Highlights:

[...] Therefore, it is in their interest to mate with as many females as possible to increase their chance of passing on genetic material. This is one hypothesized reason for males having two penises instead of one: as each hemipenis is associated with one testis and only one side can be used during mating, having a second hemipenis functions as a "backup" and ensures that mating can continue even if one side were to run out of sperm.

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The surface of hemipenes is one of the most interesting and unique features, and is often covered in sharp spines and spicules that are organized in formations called rosettes.

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Hemipenes are usually held inverted within the body, and are everted for reproduction via erectile tissue, much like that in the human penis.