Sex wars

by Rebecca Priestley / 21 August, 2010
A Wellington professor takes issue with the arguments about monogamy in <i>Sex at Dawn.</i>

Measuring ape ­testicles is not top of every­one's list of career choices, but for primatologist Alan Dixson, it's just part of the job. Dixson, professor of biological sciences at Victoria University and one of the scientists quoted in Sex at Dawn: The Prehistoric Origins of Modern Sexuality, boasts a 30-year career working with primates in captivity (including at San Diego Zoo) and in the wilds of Came­roon, Gabon, Rwanda, the Democratic Republic of Congo and Brazil. And it's these 30 years of observation and fieldwork that leads Dixson to take issue with some of the central premises of Sex at Dawn, touted as the most important book about sexuality in the past 50 years.

As described in a recent Listener article ("Sex at Dawn", August 7), evolutionary psychologist Christopher Ryan and co-author Cacilda Jethá argue that sexual monogamy is a recent thing. To help understand contemporary humans' fascination with sex and difficulty with monogamy, we need to look at our evolutionary roots, which they say lie in a multi-male, multi-female mating system.

As evidence for this, they cite our genetic and physiological similarities to our closest evolutionary relatives, the chimpanzee and, in particular, the bonobo. These endearing creatures, say Ryan and Jethá, have a multi-male, multi-female mating structure, are very peaceful and, like humans, use sexuality for social purposes, such as tension reduction, bonding, conflict resolution and entertainment. If we acknowledge that's the sort of system we evolved from, they say, we can better understand contemporary human sexuality.

Dixson's work on primates is quoted in Sex at Dawn, but his most recent book, Sexual Selection and the Origins of Human Mating Systems, published in 2009, is not cited. Dixson has not read Sex at Dawn, but when the book's arguments are put to him at his Wellington home, he seems bemused. As established in Sexual Selection, Dixson sees all the physiological evidence pointing towards humans having evolved not from a multi-male, multi-female mating system but from a polygynous mating system, where one male has several females - meaning our distant ancestors lived in communities more like those of hulking Rwandan gorillas than cute Congolese bonobos.

Dixson takes issue with two key arguments. The first is Ryan and Jethá's citing of the small (20%) difference in average size between human males and females as being comparable to that between male and female chimpanzees or bonobos. In contrast, male gorillas are twice the size of females.

"It isn't just a matter of body size," says Dixson, "it's a matter of body constitution. Men have far more muscle, and women have far more fat on their body than men. So the real sex difference is bigger than it appears on body weight alone. As well as that, our faces are dimorphic and men have secondary sexual traits, like beards."

But Dixson's main argument is about reproductive anatomy. "Now let's assume we've always had a mating system with everyone sharing everyone else. In that kind of system you get something called sperm competition. Chimps have multi-partner mating. Females will mate with many males, males will mate with as many females as they can get access to. There is competition between males and the alpha-male chimpanzee does better than the other ones. They have testes three to four times larger than a human testis."

What, in proportion to body size? "No, actually, they're enormous!" says Dixson. These enormous testicles, up to 80g each and bigger than a chicken's egg, are evidence of sperm competition: when a female mates with a number of males, there is competition between the sperm of two or more males for the fertilisation of an ovum. Sperm competition leads to large testes. As for sperm competition in humans, Dixson sees no sign of it. "Human testis size is perfectly ordinary, quite small in relation to body weight," he says.

But Ryan and Jethá have a comeback. They say that 10,000 years of monogamy - a system that emerged with the advent of agriculture and private property - has shrunk men's balls. Dixson throws his head back and laughs. "Right, and it did the same thing for the sperm mid-piece, and the size of the accessory reproductive glands, and the muscularity of the vas deferens? So all these things over the last 10,000 years have just shrunk, have they?"

As for Sex at Dawn's final chapters, which basically tell us to lighten up about sex (but as with most evolutionary psychology arguments I've read, focus on telling women to lighten up about male infidelity), Dixson shrugs. "Far from what these authors claim, human beings have a profound disposition to form long-term relationships, not just for sex but for reproduction and the mutual care of child­ren. The few surviving pre-agricultural societies, such as the Hadza hunter-gathers of Tanzania, do this.

"It is nonsense to claim that monogamy is a recent phenomenon, or that it developed because of the rise of agriculture 10,000 years ago. Anatomically, modern humans existed in Africa more than 195,000 years ago. They probably had monogamous and polygynous mating systems."

And don't bring testicle size into the argument. "I look at the degree of sexual dimorphism in human beings and it would be consistent with either some degree of polygyny or a multi-male, multi-female system. But if you look at the reproductive system, it's completely incompatible with a multi-male, multi-female system. Our reproductive anatomy is completely inconsistent with a past that involved sperm competition."

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