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. …

What they found was that a critical gene called Bmp4 switches on, causing developing genitals to wither away. In other birds like ducks and emus, that gene stays switched off, allowing their penises to grow fully. (In some birds, they grow a little too fully: certain species of water fowl, like ducks, have such large phalluses, they can exceed the length of the body.) …

Says Ana Herrera, UF graduate student and lead author on the study: “One idea is that choosing to mate with males who have a reduced or absent penis confers some advantage to females by giving them control over copulation and the paternity of their offspring.”