This article is interesting because it shows the biology and co-evolution between the male and female waterfowl. The male waterfowl has evolved to have forced copulation with the female. The male has a large corkscrew penis to efficiently penetrate and inseminate the female, but female evolved with the male to avoid unwanted insemination. The female can tighten her cloacal muscles to prevent fertilization of her eggs from an unwanted or undesirable male. The female's vagina has evolved to turn in the opposite direction of the male's penis and has dead end cavities inside. When a female tightens her cloacal muscles, her vagina becomes difficult for the male to fertilize, but she can relax her muscles to allow a desired male to fertilize her
