Propagating the species is a much larger investment for females than males for almost every species. While most males have basically an unlimited amount of gametes to spread their genes onto the next generation, the ladies have to make (relatively) gigantic eggs which take up a lot of resources. Many mothers then have to carry their young inside them or find a safe place for the eggs to develop, sometimes providing constant protection. Depending on the species, the mother also has to provide for her young for a considerable amount of time after it is born, while the father may or may not be involved.

Being a mom can be fairly exhausting, which is why females typically show much more discretion when choosing a mate. If you’re going to go through all of that, the guy (and his genes) had better be worth it. Females typically respond best to the most virile males as a means of ensuring that her babies will be in good health with a better chance for survival. It would be a shame if all of that effort and resources were all wasted.

If all other aspects in life were equal, which parental role would you choose to take on? Sure, as humans we could rationalize the situation and make the decision based a variety of factors like who wants to feel the baby kick or stay at home to breastfeed. But, if we look at this objectively at simpler lifeforms, the answer is clear: being the male is the better way to go because you still get to continue your lineage with considerably less effort.

Flatworms are hermaphroditic and can take on either parental role, based on who does the inseminating. Unable to have a logical conversation about the matter, the flatworms solve the problem in the only way they know how: by trying to stab the other with their penis and the first to inseminate wins. Now, the idea of love or intimacy during copulation is hands down a complete rarity among animals, but it’s not exactly the epic battle that it is with flatworms.

When they approach one another and are ready to mate, it is instantly game on. They sit halfway up and get their double-headed penis ready to strike. This also leaves them exposed to getting hit themselves. Penis fencing can last an hour and result in the flatworms being stabbed multiple times. Ultimately, one deposits sperm into the other and emerges victorious, completing the paternal obligation. The flatworm that lost immediately begins to search for food to make up the resources required for making the eggs.