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SIZE isn鈥檛 everything. Big testes are usually a telltale sign of strong sexual competition between males. But it seems mice can quickly evolve testes that produce more sperm without growing larger.
When many males mate with a female, their sperm compete with each other to fertilise her eggs. Having larger testes allows males to produce more sperm as they bid to pass on their genes. 鈥淭here is often a raffle element to fertilisation,鈥� says at the University of Oxford. 鈥淚f you buy more tickets, you鈥檙e more likely to win.鈥�
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聯There is a raffle element to fertilisation. If you buy more tickets, you鈥檙e more likely to win聰
at the University of Western Australia in Perth and her colleagues found that, when mice evolve with high sperm competition, the males produce more sperm 鈥� but without developing bigger testes. 鈥淲e were wondering how the mice had increased their sperm production in the absence of a change in testes size,鈥� says Firman.
Studies by researchers like of Syracuse University in New York and his colleagues had suggested that species under intense sperm competition have more sperm-producing tissue. But these studies only showed a correlation between sperm competition and the density of sperm-producing tissue, and couldn鈥檛 prove that one leads to the other.
To test what was happening in mice, Firman鈥檚 team allowed them to evolve 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 all mated with the same group of females. Just 24 generations later, testes from polygamous males contained more sperm-producing tissue than those of monogamous males (Evolution, ).
L眉pold says this is the clearest evidence so far that the level of sperm competition can affect the architecture of testes, and do so in such a short time. 鈥淭his shows that size isn鈥檛 everything,鈥� says Wigby. He points out that blue whales have bigger brains than humans, but aren鈥檛 more intelligent.
The findings are unlikely to overturn our understanding of sperm competition and testes size, Wigby says. 鈥淥verall, you鈥檇 still expect bigger testes in species or populations with much more sperm competition.鈥�
This article appeared in print under the headline 鈥淔orget size 鈥� it鈥檚 what鈥檚 inside that counts鈥