"Sperm Wars": Voles Follow Their Noses to Win

James Owen
for National Geographic News
September 22, 2004
A male meadow vole faces a big problem: How can it become a father when a female meadow vole is likely to mate with every other male in the neighborhood?

Sperm competition is a familiar challenge for thousands of male animals. Many males from species as diverse as fruit flies, salmon and chimpanzees are locked in "sperm wars" with their rivals. Because females of these species mate with more than one male, each male's sperm must fight with a competitor's to fertilize a female's eggs.

Success is measured by the number of babies each male sires.

Tactics vary enormously. The penis of the male black-winged damselfly, for example, is adapted to act like a scrubbing brush. It can remove up to 100 percent of previously deposited sperm.

This option isn't open to the male meadow vole, or any other mammal. But a new study, published tomorrow in the science journal Nature, suggests the male vole employs another ingenious tactic: It uses its nose to sniff out potential rivals. If they are detected, the male reacts by unleashing an extra dose of sperm while mating.

The researchers behind the study believe it's the first one to make the connection between smelling and sperm competition. They add that many other mammals may also use their sense of smell to assess the risks of sperm competition.

"There are no similar studies done with other species," said co-author Javier delBarco-Trillo, a biologist at the University of Memphis, Tennessee. "However, we can make the assumption that what we have found for meadow voles may be applied to many other mammalian species."

The reason, delBarco-Trillo said, is that olfaction is the main form of communication among mammals, while sperm competition is also widespread.

"Less than 5 percent of mammalian species are monogamous. The rest show different degrees of promiscuity," he added. "That means that in most mammalian species, females are likely to copulate with more than one male during each reproductive bout."

Male Odor

In the study, male meadow voles were paired with sexually receptive females in two cages. One cage contained the scent of another male, while the other cage did not. All ten of the male voles tested were seen to investigate this odor (a testosterone-related compound) before and during copulation.

The researchers also found that males that copulated in the scented cage produced a lot more semen. The increase came in the number of sperm allocated in ejaculations rather than in the number of ejaculations.

Males in the "no odor" cage ejaculated an average of 98 million sperm, against 169 million sperm when responding to the threat of competition. "That is a very significant difference in numbers," delBarco-Trillo said.

In mammals, scientists believe, sperm competition is mainly a numbers game, the researcher said.

"In many species of insects, males are able to remove the sperm of previous males prior to allocating their own sperm," he said. "In mammals, sperm competition is more like a raffle: The male that allocates more sperm has more possibilities of fathering most of the offspring."

Scientists believe this is the reason why many mammals, such as rats and chimpanzees, have larger testicles relative to less-promiscuous species.

According to Jack Palmer, co-author of Evolutionary Psychology: The Ultimate Origins of Human Behavior, 99 percent of a chimpanzee's sperm actually consist of "kamikaze sperm," which function to prevent the sperm of other males reaching the egg.

By contrast, he writes, "The testicles of gorillas are relatively tiny compared to those of chimpanzees. This is because of the sexual monopoly that is obtained by a single silverback male gorilla over his harem of females. In this type of mating system sperm competition is almost nonexistent."

But for most male mammals, the more sperm, the better.

Mice Mates

DelBarco-Trillo notes, for example, that a female mouse that copulates with six males may produce six pups, all by different fathers. He added, "Say that female copulated with only two males, and one of them allocated more sperm than the other male. You would expect the male that allocated more sperm to father maybe four, five, or even six out of the six pups."

In the case of meadow voles, females would normally copulate with four to six males while sexually receptive.

Various possible motives have been identified to explain why female mammals tend to be promiscuous. These include increased chances of successful fertilization and increased genetic variation of offspring.

And it's possible, delBarco-Trillo says, that male mammals may also use their senses of smell on females when weighing the risks of sperm competition.

"Males normally smell the anus and genital area of the female prior to copulation," he said. "So it is possible that males are able to detect the presence of sperm within the reproductive tract." The scientist adds that no data currently exists to support this hypothesis.

Mammals, however, appear to have a long way to go to match the ingenuity of many other organisms when it comes to sperm wars between males.

Males of the South American butterfly Heliconius erato inject females with an anti-aphrodisiac, which repels other potential mates for several weeks. While the parasitic worm Moniliformes dubius basically cements up a female's genital opening after mating.

Chastity belts, it seems, aren't a human invention after all.

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