Multiple partners may be the key to agile sperm that get the job done, suggests a new study in mice.

After only 12 generations, mice brought up in a polygamous mating group developed stronger and faster sperm and had better mating success than their monogamous counterparts.

The finding sheds light on sexual selection, which posits that showy traits like a peacock's vibrant tail or the bright colors of guppies provided an advantage in wrangling mates and passing on one's genes. Recent research is showing that evolution of these sexually selected traits happens at the molecular and cellular levels, too – and can even happen after breeding has occurred. Sperm competition between males, originally proposed in the 1970s, is a type of sexual selection for sperm traits that would increase the likelihood of fatherhood.

Sperm competition happens more often during polygamous mating than monogamous mating, since more potential mates and their sperm are involved. For a male to pass on his genes to offspring, he needs better, faster and stronger sperm. Earlier research has shown that polygamy in Drosophila, the fruit fly, increases sperm competition and likelihood of parentage, though fruit flies are invertebrates with some very specialized reproductive plumbing.

When looking at sperm competition in mice, "males evolving with sperm competition (polygamy) had a significant paternity bias over males evolving without sperm competition (monogamy)," the researchers wrote in the Jan. 20 issue of the journal BMC Evolutionary Biology. So those mice with multiple mates, over several generations, were more likely to father children than the monogamous males, the first time this has been seen in vertebrates.

Mouse mating

Researchers Renée Firman and Leigh Simmons at the University of Western Australia created the mice by preferentially breeding them in either polygamous relationships or monogamous relationships for several generations.

After eight generations they noticed that the sperm of the polygamous mice were stronger and faster than the sperm from monogamous mice.

"In the eighth generation, we observed genetic divergence in ejaculate quality; males evolving under sperm competition had higher sperm numbers and better sperm motility compared to males with a selection history of monogamy," the researchers write.

Next, they wanted to see how the sperm differences affected a male's ability to compete for parentage, so they pitted the monogamous mice against the polygamous mice.

When a female mates with two males, the first to mate usually has an advantage over the second. To test if the polygamous males could out-compete their monogamous counterparts the team randomized the order of mating. Even when they were the second to mate, the polygamous males were the sole father of 33 percent of the litters, while the monogamous line took the litter only 14 percent of the time. The rest of the litters were of mixed parentage.

"The polygamous males outcompeted their rivals by reaching the ova first and/or by penetrating the ova first, and thereby gaining greater numbers of fertilizations," they write.

Choices, choices

While it's understandable that higher-quality sperm mean greater mating success, the fact that it evolved so quickly (in just 12 generations) in the differentially mated populations is pretty surprising and reinforces in important role that sexual selection plays in reproduction.

"Sexual selection, and in particular this post-copulatory sex selection, must be extremely strong to drive such rapid change in such a small amount of time," said Heidi Fisher of Harvard University who was not involved in the study.

When female mice breed with multiple partners, the males are competing for parentage. Stronger, faster sperm from the male are likely to be more successful at reaching and penetrating the ova, but past research has shown that the female also has an ability to preferentially pick and choose the father of her children, even after copulation.

This "cryptic female choice" isn't well understood, though it's possible that in addition to males manipulating their sperm, the females may also be able to preferentially implant eggs fertilized by males from the polygamous lines.

"Most females in nature do not mate monogamously, and since they will usually mate with many males, it opens an ability to manipulate the parentage," Fisher told LiveScience. This manipulation can happen in many ways, including changes to sperm seen in this new study, which "showed that those differences actually translate into different reproductive successes."

This article was reprinted with permission from LiveScience.

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