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Penises are invariably interesting. They are also interestingly variable. Cockly varieties aren't just those apparent in the shower at the Y-- differences of size, proportion, shape, and color. From beetles to baboons, penises vary strikingly between species, say students of genitalia. And within species, penises change at a faster rate than other structures. The dean of evolutionary theory, Charles Darwin, weighed in on the penis problem. He favored an explanation in terms of locks and keys-- penises change, he said, in an ongoing arms race to more uniquely fit the corresponding female sex organs, the better to exclude cocks of closely related species. Now a Swedish biologist, writing in Nature, has announced a sexier explanation. Penises evolve and specialize because, like peacock feathers, females find them beautiful. And what's beautiful is subject to fashion's whimsy.

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To probe the penis question, University of Umea biologist Göran Arnqvist got on his hands and knees-- and looked to insects. Human dicks may have intriguing helmet-shaped glanses and foreskins (or not have them); monkey penises, flanges that look like wings; feline phalli, a carpet of tiny spikes-- all superfluous to the basic function of delivering sperm to egg. But for a truly accessorized phallus, the insect world has larger creatures beat. Some kinds of crickets and cockroaches, for example, have penises with titillators for stimulating a female's G-spot equivalent, clasps for holding a wriggling mate in place while the deed is done, brushes for sweeping from the female's reproductive tract a rival suitors' sperm, and such intriguingly-named parts as cerci, genotremes, epiprocts, and pariprocts. One sort of roach even has a phallus that unfolds like a Swiss-army knife. Getting these penile elements straight is sometimes the only way-- for humans anyway-- of telling apart otherwise identical looking species.

On the lock-and-key theory of penile variation, these embellishments evolved to prevent a male from a closely related species from mating across the special divide. The hybrid offspring that are the progeny of such pairings often are sterile or genetically compromised. And cross-species matings deprive same-species males of a chance to tryst. Arnqvist reasoned that if the lock-and-key theory were right, then in species where females mated with only one male, penises would be more unique, given that the stakes of that one pairing were so high. But in species where females sleep around with many males, the genetic stakes in any particular pairing would be less. So correspondingly, he anticipated, would be penile uniqueness.

To test his theory, Arnqvist needed an objective way to measure penile morphology-- readers will appreciate the enormity of the task-- and data on insect mating patterns. When all was said and done, the results were conclusive. Penile variation was far greater in species where females were promiscuous-- the result one would expect if female choice were driving penile changes.

There is a penile arms race, Arnqvist contends, but it's fought with the goal of attracting females with a pretty cock-- as decided by feminine taste-- not keeping rival species' penises from fitting into females. Penile form, that is to say, is a result of what's known as sexual selection-- changes to the gene pool wrought by what individual creatures find attractive in a mate. Natural selection, by contrast, does evolution's primary work-- by killing off unfit individuals before they can pass on their genes. But sexual selection changes species through mating choices of the reproductively fit, with females being generally the choosier sex. If natural selection makes creatures fit to catch their dinner and weather desert dryness or winter snows, sexual selection helps make them beautiful. A peacock's plume may not help him find bugs to eat or escape a fox, but it makes him attractive to peahens-- and often admiring humans. By nature's own lights, Arnqvist's research indicates, penises exist for pleasure and ornament.

It turns out Darwin wasn't so wrong. The secondary sexual characteristics, (pubes, axial hair, pheromone-rich sweat) are often chalked up to the effects of sexual selection, as opposed to the naturally-selected primary sex characteristics (penises, testes, ovaries, breasts, etc.). But Darwin insisted that the line between the two was fuzzy. Even Charles Darwin, eminent Victorian, was prepared to grant that penises are beautiful. Now there's proof. **

Editor's Note: from The Guide, August 1998