Sex Wars

Dr Martin Brookes

July 2003

The male Mediterranean rabbit flea is, by common scientific consent, a real record breaker. Superficially, it's an unremarkable little beast but when a female flea jumps into his orbit we get a glimpse of his extraordinary charms. Only then, when aroused, does he reveal what is widely regarded as the most complicated penis in the world, a stand-alone tool kit, complete with hooks, barbs, levers and springs.

Outstanding it may be, but the animal world is riddled with similar kinds of devices. Some species of dragonfly, for instance, have clubs and whips attached to the ends of their penises, while others have inflatable penis horns. In ferrets the penis is hooked like a hockey stick, while in cats it is stippled with spines. If the basic function of the male's penis is to introduce sperm into the female, then why does it look more like a weapon of war than an instrument of love?

Well, the penis is not alone in its perversity. The animal kingdom is full of bizarre structures that initially seem to make no sense. The grotesquely enlarged jaws of male stag beetles, the giant antlers of red deer, and the elaborate plumage of the male peacock all seem to be a hindrance to survival rather than a help. But as Charles Darwin realised more than 150 years ago, survival means little in evolution if it doesn't lead to reproductive success. Many fascinating features evolve not because they are advantageous to survival, but because they increase the chances of having offspring.

Sexual selection

Darwin first aired his idea of sexual selection in On the Origin of Species, in 1859, but it was in the Descent of Man, and Selection in Relation to Sex, published in 1871, where his argument reached maturity. Sexual selection, he believed, operated in two different, but interrelated ways. On the one hand, there was the struggle between the individuals of one sex (usually but not exclusively males) for access to the other. It was this innate desire to outdo each other, he argued, that was the driving force behind the evolution of typically aggressive male characteristics and behaviours.

The second way was through female choice. Darwin believed that just as pigeon fanciers could accentuate characteristics they admired through selective breeding, female choice of mate could drive the evolution of elaborate colours and ornamentations that were typical of many males. Darwin had no clear understanding of why males fought and females chose, rather than the other way round, but evidence gathered from across the animal kingdom confirmed the general pattern.

Initially, Darwin's views on sexual selection were greeted with scepticism. While some accepted his interpretation of the male side of the equation, few were willing to accept that females could have such a powerful evolutionary role. Even Darwin's evolutionary ally, Alfred Russell Wallace, doubted that female perception could be so discerning as to mould male characteristics in such an extravagant way.

Revived theory

For the next 100 years, the theory of sexual selection more or less went into hibernation. The empowering concept of female choice jarred against the prevailing sexual politics of the age and most biologists chose to ignore it. The idea did, however, make a brief reappearance in the 1930s when the mathematical biologist Ronald Fisher came up with his theory of runaway sexual selection. By assuming that female choice was under genetic control, Fisher showed the evolutionary plausibility of exaggerated characteristics like the peacock's tale. It was an intriguing theory and now a widely accepted one. But few people took much notice at the time, and the idea was quietly put back to bed.

The renaissance came in the early 1970s, when biologists finally woke up to the fundamental conflict that lies at the heart of sex. The essence of the dispute is the differential investment in gametes. Females produce small numbers of large eggs while males produce millions of tiny sperm. Both sexes want the same thing – to sire as many offspring as possible – but these basic biological differences mean that they have evolved entirely different ways of going about it.

Different strategies

For males, the best strategy is to mate and move on, in an attempt to breed with as many females as possible. Females, however, are more interested in quality than quantity. Because their potential fertility is much lower than males, it pays for them to shop around for the best male deals available.

With the conflict of sexual interests providing a solid theoretical foundation to Darwin's original observations, studies of sexual selection mushroomed. Today, biologists recognise that the battle of the sexes is a war fought on many fronts. A large set of antlers may beat off your rivals as you struggle to protect your female assets or an elaborate tail may grab the attention of a judicious mate, but as war games go, this is only the initial assault.

Sexual selection doesn't always end at insemination. The struggle can continue inside the female, and there seems no limit to evolution's ingenuity. The penis of the Mediterranean rabbit flea is not just an organ of penetration. It's an elaborate tool that can scrape out the sperm of a rival male. The males of some species insert genital plugs into the female reproductive tract to deter rivals from mating. Others provide nutritious sexual gifts in an effort to convince the female to accept their sperm. Fruit flies have extended the war even further. The male's seminal fluid contains chemicals that can mimic female hormones and induce her to lay eggs after he has mated. But females can fight back with chemical weapons of their own. Even with the sperm of many males inside them, they can still exercise some element of choice.

