There was a time when seahorses meant little to me. They were pleasant to look at in an aquarium. They seemed to show up a lot on the walls of restaurants near beaches. But as is so often the case in nature, there’s bizarre biology lurking under the surface. Specifically, inside the male seahorses. When it’s time to make new seahorses, the male seahorses get pregnant.

Their pregnancy seems bizarre because it is rare. In most species that keep their young inside a parent, the job goes to the mother. But there is a deep symmetry to these two ways of reproducing. That’s a general rule when it comes to evolution: time and again, biologists find the same underlying principles driving