The uncomfortable truth of duck copulation

How many of us have gone to a pond and thrown chunks of bread into the water to feed the ducks? Now how many of us have ever wondered how ducks copulate? Well sadly ducks display a rather aggressive approach towards their potential partner and often rape, not only by the chosen male but by other “want to be donors” is the likely outcome.

Image: 3 male Argentine Lake ducks accosting a
female. photo:mentalfloss.com 

Ducks are one of only 3% of avian species to pose an external phallus.  It is believed this is due to the fact that ducks are aquatic birds and the potential for their semen to be damaged by water is high (Brennan et al., 2000). So unlike most other birds who just rub their cloaca’s together for fertilization, male ducks have evolved to obtain an external phallus and use penetration to ensure greater potential for fertilization (Coker et al., 2002).  Now where I think things become really interesting is that the female duck have also evolved a rather unique reproductive tube to allow her to choose which semen she would like to use for her next offspring.

The male Argentine lake duck has one of the largest penises in proportion to body size of any animal. Not only is it rather large it is also coiled and also contains barbs at the base of his penis and at the tip are tiny spines that resemble a bottle brush flower. These small spines are believed to help push out other males semen. The female Argentine lake duck ovarian tubes accommodate for the unique penis and are also coiled though in the opposite direction. As it coils around there are also small chambers and pouches known as “dead ends” that function to exclude the phallus and help eject unwanted semen (Brennan et al., 2000).

Image: Arrow shows the female vagina,
 star shows male phallus. Image: Brennan et al., 2000.

It is believed that the genital morphology has evolved by an arms race in copulation behaviour and inter-specific selection by the female (Brennan et al., 2000). The larger the male duck’s phallus the more aggressive copulation is on the female.  As the male becomes more aggressive towards the female the female has evolved to counteract this behaviour by a complex vagina, allowing the female to retain control of fertilization.

References:

Brennan, P.L.R., 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, vol. 2, no. 5, pp. 418.

Coker, C.R., McKinney, F., Hays, H., Briggs, S.V. & Cheng, K.M. 2002, "Intromittent organ morphology and testis size in relation to mating system in waterfowl", The Auk, vol. 119, no. 2, pp. 403-413.