A plains mouse (Pseudomys australis)Roland Seitre/Minden Pictures/Alamy
Male rodents with shorter breeding seasons tend to have larger testes, probably because the time pressure increases sperm competition between males.
Studies have found that males typically have bigger testes when the females of their species have multiple mating partners. This is because their sperm must compete within the female reproductive tract to fertilise the eggs, meaning it is advantageous to have bigger testes that can produce more sperm.
“It’s like a raffle – the more tickets you have, the more likely you are to win,” …

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