Male rodents with less time to breed grow bigger testes

A plains mouse (Pseudomys australis) Roland Seitre/Minden Pictures/Alamy
Shorter breeding seasons lead to larger testes in male rodents. This is probably due to increased sperm competition among males.

Research has shown that males have larger testes than females with multiple partners. Because their sperm must compete within a female reproductive tract to fertilize eggs, it is beneficial to have larger testes that can produce more.

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