Biology
Mating Systems and Parental Care in Seahorses and Pipefish
Quick fact
Male seahorses and pipefish are the only vertebrates that experience true pregnancy, with some species brooding hundreds of embryos in specialized pouches—a phenomenon that flips the usual sex roles and drives intense sexual selection on females.
Why this is interesting
In the animal kingdom, it's usually females who invest heavily in offspring. So how did a group of fish end up with males that get pregnant?