Talking Dirty, Talking Truth
Tim Birkhead
In the drier parts of southern Africa lives a starling-sized
bird whose sexual anatomy and behavior almost defy belief. The buffalo weaver is unique. In terms of sex no other bird in the world comes close. The male possesses a two-centimeter long false penis, which, during an extraordinarily protracted coupling, is rubbed against the female’s genitalia, leading eventually to a wing-shuddering, foot-clenching orgasm as the male ejaculates.
Sallie Tisdale suggests I might be unhappy about this. But, on the contrary, I am excited to tell you about it, because it symbolizes the role that academic research on reproduction has played in releasing us from our sexual inhibitions.
But she’s right to suggest that others will be unhappy getting this news.
For a full century following Darwin, evolutionary biologists and naturalists alike assumed female animals to be like good Victorian wives: strictly monogamous. Darwin asserted that this was true, even though he knew it wasn’t, and, for a full century, other biologists followed like sheep. For Darwin, female promiscuity was forbidden territory: an inhibition motivated by the sensibilities of his wife Emma and, in particular, by his daughter Henrietta, who acted as his proofreader and sexual censor. The myth of the monogamous female persisted, largely unquestioned until the revolution in evolutionary theory that gave rise to the “selfish gene.” Under this new paradigm, pioneered by George Williams at Princeton University in the 1960s and popularized by Richard Dawkins at Oxford and E. O. Wilson at Harvard in the 1970s, the reproductive roles of males and females were primarily selfish. Prior to this, sex had been viewed as a cooperative kind of affair between partners. The new focus on “individual selection,” as biologists call it, was the kiss that awakened the sleeping beauty of female promiscuity.
The evolutionary consequence of female promiscuity is the phenomenon of “sperm competition”—the competition between the sperm of different males to fertilize a female’s eggs—an unconscious process controlled ultimately by its genetic outcome. The truly wonderful thing about the notion of sperm competition is that it explains so much of what we didn’t understand about sex. Why, for example, does one species have huge testicles while another similar in body size, has tiny ones? Why do male birds stick like glue to their social partner during the time she is fertile? And why do females “paired” to one male occasionally sneak off to copulate with another?
The first studies of sperm competition focused on insects—on dungflies, to be precise—and for a while it was assumed that talking dirty might be confined to animal low life. But studies of birds—until then the ultimate model of monogamy—showed that even in pair-bonding species like ourselves, female promiscuity was virtually ubiquitous. Yet when I first started to explore the inner recesses of this topic by going back through the vast ornithological literature, I found most authors to be amazingly coy about the sexual habits of birds. Descriptions of copulation were evasive, full of euphemisms and, from a scientific perspective, frustratingly vague. This wasn’t surprising with the older literature, but was true even for those studies completed during the liberated 1960s.
Science is often serendipitous, and it was a lucky chance that the new academic interest in female promiscuity (and its evolutionary causes and consequences) coincided with the new molecular tools, like DNA fingerprinting, that could unequivocally assign paternity to particular males. Prior to the molecular revolution, humans relied on blood tests to eliminate putative fathers, but these tests lacked so much precision they were quite incapable of pointing an accusing finger at anyone in particular.
Critics of evolutionary biology say that Darwin took the magic out of life, but nothing could be further from the truth, as the study of female promiscuity and sperm competition proves. Before sperm competition there was much in reproduction—like the buffalo weaver’s pseudo-phallus—that didn’t make much sense. But evolutionary thinking has allowed us to better interpret what we see. And it turns out that the female buffalo weaver is among the most promiscuous of birds, and the male’s sexual attributes are the adaptations that enable him to compete for fertilizations. As this area of academic endeavor has gone from strength to strength, it has become easier and easier to discuss topics like ejaculate size and the significance of orgasms.
What was once perceived as “talking dirty” is now talking truth.
Tim Birkhead is a professor of evolutionary biology at the University of Sheffield in England. His book Promiscuity (Faber & Faber, 2000) explores the issues raised in this brief essay. His most recent book is The Red Canary (Weidenfeld & Nicolson, 2003). In the U.S., it was released as A Brand-New Bird (Basic Books, 2003) and has been widely praised. Birkhead frequently contributes to such venues as New Scientist and BBC Wildlife. He is the winner of the Consul Cremer Prize for The Red Canary and is the past president of the International Society for Behavioural Ecology.
Responses from:
Tim Birkhead | Paisley Rekdal | Bruce Begemihl | Melanie Domenech Rodriguez | Dorianne Laux | David Shields | Gary Paul Nabhan