Would you rather build and house and sing to attract a mate, or sneak into someone else's house? Young male midshipman face this choice early in life and must commit to a lifelong strategy. That's the plainfin midshipman (a fish), not naval cadets.
Some male midshipman opt for the straightforward competition of singing and letting females choose the best singer. But smaller "sneaker" males wait for a female to select a male and then sneak into the mating party and try to win by what's called "sperm competition."
To help, the small sneaker males have one body part that's much larger than the singers, the testes. The testes of sneaker males are 15% of their body weight, that would be 25 pounds in an average human male. This allows the sneakers to produce more sperm than the singers in an attempt to win the sperm competition. Improbable? Scientists believe that sperm competition is a part of human biology, although 25 pound testes are thankfully not part of it.