The animal kingdom is rife with different mechanisms for sexual reproduction, but the one we are most familiar with (as it was so eloquently stated in Kindergarten Cop) is that boys have a penis, and girls have a vagina. Female hyenas have pseudopenises, but that’s mostly just for show. There have not been any known species where the female naturally has a true, functional penis until now.
However, male and female isn't defined by genitals, it's defined by gametes. Males produce sperm, females produce eggs.
In a cave in Brazil, an insect has been discovered that has completely swapped genitals. The new genus has been dubbed Neotrogla, and the insects themselves are about 3 mm long and kind of look like flies. Not only does this female have such a relatively large piece (called a gynosome) but she knows how to use it. The results of this discovery have been published in Current Biology.
When it is time to mate, the female mounts the male, inserts her gynosome into his vagina-like genitalia, and they begin a copulation session that is about 40-70 HOURS long. After it has been inserted, the gynosome curves inside of the male, locking the two together. This connection is so strong that a researcher who tried to pull the two apart during the deed actually ripped the male’s body in half, but didn’t separate their genitals.
When the male transfers ejaculate over to the female, it is believed to come with loads of nutrients to help the female with the eggs. In caves where food isn’t always plentiful, that can be a huge prize to be won for the female, and is very taxing for the male. As a result this has caused Neotrogla females to compete for males to get those precious resources.
Typically in the animal kingdom, the male is able to make and spread gametes like crazy, since most males don’t play any role in rearing their offspring. For the female, it’s much more energetically taxing to make eggs and care for them, leaving the females to be choosier about their partners. This is reversed in Neotrogla due to that nutritional gift from the male to the female, leading the male to be more selective about his mates. The researchers believe that this extreme choosiness actually led to the eventual reversal of the genitalia.
Graphical abstract from the paper:
Image credit: Yoshizawa et al., Current Biology (2014)