Generals have said that throughout history


Randy Thornhill




Together with Craig Palmer, Randy Thornhill (University of New Mexico), wrote 'A Natural History of Rape, Biological Bases of Sexual Coercion'. The following interview took place in Austin, June 2005.

Individuals of most species reproduce sexually. It seems much easier to just make clones of oneself, why don't they do that?

The question of the function of sexual reproduction is an important one in biology. It looks as if sexual reproduction functions to diversify the genetics of offspring to deal with biotic enemies, because you get the distribution of sexual reproduction that is consistent with biotically harsh environments that offspring will face. Specifically harsh biotic environments, as opposed to harsh physical environments. So you get a preponderance of asexual reproduction in physically harsh environments, but you get a preponderance of sexual reproduction in biotically harsh environments. Biotic harsh environments are environments in which there is a lot of predation, and competition between species. With high species diversity as in the tropics you get more sexual reproduction, compared to more a-sexual reproduction at higher latitudes. And in addition, disease is more prevalent in certain ecological settings than in others, and that contributes to the harshness of the environment that offspring will face. All these biotic elements, predators, competitors and parasites, they co-evolve with the host and make the requirement of genetic diversity in the offspring. And only sex can rapidly provide the genetic diversity in the offspring. So the ecological distribution of sex relative to asex, is very consistent with harsh biotic environments being the driving selective pressure for the evolution of sexual reproduction.

Why are there generally only two sexes?

The leading idea is that there are two sexes because a third sex will be selected against. Each sex is a specialist. Males specialize in finding females and females in parental investment. A third sex cannot be favored because the other two fill all relevant niches.

Why are there generally as many males as females in a species?

The sex-ratio is usually about 50-50, because each individual has a mom and a dad. That means the total reproductive success of males and females is equal. So if there is a deviation toward a greater preponderance of one sex, that selects for individuals that produce a majority of that underrepresented sex, because they will have more grandchildren. So you go back to a 50-50 ratio. On top of 50-50, you have some local deviations of sex ratios, based on mothers' condition, genetic quality of fathers, inbreeding, etcetera.

What is the difference between males and females?

The basic difference is gamete size. You get an optimum for offspring nourishment, the big gametes, and an optimum for finding those big gametes. A stable two gamete situation, because an intermediate size is not going to be good at either one of those optimums. By definition, females produce the big gametes, males the small gametes, that's the definition of male versus female. And then there is a lot of baggage with that. So if you are the male, you are producing small gametes, then you are a specialist in finding lots of mates, and you tend to be rather indiscriminative. And if you are female, then you are the investing sex and all the baggage associated with that, greater parental care and discriminative mate-choice, because you have more to lose if you make the wrong choice. Sometimes the sex-roles are reversed, and that follows the same rule. Where males do a lot of parental investment, as in seahorses and some other species, you get stronger competition among females for males, and you get greater male than female choice.

You wrote together with Craig Palmer a book on rape. How did you come to study this subject?

I got interested in rape early on, because of my interest in sexual selection. Sexual selection is an aspect of natural selection, and is about differences in the ability to best members of one's own sex in competition for access to the members of the other sex. What rape does is increasing mating success of some males at the expense of others, so that is in the domain of sexual selection.

One of the things you describe in your book is males of certain kinds of scorpionflies who obtain a mating with a female either by force, that is by raping a female, or by offering her food. Which method do males prefer, and how do you know?

Male scorpionflies prefer to use the nuptial gifts to get females, give them the food-gifts in exchange for a mating. I determined that by taking a cage, and I put a dozen male scorpionflies and six dead arthropods in there. So the scorpion fly males fight to get the six dead arthropods. Those males that don't get the arthropods then become the males that start looking around for females and for forced mating on the female. These males tend to be smaller than the males that do get the arthropods. If you put more dead arthropods in the cage, then all the males will use those. Females like the males that deliver food to them in exchange for mating, but females try to avoid very actively the males who don't have a nuptial gift. So there is a male preference for using nuptial gifts because it pays off for males in terms of female preference; females prefer these guys with the gifts and avoid males without gifts.

Are male scorpionflies adapted to rape?

