While it’s common knowledge that Pokémon are hatched from eggs, nobody really knows exactly how are these eggs formed in the first place.
Aside from, of course, knowing that eggs are produced after leaving two same-species Pokémon of opposite genders at a Pokémon daycare, not much is known on what actually goes on behind closed doors in the Pokéworld.
Nobody knows since the games and the cartoon series did not actually elaborate on the subject.
Anime writers and the game designers of Pokémon may have probably left that particular detail out since the intended audience, (at the time) were kids.
But while details are scarce, there have been theories.
“I suspect there’s a lot of different ways that different Pokémon go about [having sex],” evolutionary biologist and zoologist T. Ryan Gregory told Vox.
The scientist, who runs the Gregory Lab of Genomic Diversity at the University of Guelph in Ontario, gave his thoughts on the matter pointing out how it made sense that all Pokémon lay eggs.
He explained that since Pokémon spend their time moving around in search of other Pokémon, “a prolonged period of gestation would be very difficult. It would make a lot more sense to have external eggs.”
He further noted how this fact gives a hint on why it was possible for different types of Pokémon breeds to cross reproduce.
“They all go through a very similar early stage—the egg stage—and then their differences arise later in their development,” Gregory was quoted as saying.
Gregory then posits the possibility that most Pokémon have sex in either one of two probable ways.
For most Pokémon, which are “bilaterally symmetrical”, or as Gregory describes it, “two matched sides with a head at the front and a butt at the back,” their mating behavior are most probably similar to common animals. Simply put, any Pokémon that has legs, arms, and a head may probably have genitals and are used the same way we are most familiar with.
For other Pokémon types that don’t have such noticeable parts, Gregory thinks they probably have sex via external fertilization. In such manner, a female Pokémon lays an egg for the male to fertilize later. Male Pokémon can either shoot his sperm into the egg or via a technique called “broadcast spawning”, wherein males shoot “massive quantities of sperm everywhere, which females then take in”.
Interestingly, the Pokémon egg groups, the network which connects physically similar Pokémon and identifies which can mate with each other within the game, also reveal how such different reproduction techniques may exist in the universe.
According to Gregory, the egg groups are quite similar to the real-world phenomenon called ring species, which is basically “the series of connections that unify two things that appear to be quite different”.
An egg group represents a “ring” in the network, he said, and if you were to pick two Pokémon at random, they might not be able to breed together. However, linking them together through a series of Pokémon who could breed together is possible.
When it comes to physical anatomy, it is not really that hard to imagine what different types or how weird-looking Pokémon genitalia can get, considering the ones we have in our own animal kingdom can be quite odd as well.
As Gregory points out, real-world animals can be pretty crazy too: “Duck penises are curved and really long, and cat penises are spiky. If you look at certain insects, the females have special shapes for scooping out sperm from the males, while other insects’ penises break off.”