The birds, boys and female clothes

A plea for tolerance of all human sexual behaviours attacks the wrong targets, argues Mark Pagel.

Evolution's Rainbow is a 400-page disputation on popular and scientific views of gender and sexual diversity in animals and people. Joan Roughgarden indicts generations of academic biologists and anthropologists for creating an orthodox and restrictive view of human sexual behaviour that marginalises people whose sexual and gender roles deviate from conventional heterosexual behaviour. In the orthodox world that Roughgarden attributes to the conspirators, ardent males pursue coy females, and family life consists of nurturing females raising the offspring while the hunter-warrior male acquires food and fends off competitors. This is a binary male-female heterosexual place with scant room for diversity in sexual orientation.

Roughgarden delves into the evolutionary biological literature to support her charges, documenting an impressive range of sexual practices and styles in animals that go well beyond those that might be considered conventional. In a reversal of sex roles and morphology, the males of some sea horse species brood the young in pouches on their abdomens then rear the newborns. In many bird species, young males forgo reproduction to remain behind as helpers at the nest. Fish known as simultaneous hermaphrodites have male and female gonads and switch continuously between producing sperm and producing eggs. The males of some hummingbird species mimic the coloration of female hummingbirds.

Males are not responsible for all of the diversity. Female bonobo chimpanzees engage in breathtaking and prolonged orgies of what observers delicately refer to as G-G rubbing. An all-female clonally reproducing lizard species has elaborate lesbian courtship rituals, and in the avian Alpine accentor species, females, far from being coy, harass males for sex.

The females of many bird species divorce males if matings with them fail to produce offspring, and a phenomenon known as extra-pair copulations, in which females seek extra mates, is common.

Hundreds more examples can be adduced to support Roughgarden's thesis that what we may consider to be "normal" heterosexual behaviour is but a fraction of the range of sexual orientations and behaviours found in nature. Were Evolution's Rainbow a book on the variety and natural history of mating systems, its first few chapters would provide an engaging review.

But Roughgarden shows a surprising willingness to bite the hands that feed her. In reporting the literature on sexual diversity, she fails to acknowledge that almost everything that is known about the subject in animals derives from research by the very people she derides as conspirators. If it were not for evolutionary biologists and anthropologists, we might just hold the simplistic views Roughgarden claims we do.

The upshot is that her attacks on contemporary evolutionary scientists, not to mention a curious and misplaced attack on Darwin, are gratuitous, opening her to the charge of having written a piece of propaganda against an enemy that does not exist. This is a shame because Roughgarden is an incisive and clear-headed writer with a strong reputation. But in Evolution's Rainbow her lucid passages frequently give way to opinionated diatribes about sexual diversity and sexual politics. These are unnecessary, as her social agenda is unassailable: to argue for tolerance and understanding of the ways that humans express sexuality and gender, including gays and lesbians, intersexuals, transsexuals and transgendered people.

To make the case for tolerance of human sexual behaviours, Roughgarden - mistakenly in my view - seeks to find analogous behaviours in nature. She likens human transgendering, for example, to the male's mimicry of females in hummingbirds. Too clever to be caught in the trap of committing the "naturalistic fallacy" - arguing that human practices such as polygamy and incest can be morally justified by similar practices in animals - Roughgarden adopts a riskier tack. She seems to suggest that the many alternatives to conventional human heterosexual behaviour would not exist at the frequencies and in the varieties that they do unless they served some biological function.

The reader is lulled rhetorically towards the view that human sexual orientations such as transsexuality and transgenderism have some basis in promoting reproductive success - if not one's own, then possibly a sibling's or a parent's.

The danger in adopting this stance is that it may patronise some groups, and if real advantages in promoting reproductive success cannot be found for particular sexual orientations, then the people who have them are left somehow looking "abnormal" - the very situation Roughgarden seeks to overturn.

To show how difficult it can be to find advantages for human behaviours, consider that human females become non-reproductive around the age of 50.

This is just about unique in all of animal life and could be classified as an alternative to "conventional" heterosexual behaviour.

A prominent view of the menopause is that older women forgo their own reproduction to help their daughters. It has proved difficult to find evidence that a woman can increase her genetic representation in the future more by helping her daughter than by having additional children of her own.

Whereas just about everyone has a grandmother, making it easy to collect data on this issue, fewer people have gay or lesbian siblings, and yet fewer a transsexual or transgendered relative. To my knowledge, there is no good evidence that any of these sexual orientations promote anyone's reproductive success. A recent report suggesting that sisters of gay men have more offspring received much attention but it is flawed.

Interesting, even fascinating, as it is to try to understand human sexual diversity as different ways of obtaining reproductive success, we have moved on, giving Roughgarden's social and political arguments an anachronistic feel. Western society has been making progress towards understanding and accepting diversity of sexual behaviour in some of its most conservative institutions for the past 20 to 30 years: homosexual marriages and adoption, homosexual clergy, police, government and military personnel are all losing their shock value.

Ironically, it is San Francisco, where Roughgarden lives, that leads the way in many of these changes. Institutional changes mirror those on the street. A novel about a gay man seeking romance in London has recently won Britain's prestigious Man Booker Prize, and the European Union's newly proposed commission fell even before being voted on because it contained a homophobic and misogynistic candidate. Yes, societies change slowly, but past changes suggest more are on the way.

Of the many forms of human diversity - size, shape, colour and personality - sexual diversity is somehow treated differently. Why is this? Evolution's Rainbow succeeds in making the case that the range and degree of sexual diversity in nature and in humans is far reaching. It is courageous in springing from the author's own deep personal involvement with issues of sexual diversity. But as a political treatise and as a call to arms, it is flawed by attacking the wrong institutions and people.

Mark Pagel is professor of evolutionary biology, Reading University.