Sex is far from a perfect way to reproduce. It imposes a huge cost on a species, and that cost is called “males.” If roughly 50 percent of a species is made up of males who are incapable of producing babies, it is at a serious reproductive disadvantage relative to another species made up mostly of females capable of reproducing on their own.

And an animal that reproduces by herself has a big advantage when moving into new territory, because she doesn’t need a partner to be fruitful and multiply. Every single one of her babies will also produce its own offspring. Sexual reproduction “seems like a simple thing, but from an evolutionary perspective, it’s so inefficient,” says Rob Denton, who studies unisexual salamanders at Ohio State University. “It’d be so much easier if everyone were female.”

Brains don’t come pre-wired to act male or female, but are organized by sex chromosomes, hormones, environment, and experience.