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But how is the role of each sex determined in nature?Why do males generally compete for access to females?

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These exceptions enhance our knowledge of how nature and evolution work.

Sexual stereotypes are not as universal as traditionally thought.

Males frequently exhibit adaptations in the genitalia or the ejaculates (for instance, regarding seminal fluid proteins) to improve sperm transfer, stimulate a female's uptake of sperm, or to outcompete rivals.

In some species, such as in some damselflies, males even remove the sperm transferred to the female by previous males.

According to Trivers and the Bateman principle, sexual selection is stronger in the sex that allocates fewer resources to parental investment.