Wars are sometimes difficult to justify, but in the battle of the sexes we're looking for weapons of mass reproduction rather than mass destruction. It's a more subtle kind of conflict. And thanks to Darwin's original insight, the evidence supporting it continues to grow.

Discover the differences between men and women at Channel 4's Battle of the Sexes site.

Find out more

Channel 4 is not responsible for the content of third party sites

Websites

The Nature Journal
www.nature.com/nature/
Use the search option to uncover a huge number of informed and detailed studies, reviews and articles on everything from butterfly inbreeding to the battle between multiple wood mouse spermatozoa competing for fertilization of ova.

The Darwin Foundation
www.darwinfoundation.org
Comprehensive site dedicated to the variety of work carried out by the foundation and their continuing efforts.

Charles Darwin: British Naturalist
www.lucidcafe.com/lucidcafe/
library/96feb/darwin.html
Containing a biography, bibliography and links to numerous other Darwin-related sites, including where to buy video documentaries and books.

Mounting evidence for swinging beetle success
www.bbc.co.uk/nature/news/106news3.shtml
This article highlights a recent study into the lengths certain beetles will go to attract a mate. Females have been observed mounting each other in order to attract greater male attention. For when a male discovers a 'mating' pair he assumes one is male and attempts to break up the couple and take over mating.

Obituary of Darwin, 1888
http://aleph0.clarku.edu/huxley/
CE2/DarwObit.html
An exhaustive text chronicling the life and times of Charles Darwin. Includes details of his family background, insights into his work and further comment from the author, Thomas Henry Huxley, then President of the Royal Society, who devoted most of his career to defending Darwinism.

Darwin Correspondence Project
www.lib.cam.ac.uk/Departments/Darwin/
'One of the triumphs in scope and excellence of post-war publishing in England' said The Sunday Times. The Darwin Correspondence Project exists to publish the definitive edition of letters to and from Charles Darwin, the most influential naturalist of the 19th century.

Books

	The Origin of Species by Charles Darwin (Wordsworth Editions Ltd, 1998) One of the most important and influential books ever written. Covers everything from natural selection, via the struggle for existence, to the survival of the fittest. Get this book
	Descent of Man by Charles Darwin (Gibson Square Books Ltd, 2003) Charles Darwin's seminal book is reissued with an introduction by best-selling author Richard Dawkins. Dawkins' work has been profoundly influenced by the father of evolution, Charles Darwin, and his introduction reveals a number of startling discoveries that mean books about Darwin will have to be rewritten. Get this book
	Almost like a whale by Steve Jones (Black Swan, 2001) An exhilarating update of Darwin's Origin of Species for the modern reader including modern genetics and its implications for evolutionary theory. Get this book
	River Out of Eden: A Darwinian view of life by Richard Dawkins (Phoenix Mass Market, 1996) Nearly 150 years after Charles Darwin formulated it, the theory of evolution is still the subject of considerable debate. Oxford scientist Richard Dawkins is among Darwin's chief defenders. Get this book
	Genetical Theory of Natural Selection by RA Fisher (Oxford University Press, 1999) 'A book that I rate only second in importance within evolution theory to Darwin's Origin of the Species': WD Hamilton FRS, Department of Zoology, University of Oxford. Get this book
	Charles Darwin's 'The Origin of Species': New interdisciplinary essays edited by David Amigoni and Jeff Wallace (Manchester University Press, 1995) From its inception, Darwin's central theory of natural selection could not be contained within the parameters of the natural sciences. Contains contributions from the fields of history, literature, sociology, anthropology and history of science. Get this book
	The Red Queen: Sex and the evolution of human nature (Penguin Books, 1994) This readable book discusses sexual Darwinism, demonstrating how the evolutionary struggle between men and women has shaped our culture and behaviour. Get this book
	The Forgotten Naturalist: In search of Alfred Russell Wallace by John Grenfell Wilson (Robert Hymans, 2000) Biography of Alfred Russell Wallace, who, despite being Darwin's evolutionary ally, still questioned his early theories on female sexual selection. Get this book
	Sexual Selection by James L Gould and Carol Grant Gould (Scientific American Library, 1989) Insights into the many mating systems and strategies practised by species ranging from micro-organisms to insects, birds, mammals and humans. Get this book

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