Between the ducks of reddish colorthe new vegetation exhibits variations in its length, depending on the competition between males, says Patricia Brennan of the University of Yale, in the United States.

The conflict between the sexes may have unexpected effects, at least in the case of these birds. The reproduction of the ducks is the focus of an investigation intriguing that aims to clarify the unusual conformation of their reproductive organs, significantly different to those of other avian species.

In species where the copulation forced is common, the males have evolved penises of greater length, while females have adapted their vaginas in a helical shape opposite to that of the penis, making it difficult for penetration, says Brennan.

This study originated in the wake of another research of Yale conducted in 2007, in which he described for the first time the peculiar genital anatomy of the ducks. Only three percent of the bird possesses a phallus, as is the case of the ostrich and the ducks. During the act of copulatory, these animals extend your penis, which remains hidden within his body.

Certain species, such as ducks, are known for their “explosive erections,” achieved by filling their penises with lymph to rapidly increase pressure for a short period. Their considerable size gives them an advantage during forced copulation. Researchers conducted several high-speed recordings and observed that the copulation process in ducks takes only half a second.

After conducting numerous experiments using glass tubes that mimicked the shape of a female duck’s vagina, the researchers concluded that this anatomy makes forced copulation difficult, which, according to them, offers new insights into the sexual conflict from a reproductive perspective: “The evolution of duck anatomy to regulate reproduction is another illustrative example of sexual conflict in the animal kingdom,” concludes Brennan.

The study indicates that, in several duck lineages, females have independently developed more complex oviducts, just as males have evolved larger penises. In other lineages, females have experienced a reduction in the complexity of their oviducts, just as the penis has evolved to smaller sizes.

According to the hypothesis presented by Brennan, this dynamic evolution suggests that the females of waterbirds have developed these anatomical features particular as defensive measures, in order to avoid attempts by male master playback.

According to Prum, “despite the fact that the majority of the aquatic birds form junctions monogamous, the copulations forced by other males, which are the equivalent of avian rape, are common in many of these species; the length of the phallus in each of the species is strongly related to the frequency of these copulations forced.”
Brennan notes, “In response to efforts by forcing males to assume paternity, waterfowl, females may be able to exercise their own methods of behavior and adaptations anatomical to procreate their offspring.”

The authors propose that the sexual organs complexes and the oviducts of females have evolved in response to the dynamics of the males, who have developed the most complex structures to overcome the barriers of exclusion, and vice versa. These physical structures, according to speculate the authors, can be easily overcome when the females collaborate during the process of copulation, but in the absence of cooperation only serve to prevent approaches unwanted.

Brennan is extending these findings by using a more in-depth research on the development and evolution of the genital organs in birds. “I am convinced that there are more surprises for you to discover,” he says. His future research will focus on examining different lineages of ducks and other birds that exhibit variations in the reproductive biology and the complexity of their genitals.

Here is the link to the study.