Small, but very efficient (Image: Redmond Durrell/Alamy)

Size isn’t everything. For decades, we’ve known that bigger balls are a sign that there is strong sperm competition between males. But now a laboratory experiment has shown that when many male mice mate with the same females, their descendants can quickly evolve testes that can produce more sperm even though they’re not any larger.

Primate researchers in the 1970s observed that the size of an animal’s testes seemed to be linked to its mating system. When many males mate with the same females, males of that species tend to have larger testes. This enables them to produce more sperm, to help them outcompete other males who might have partnered with the female and increase the chances of passing on their own genes.

“There is often a raffle element to fertilisation,” says Stuart Wigby at the University of Oxford. “If you buy more tickets, you’re more likely to win.”


Is bigger better?

This theory of bigger being better for promiscuous species was supported by a now iconic study from 1981 that found evidence across primates.

Now, real-time evolution played out in lab-reared house mice has added a new twist to the tale.

Renée Firman at the University of Western Australia in Perth and her colleagues had previously found that, when mice evolve in conditions in which there are three females and three males, the males produce more sperm – but they somehow manage to do this without developing bigger testes.

“We were wondering how the mice had increased their sperm production in the absence of a change in testes size,” says Firman.

What’s inside counts

Studies by researchers like Stefan Lüpold of Syracuse University in New York and his colleagues, have suggested that there’s more to it than meets the eye.

His team has found that bird species under intense sperm competition had more sperm-producing tissue in their testes. But these studies just showed a correlation between sperm competition and the density of sperm-producing tissue, and could not prove that one caused the other.

To test what was happening in mice, Firman’s team put the animals in two different mating systems: a monogamous system in which males did not have to compete for females, and a polygamous one, in which males shared the same group of females – a situation closer to what would happen in the wild.

Just 24 generations later, testes from polygamous males contained more sperm-producing tissue than those of monogamous males.

More productive

“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,” says Firman.

Lüpold agrees. “This study provides the clearest evidence so far that the level of sperm competition can affect the architecture, and likely the function, of testes, with significant changes seen after just a few generations of selection,” he says.

Wigby compares the finding to brain size. Blue whales have bigger brains than humans, he says, but aren’t more intelligent. “This shows that size isn’t everything,” he adds.

But he says that Firman’s findings are unlikely to overturn our understanding of sperm competition and testes size.

“There are only so many efficiency savings a species can make. Overall, you’d still expect bigger testes in species or populations with much more sperm competition.”

Journal reference: Evolution, DOI: 10.1111/evo.12603