They are. They have a morphology for rape, and this morphology is encoded by genes. It is a complex clamp on the male's abdomen, it is a structure that works like a clamp and holds the female's wing during forced mating. I discovered that if you cover the clamp, so that it doesn't work, then males absolutely cannot rape. They grab females, but they can't actually hold her well enough to rape, the females always get away. In experiments I used bee-wax and other materials that are not toxic to the males, to cover the clamps, to render it non-functional. So far as I know, rape is the function of the clamp. I did other experiments where I looked at whether the clamp serves to prevent other males from taking over a female. So once a male is mating with a female, then the clamp could potentially prevent other males from taking over the female. And that did not work. So the clamp specifically is about holding his mate, to allow him to rape the female.

If human males are given the choice, like the male scorpionfly, to have consensual sex with a woman or to rape, what is the usual preference of the human male?

Well I haven't done experiments that are quite comparable to the scorpionflies, but one would surmise that since males like female sexual interest in them, and high arousal and all that, it is very desirable to men. It is not required for men's sexual interest obviously, because men rape. But men like women to be excited, so I assume that the preference would be consensual sex, and that men who cannot get consensual sex from females are more motivated to rape.

It does not seem as simple as in the scorpionfly, because rich men sometimes rape, and poor men often don't rape.

Yes. But there is a social-economic relationship there. Such that more male rapists come out of low social-economic environments than middle and upper social economic environments. It is not to say that only poor men rape, no, it is to say that many men in the right circumstances, where the perceived costs of rape to the men are low, would rape, regardless of whether he is rich or poor.

Given the 'right' circumstances, low costs, a warlike situation, extreme vulnerability of women, no moral or social restrictions, is the average human male going to rape?

I would say the general answer would be yes, the average male would. I mean, that's why rape is so prevalent in warfare. You got low costs of rape, the absence generally of social restrictions, you got very vulnerable females because you have taken over the village. The protectors of the females are dead or gone. And the young women are the vulnerable ones, the sexually attractive ones of course, that follows the general pattern across rape inside and outside warfare, where young women are the primary victims. The U.S. government has recently taken raping more seriously, for the first time, very recent though. They tell the soldiers, don't do it, we're not put up with you raping in warfare. But that's not been the tradition of American warfare, or of warfare anywhere else in the world.

I somewhere read about a Japanese World War II general who said: "If you are a real man, then you can rape".

Generals in general have said that throughout history, it is not just this Japanese general. Rape in war is often encouraged, and it is used as a tactic to manipulate your enemy. So if you're a guy out there in the war, and you think your daughter, wife, sisters being raped somewhere, that can be reason for you to desert the war and go back to take care of them. Although it could inspire more violence in the soldiers, but it seems to have a negative effect on the soldiers willingness to participate in the war.

You report about an experiment where they found that the sexual arousal of many males diminishes by seeing violence toward the victim, and by signals of her pain and humiliation. What is 'many'? 10%, 50 %, 90 %?

Depends on the study, there are multiple studies. The pattern is that if you have what researchers call gratuitous violence, that is violence in excess of what people perceive is needed to get something you want, so that is guys beating up the females, men are turned off by that, unless they are psychopaths. But that is a low frequency, that's three percent of all men. They are more tolerant of violence. Guys who end up in prison for violent crimes, and that would include a lot of rapists, psychopaths are over-represented there, it might make up thirty percent of those guys. But too much violence for the vast majority of men turns them off. But sex being coerced, without gratuitous violence, is in general sexually attractive to men.

There is a story in your book about a woman being raped by a male orangutan. What is the point being illustrated?

This event illustrates the general pattern that rape of a woman by a man other than her partner lowers the partner's paternity confidence and thereby threatens her relationship with her main partner. In this case, there was no man involved as rapist. The husband of the victim, when asked what he thought, commented that it was ok, because the rapist was not a man.

I have observed the rock-pigeon for many hours, and I have never seen anything that looks like rape. Is it because they lack the physical ability to grab or threaten the female?

That's a constraint. You get rapes in some birds and not in other birds. Male ducks have a penis for example, pigeons don't have a penis, they have a cloaca. But in the common house sparrow, males gang rape, and they don't have a penis.

Can't the female just fly away?

Well, no. There are several males that gang up on the female, holding her down and taking turn to raping her.

You also describe the female dung fly who behaves as if she is looking for the best rapist.

Yes, could be, that's an interpretation. A male that can physically control her, that's one way females can test male quality. You expect that to be a pretty limited way for females to do it, because there are always other, low cost ways for the female to do it. Like she could smell the males or something, or visually inspect them. She doesn't have to be beaten up by a male to inspect his quality. So you expect the assessment by females of male genetic quality based on male physical strength toward the female to be a pretty limited pattern across species. But some have claimed it occurs in certain systems.