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The mystery of the four-headed echidna penis

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Monotremes are the only egg-laying mammals, but they also have a number of other unique reproductive characteristics.

For the males, their testes never descend, they have no scrotum, when not in use, their penis is stored internally and their ejaculate contains bundles of up to 100 sperm that swim cooperatively until they reach the egg.

In most other species, sperm swim individually and it's every sperm for themselves. Unlike other mammals, the monotreme penis is used only for mating and never carries urine.


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