Arms Race: Sexual Conflicts in Ducks

Until now, the assumption for ducks (Anatidae) was «Larger penis, forced copulation, larger eggs.» A recent study led by the Konrad Lorenz Institute of Comparative Ethology at the University of Veterinary Medicine Vienna has now been able to refute this arms race hypothesis.

According to the findings, the opposite appears to be the case: 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 between forced copulations on one side and anatomical countermeasures on the other cannot continue indefinitely in ducks.

Ducks: Vaginal intercourse instead of cloacal kiss

In most birds, the sex act consists of a cloacal kiss. The male and female press their shared bodily opening — used for urine, waste, and reproduction — together, allowing sperm to enter the female’s cloaca. Ducks are different: the drake mounts the female and pushes her underwater. What is not visible: unlike most other birds, a penis is everted from the cloaca during the sex act. Another major difference: these copulations are not infrequently forced and often lead to serious complications for the females — up to and including death, for example by drowning.

These forced copulations are scientifically explained as an overt sexual conflict and an evolutionary arms race between the sexes. For instance, females have, as a defensive measure, coils in the female reproductive tract that turn clockwise, while the coils on the drake’s penis run counter-clockwise.

The battle of the sexes

A larger dataset than in previous studies allowed researchers to test the relationship between penis size, forced copulations, and the size of eggs laid. The results contradict previously held assumptions, as Hans Winkler from the Konrad-Lorenz-Institut für Vergleichende Verhaltensforschung at Vetmeduni Vienna explains: "The results of our study compel us to reject the previous hypothesis, as egg size correlates negatively with the length of corkscrew-shaped penises and the number of vaginal spirals. The apparent trade-off between egg size and the morphological defense of females — counter-spiraling vaginal tracts — is particularly pronounced in monogamous species." Accordingly, the arms race involving large drake penises and the associated forced copulations and vaginal spirals cannot escalate without limit. Overall, the researchers assume that there are factors establishing a lower limit for egg size. These also constrain the morphological defense of females (the number of vaginal spirals relative to the length of the average drake penis) and thereby the arms race between the sexes.

A Fascinating Field of Research

Many aspects of the reproductive biology of waterfowl are well studied due to their commercial value. Nevertheless, important details remain unclear. There is, for example, still much work to be done to better understand the morphology, defenses, and mate selection mechanisms 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 yield exciting results and insights into the evolutionary dynamics of sexual conflict," explains Hans Winkler, regarding his research just published in the journal "Journal of Avian Biology" with lead author Bernd Leisler from the Max-Planck-Institut für Ornithologie, Radolfzell, Germany.

Why Do Females Accept Forced Copulations?

As a further defensive measure against forced copulations, females could also abandon eggs resulting from such a mating act and not incubate them. This is not the case, however. Why they do not abandon their clutch has previously been explained by science (Briskie and Montgomerie 1997, 2007) as follows: females that produce large and thus “costly” eggs relative to their body mass are not inclined to abandon their “investment” — i.e. a large egg — even if they were victims of a forced copulation. This would, from an evolutionary perspective, essentially invite males to continue developing large penises and maintaining forced copulations. An assumption that has been refuted by the study now available. More about the fascinating biodiversity of the animal world at wildbeimwild.com.