Sexual conflicts in ducks – an evolutionary arms race

© Thomas Schilling

© Thomas Schilling  1

Until now, the going assumption for ducks (Anatidae) was "large penis, forced copulation, large eggs". A current study led by the Konrad Lorenz Institute of Ethology at the University of Veterinary Medicine Vienna has now been able to refute this hypothesis. The opposite actually seems 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 ducks´ evolutionary arms race with forced copulations on one side and anatomical countermeasures on the other cannot continue without restrictions.

Ducks: vaginal intercourse instead of cloaca kiss

In most birds, the sexual act consists of a cloaca kiss. Males and females press their common body exit for urine, intestine and reproduction against each other, whereby the sperm get into the cloaca of the female. Not so with ducks: the drake mounts the female and presses it under water. What you don't see: In contrast to most other birds, drakes push a penis out of their cloaca during sexual intercourse. Another big difference: Forced copulations are not unusual and often lead to serious complications in the females - even death, for example due to drowning. Such compulsive copulations are scientifically explained as an open sexual conflict and an evolutionary arms race between the sexes. As a defense measure, for example, females have turns in the female genital tract that rotate clockwise, while the turns on the penis of the drake are counterclockwise.

Sexes arms race

A larger data set than in previous studies enabled the researchers to test the relationship between penis size, forced copulations and the size of the eggs laid. The results contradict previous assumptions, as Hans Winkler from the Konrad-Lorenz Institute of Ethology at Vetmeduni Vienna explains: "The results of our study force us to reject the previous hypothesis, since the egg size is negatively correlated with the length of the corkscrew-like penises and the number of the of vaginal spirals. The obvious compromise between egg size and the morphological defense mechanism of the female, the contra-spiral vaginal tract, is particularly strong with monogamous species. ”

Accordingly, the arms race with large drakes penises and the associated forced copulations and vaginal spirals cannot escalate indefinitely. Overall, the researchers assume that there are factors that set a lower limit for the egg size. This also restricts the morphological defense of females (higher number of vaginal spirals the longer the average drake penis is) and thus limits the arms race between the sexes.

Exciting research area

Because of their commercial value, many aspects of the reproductive biology of waterfowl are well researched. However, important details remain unclear. For example, there is still a lot to do to better understand the morphology, defense and selection mechanisms of females. “We can expect a lot of surprising insights once we learn more details about the behaviour of other waterfowl species in the wild. We assume that studies on the behavioural ecology of waterfowl will continue to provide particularly exciting results and insights into the evolutionary dynamics of sexual conflicts, ”says Hans Winkler about his recently published article with Bernd Leisler from the Max Planck Institute of Ornithology in Radolfzell, Germany.

Acceptance of forced copulations

One further defense measure that females could employ against forced copulations would be to abandon eggs resulting from such mating acts and not incubate them. However, this is not the case. Scientists (Briskie und Montgomerie 1997, 2007) had hitherto explained why females do not give up their clutch with a reluctance by females to abandon their “investment”, i.e. the production of relatively large and therefore "expensive" eggs, even if these eggs are the result of a forced copulation. From an evolutionary point of view, this would then “invite” the males to continue to develop large penises and to maintain forced copulations as a mating strategy. This explanation has however been refuted by the present study.

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