for National Geographic News
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.
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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."
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.
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