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Thursday, Feb. 14, 2002
Of arms races and sex battles
By ROWAN HOOPER
On Valentine's Day, what better subject to tackle than sex? Well, maybe love, but that's not what gets evolutionary biologists all hot and bothered. Sex is where it's at -- the battle between the sexes. Males and females interact like two superpowers engaged in an arms race -- each escalation in arms is matched by escalation on the other side. A relationship of apparent mutual harmony might in fact be more one of mutually assured destruction.
John Maynard Smith at the University of Sussex said it was an "evolutionary scandal" that biologists still couldn't explain the purpose of sex -- why males and females mix DNA. After all, many species of insects, amphibians and reptiles reproduce without having sex -- they clone themselves. Mark Ridley of the University of Oxford called sex "the ultimate existential absurdity."
The problem is this: Sexual reproduction forces us -- and all other sexual organisms -- to reproduce at half the rate that we could by parthenogenesis, that is, by cloning. Half the rate, because sexual organisms have to waste half their resources in making males -- clonal organisms like aphids don't need sperm, so they just produce females. Since natural selection automatically favors organisms that reproduce most efficiently, why haven't clonal reproduction methods replaced sexual ones? There must be some advantage to sex, that offsets the numerical advantage gained by cloning. Just what that advantage is remains the subject of intense research.
So evolutionary biologists have a good excuse for their sex obsession. There is no obvious reason why sex should exist, yet as we all know, it does, and forms a large part of our lives. It is one of the most mysterious of biological processes and centrally important to evolution. It throws up many other, deeply rewarding areas of research. Why, for example, are there differences between males and females?
At the root of the evolutionary power of sex, and of all differences between males and females, is the size difference between eggs and sperm. Sperm are tiny, so males can produce more of them than females can large eggs. This means that the number of offspring a male can make is limited only by the number of females he can gain access to. Conversely, females can get all the sperm they need from one male, so in theory their reproduction is not limited by the amount of males they come across. (Even a woman who became pregnant as often as possible could produce only a maximum of about 30 children in her life. A man who had unlimited access to fertile women could father thousands of children.)
This is Darwin's theory of sexual selection. It means that, in general, males will compete for females, and females will choose among males. Males will want to have sex with any female, while females will want to be choosy -- this antagonism between the sexes is responsible for many of the differences between males and females. (Readers in a combative mood may wish to substitute "men" and "women" for "males" and "females," though it might not make a good prelude to a Valentine's dinner.)
Whenever the aims of two sides are opposed, there is the potential for an arms race. In a military sense, a build-up in arms on one side is quickly matched by the other. The balance of power, however, remains relatively unchanged, because both sides are equally well-armed. For biologists, this means that arms races and their consequences may be difficult to detect. But now Goran Arnqvist of the University of Uppsala in Sweden and Locke Rowe of the University of Toronto, Canada, have for the first time demonstrated the existence and dynamics of a sexual arms race. Their paper is published today in Nature. Their study animal: the water strider, a semi-aquatic insect often seen skating on the surfaces of ponds.
Water striders engage in a violent premating struggle, in which males attempt to harass females into having sex, and females try to dislodge them. It pays males to have sex whenever they can, but mating is costly for females: It uses up valuable energy, it increases their vulnerability to predators and decreases the time they can spend foraging for food. In short, females would rather have sex when it suits them, and with a male of their choice.
Male water strider "arms" include specialized genitalia that they use to clasp hold of females, and a flattened abdomen which allows them to hold on more firmly. Females have responded to this with morphological counteradaptations of their own, such as abdominal spines to weaken the male grip.
What Arnqvist and Rowe looked at was how deviation from the arms-race stalemate would affect the success of each side. By comparing 15 species of water striders, each with different levels of armament, they found that the balance of power continually shifts. When males have the upper hand (when their armaments are better than the females') they get what they want: a high rate of mating. When females hold the balance of power, mating rates go down.
Despite being such an important part of sexual selection theory, direct evidence for biological arms races has been hard to come by.
"Perhaps it's human bias -- our Victorian heritage still affecting the way science is done," said Arnqvist in an e-mail interview, "but arms races are 'hidden' because of their balanced nature. Interactions may remain the same across populations or species, leading us to believe that these things are not very potent forces."
And here's something else to ponder across the candlelit dinner table. "Many 'arms' probably don't look like 'arms' at all," said Arnqvist. "Many showy male traits, for example, may actually be antagonistically coevolving with female resistance to such male seduction/stimulation traits."
In other words, arms don't have to be weapons. A showy trait might include that expensive bottle of wine that your date pours for you this evening. For some women, that might be enough; others might not be impressed -- they might resist the wine-seduction ploy and hold out for diamonds, while batting mascara'd eyelashes at the man on the next table.
E-mail Rowan Hooper at firstname.lastname@example.org