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.
