I recently learned that in ducks, the penis and vagina have coevolved: the vagina of the female ducks gets dead-end sacs and clockwise coils; the penis of male ducks become longest and its morphology changed to adapt to the female vagina. [1] This antagnostic coevolution happened due to the evolutionary arms race between male and female ducks.

This seems counter-intuitive when we think about natural selection. Indeed, if female ducks are forcibly copulated, they are more susceptible to reproducing themselves and sharing their genetic traits.

From [2], it appears that only 3% of forced copulations results in fertilization. If this is the case, why has the male phallus evolved this way? Indeed, it does not appear to me to be a good evolutionary strategy.

If [1] and [2] are correct, I would think that the duck vagina and phallus shouldn't have coevolved that way. What am I missing?

[1] Brennan, P. L., Prum, R. O., McCracken, K. G., Sorenson, M. D., Wilson, R. E., & Birkhead, T. R. (2007). Coevolution of male and female genital morphology in waterfowl. PLoS one, 2(5), e418.

[2] Burns, J. T., Cheng, K. M., & McKinney, F. (1980). Forced copulation in captive mallards. I. Fertilization of eggs. The Auk, 97(4), 875-879.

  • $\begingroup$ Your question is unclear what don't you understand, preferential mate selection? That male and female genes can have an antagonistic relationship? $\endgroup$
    – John
    Oct 6, 2019 at 3:36
  • $\begingroup$ @John I don't understand why phallus AND vagina evolved that way. According to me, if females are forcibly copulated, their genes should spread (so no dead-end sacs vagina). But if forced copulations don't result into fertilization, it is a bad reproduction strategy for males and they should evolved in a non-rapist species. However, it is not what we see and that is what I don't understand. $\endgroup$
    – Pierre
    Oct 6, 2019 at 7:42
  • 1
    $\begingroup$ There is a difference between spreading your genes and spreading your genes in the most effective way, it is the same reason animals are picky about mates, A male who mates with a female can go find a different female the next day, an impregnated female now can't mate even if better males are available. $\endgroup$
    – John
    Oct 6, 2019 at 13:13


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