A team of researchers at the University of Florida has solved the mystery of what some call “one of the most puzzling events in evolution”: the reduction and loss of the penis in most male birds.
About 10,000 species of birds have reduced or absent external genitalia as adults. Many have normal penises as embryos, but as they develop, their penises stop growing and shrink away. (Despite that, male birds still manage to fertilize female birds through internal insemination, just like humans. We’ll get to how in a moment.)
To study how male birds lose their penises, the UF researchers examined the embryonic development of birds with penises (ducks and emus) and birds without penises (chicks), among other creatures.