A worm that is both male and female, changes its sperm production according to competition and inseminates itself is providing new insights into male fertility.
Sperm, the male reproductive cell produced by an animal's testes and released from its penis, varies tremendously among species. Although they all share the same basic function, sperm differing in size, shape and number probably evolved due to post-copulatory competition. This arises when sperm from different males compete in the female reproductive tract for the honour of fertilising her egg.
Further details: How sperm evolve