Arms race: Sexual conflicts among ducks

Until now, the prevailing assumption in ducks (Anatidae) was "large penis, forced copulation, large eggs." A recent study led by the Konrad Lorenz Institute for Comparative Ethology at the University of Veterinary Medicine Vienna has now refuted this arms race hypothesis.

It seems that the opposite is true: penis length and egg size correlate negatively; the larger the drake's penis, the smaller the duck eggs. This suggests that the evolutionary arms race with forced copulation on the one hand and anatomical countermeasures on the other cannot continue indefinitely in ducks .

Ducks: Vaginal intercourse instead of cloacal kissing

In most birds, mating consists of a cloacal kiss. During this act, the male and female press their shared cloacal opening (used for urination, defecation, and reproduction) together, allowing sperm to enter the female's cloaca. Ducks are different: the drake mounts the female and submerges her underwater. What you don't see is that, unlike most other birds, a penis is everted from the cloaca during mating. Another major difference: these copulations are often forced and frequently lead to serious complications for the females – sometimes even death, for example, by drowning.

Scientifically, these forced copulations are explained as an open sexual conflict and an evolutionary arms race between the sexes. For example, as a defense mechanism, females have coils in their reproductive tract that rotate clockwise, while the coils on the penis of drakes rotate counterclockwise.

Gender arms race

A larger dataset than in previous studies allowed researchers to test the relationship between penis size, forced copulation, and egg size. The results contradict previous assumptions, as Hans Winkler from the Konrad Lorenz Institute for Comparative Ethology at Vetmeduni Vienna explains: “The results of our study force us to reject the previous hypothesis, as egg size correlates negatively with the length of the corkscrew-shaped penises and the number of vaginal coils. The apparent compromise between egg size and the females' morphological defense mechanism—the counter-coiling vaginal tracts—is particularly strong in monogamous species.” Therefore, the arms race with large male penises, and the associated forced copulation and vaginal coils, cannot escalate indefinitely. Overall, the researchers assume that there are factors that establish a lower limit for egg size. This also limits the morphological defense of females (the number of vaginal coils increases with the length of the average male penis) and thus the arms race between the sexes.

Exciting field of research

Many aspects of waterfowl reproductive biology are well-researched due to their commercial value. Nevertheless, important details remain unclear. For example, much work remains to be done to better understand the morphology, defense mechanisms, and selection processes of females. "We can expect numerous surprising findings as more details about the behavior of other waterfowl species in the wild become known. In particular, we anticipate that studies on the behavioral ecology of waterfowl will continue to provide exciting results and insights into the evolutionary dynamics of sexual conflict," explains Hans Winkler, describing his research paper, recently published in the "Journal of Avian Biology," with first author Bernd Leisler from the Max Planck Institute for Ornithology, Radolfzell, Germany.

Why do females accept forced copulations?

As a further defense against forced copulation, females might also abandon eggs resulting from such mating and not incubate them. However, this is not the case. Scientists (Briskie and Montgomerie 1997, 2007) have previously explained why they do not abandon their clutches as follows: Females that produce large, and therefore "costly," eggs relative to their body mass are not inclined to abandon their "investment"—that is, a large egg—even if they have been victims of forced copulation. From an evolutionary perspective, this would, in effect, encourage males to continue developing large penises and perpetuating forced copulation. This assumption has now been refuted by the present study. Learn more about the fascinating biodiversity of the animal kingdom at wildbeimwild.com.