Size isn't 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. "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."
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