There's been much talk over the last week in the US about deflated balls. But enough about American football. Penny Sarchet reports at newscientist.com that balls of a different sort are not deflated at all....
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.
Isn't science fascinating?
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.But... science is always investigating.
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.
Making better use of space is something everyone can relate to, right?
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.The poly males were rising (so to speak) to the occasion.
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.