's mating opportunity.

In 20 documented cases where the male gorilla died or disappeared and left females and babies, in every case all the babies were killed by another male.

Female dominance
Another way females can protect themselves is by dominating males. This occurs especially in lemurs. For example, in ring-tailed lemurs, male aggression is reduced or absent.

Variation across species on male aggression and sexual coercion-

Why is male coercion more common is some species than in others? The different female counter-strategies have a great deal to do with how vulnerable females are to male aggression and how often male aggression will happen.

Orangutans are good studies because they're the only nonhuman primate where forced copulations happen and also females travel alone- they have no other females to ally with, and no males to defend them. Smuts thinks this is why they're coerced; they have no protection.

Further evidence comes from a comparison of chimps and pygmy chimps, and exemplifies why social organization is an important factor.
Chimps are male-bonded. The males form coalitions against males from neighboring communities. Males groom frequently, and they reconcile one another after a fight much more than females do. The closest bond in chimps is males. Females are like 85% the size of males- large females are about the size of small males, and males' teeth aren't that much larger, so sexual dimorphism doesn't do much to explain male dominance over females. However, females invariably pant grunt to males and they don't fight back when males attack them. (Signs of submission)

Contrast with this the pygmy chimps. They also have a social system where males stay and females disperse. They show just as much sexual dimorphism. Within groups, however, social relations are really different. The closest bonds by far are among adult females who are unrelated. Not only do they spend more time together and groom each other, but they have frequent intense sexual interactions. This is called GG rubbing, and it's when females rub their sexual swellings together. Researchers are pretty sure that the females reach orgasm in these homosexual relations as well as their heterosexual ones. This behavior is common in the wild; when they find a tree, before they eat, they all have sex together and then eat. It has been hypothesized that this sex is to keep the bonds close so that females can dominate the males. It has been particularly documented by Amy Parrish, studying bonobos in captivity. In 100s of hours, she never saw male aggression on females, but repeatedly saw females attacking males or inflicting wounds on them. No one has ever seen male coercion in bonobos.

So, the bonding and coalitions between females function to shift the balance of power, protecting them against male coercion.

What evolutionary basis is there for rape and sexual assault in humans?

When she wondered about this and went to look at the literature, she found articles comparing insect behavior to humans... Not much help.

She has found some social variables that might correlate. The Yanamano people are some of the most aggressive primitive people. They are patrilineal and people have documented very severe incidents of male aggression against females, usually because of suspected adultery.

Contrast a society of pygmies in central Africa. They're traditional hunter/gatherers where men and women gather together and the males do a lot of child-care, more than in many 'advanced' societies. Anthropologists have never seen a man lift a hand against a woman in this culture.


There's a huge difference between cultures. Women are more vulnerable to coercion if they don't have close social bonds with female kin, or if they don't have female friends, or if they don't have the protection of a male.

This comparative perspective can help us think about such a difficult issue as sexual coercion in humans and it's not a case of 'oh it's built in and males are just going to do it.' We should pay attention instead to how social bonds affect things.