Long Duck Dongs: Evolution and Sexual Organs

When she first visited in January, the phalluses were the size of rice grains. Now many of them are growing rapidly. The champion phallus from this Meller’s duck is a long, spiraling tentacle. Some ducks grow phalluses as long as their entire body. In the fall, the genitalia will disappear, only to reappear next spring.

The anatomy of ducks is especially bizarre considering that 97 percent of all bird species have no phallus at all. Most male birds just deliver their sperm through an opening. Dr. Brennan is investigating how this sexual wonder of the world came to be.

Part of the answer, she has discovered, has gone overlooked for decades. Male ducks may have such extreme genitals because the females do too. The birds are locked in an evolutionary struggle for reproductive success.

Dr. Brennan was oblivious to bird phalluses until 1999. While working in a Costa Rican forest, she observed a pair of birds called tinamous mating. “They became unattached, and I saw this huge thing hanging off of him,” she said. “I could not believe it. It became one of those questions I wrote down: why do these males have this huge phallus?”

A bird phallus is similar — but not identical — to a mammalian penis. Most of the time it remains invisible, curled up inside a bird’s body. During mating, however, it fills with lymphatic fluid and expands into a long, corkscrew shape. The bird’s sperm travels on the outside of the phallus, along a spiral-shaped groove, into the female bird.