Evolutionary Consequences
One evolutionary response to sperm competition is the variety in penis morphology of many species. For example, the shape of the human penis has been selected through sperm competition. The human penis has been selected to displace seminal fluids that were implanted within the female reproductive tract by a rival male. The thrusting action which occurs during sexual intercourse manually removes seminal fluid out of the cervix area from a previous mating.
Evolution to increase their ejaculate size in the presence of sperm competition has a consequence on testis size. Large testes can produce more sperm required for larger ejaculates, and can be found across the animal kingdom when sperm competition occurs Males with larger testes have been documented to achieve higher reproductive success rates than males with smaller testes in male yellow pine chipmunks. Male yellow pine chipmunks that had large testes fathered more offspring than males with smaller testes.
In some insects and spiders, for instance Nephila fenestrate, the male copulatory organ breaks off or tears off at the end of copulation and remains within the female to serve as a copulatory plug. This broken genitalia is believed to be an evolutionary response to sperm competition. This damage to the male genitalia means that these males can only mate once.
Read more about this topic: Sperm Competition
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