For Mice, Bigger Balls Aren't Always Better

Yes, those balls. You know the ones I'm talking about. The man rocks, the family jewels, the subject of that ACDC song we've all tried to forget about. In the great mammalian sex race, a male with big balls is likely a very successful reproducer, being both dominant and popular with the ladies. New research has found that with mice, however, that this isn't always the case. The best balls, experts are now claiming, don't always take up a lot of space.

That's at least according to a study recently published in the journal Evolution, which details how sexual selection in mice doesn't appear to encourage larger testicles, as seen in other mammals. Instead, where there are plenty of ladies to go around, male mice seem to be developing testes that can produce more sperm without growing in size.

That's a lot different from how things are in nature. As seen in lions, gorillas and other large mammals, dominant males - the ones that see the most sexual partners in their lifetime - often also boast larger testicles. This, past research argues, is so that these males can boast a larger sperm supply and "recharge time" in order to spread their seed as quickly and frequently as possible.

However, when Renée Firman, at the University of Western Australia, and her colleagues set mice up in a monogamous system where there were so many females that male mice did not have to compete with one another, the most successful breeders wound up not having massive testes, but simply more dense testes.

This density correlated with more sperm-producing tissue, but did not necessarily lead to a spike in testes size. In a polygamous situation, however, where males were in competition to spread their seed, dominant males quickly gained larger testes each generation.

"Our mouse study is the first to provide unequivocal evidence that sperm competition selects for an increase in the density of sperm-producing tissue, and consequently, increased testes efficiency," Firman recently told New Scientist.

She and her colleagues observed that even with smaller testes compared to the polygamous males, the monogamous males were still as productive for the first few generations. However, there comes a point when bigger simply has to be better. In just 24 generations, testes from polygamous males contained more sperm-producing tissue than those of monogamous males, despite that advantage in density.

That is to say, while size isn't everything like once thought, it still matters for a whole lot.

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