Any animal whose genital evolutionary pattern resembles a Cold War arms race deserves some attention from the scientific community. The complexity of male and female ducks’ reproductive organs indicates just this. Their reproductive organs have co-evolved in a “sexual arms-race” resulting in large corkscrew phalluses and complicated vaginal tracts.

Surprisingly, the female duck’s reproductive organs have not been studied in detail until recently. Dr. Patricia Brennan and colleagues started to unravel the labyrinthine nature of their vaginal tracts and published on the subject in 2007. Brennan, a post-doctorate researcher at Yale University, is one of the leading figures in the field of research revolving around the duck’s complex genital anatomy.

Research Beginnings

As with many groundbreaking scientific findings, Brennan did not start out studying duck anatomy. Before shifting her research to duck anatomy, Brennan studied the tinamou, a family of 47 species of exotic birds found in Central and South America. The tinamous have similar anatomies to ducks but in significantly more modest proportions. While they both have corkscrew phalluses, the tinamou phallus is significantly smaller in size and proportion than the duck’s. While studying the tinamou, Brennan had a hard time obtaining tinamous for dissection because the only places where they could be found were the Central and South American tropics. Noticing a similarity between duck and tinamou anatomies, Brennan decided to dissect a male duck and was astonished by the results. Brennan recounts, “I saw this large complicated penis and asked myself, ‘Why do these males have such large phalluses?’” Trying to explain the duck’s unusual reproductive anatomy, Brennan changed gears and began focusing on ducks instead of tinamous.

Duck phalluses are strange for several reasons. First of all, the size of their phalluses can be as large as forty centimeters. Some male ducks have phalluses that are larger than their bodies. Unlike mammalian phalluses, duck phalluses are not supported by blood or muscle but rather by lymph. For this reason, there is nothing to support a duck’s erection and the process of reproduction takes place within half a second. To study duck reproduction in detail, Brennan had to use a high-speed camera to capture the very short process. After initial arousal, the duck phallus extends as sperm travel in the outer layer of the penis in a corkscrew path. It takes the phallus just as much time to extend as it takes for the sperm to travel from the base to the tip of the phallus.

A Focus on the Female

By the time Brennan had entered the field, much work had already been published on male duck phalluses and their anatomy. This extensive study of the male duck provoked Brennan to ask, “What about the female?” She began studying the female duck’s anatomy with an amusing saying in mind: “For every car there is a garage.” This approach, apparently, had never been taken before, allowing Brennan’s recently published findings to represent a plethora of new information about the female duck’s reproductive biology.

In most birds, the oviduct resembles a simple tube, but in ducks, the vaginal tract not only spirals inside the duck but also has sacs and crevices that add to its complexity. This is bizarre, considering that 97 percent of birds do not have phalluses or vaginal tracts. Even among the remaining three percent that do have phalluses, like the tinamou, ducks are still an anomaly. There are several theories that attempt to explain the unique evolution of ducks’ sexual organs; however, the most popular theory explaining the reproduction organs of waterfowl states that forced copulation is the cause of the unusual structures of ducks’ sexual organs. Forced copulation, which is unlike the reproduction methods of any other birds, is not uncommon among waterfowl.

An Evolutionary Arms Race

The incidence of forced copulation among mallards, the most common species of ducks, has been confirmed in the literature since at least 1911. But the question remained: why would forced copulation influence the structure of a female mallard’s vaginal tract?

Brennan has collaborated with Richard O. Prum, Yale Professor of Ecology and Evolutionary Biology, to study the correlation between forced copulation and the complexity of duck genitalia. This study showed that the shape of the female duck’s vaginal tract explains the correlation. While male ducks have corkscrew-shaped phalluses, female ducks have anti-corkscrew-shaped vaginal tracts. That is, female ducks have reproductive organs specifically designed to be hard to penetrate. This means that only the fittest males can successfully copulate with female ducks.

To test her hypothesis, Brennan conducted an experiment on a duck farm in California. The ducks on the California farm were genetically enhanced to be more fit and have larger phalluses, which made them perfect candidates for Brennan’s research. Brennan and her colleagues wanted to see if the female duck vaginal tract was indeed harder for a male duck to penetrate. Three synthetic duck vaginas with different structures were created: two tube-like ones, like the vaginal tracts of most birds, and one that was a replica of a common female duck’s vaginal tract. The studies showed that while most of the male ducks had no difficulty penetrating the two tube-like synthetic vaginas, a significant number of ducks could not penetrate the third anti-spiral synthetic vagina. Therefore, only the fittest ducks with the largest phalluses could penetrate the complicated tract. Through natural selection, male and female ducks have co-evolved in the growth of their reproductive organs.

While many aspects of duck anatomy still remain a mystery, the last few years have brought a wealth of new information to the field—with a great impact due to Brennan’s exploration of hitherto unexplored parts of the female duck’s anatomy.