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.
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"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.
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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