2.3 Sexual conflict and forced copulations lead to the co-evolution  (Page 2/7)

Sexual conflict is the result of differing evolutionary interests between the sexes of a species (Adler 2010). This conflict is usually due to competition for greater reproductive success among members of the same sex but which often involves tactics that reduce the other sex’s success. While this is uncommon in most birds, it is prevalent in Anatidae , the waterfowl family. In fact, recent studies by Brennan et al. have shown by comparative analysis that, in ducks with greater genital complexity there is more frequently forced copulation which provides strong evidence that the organs were developed due to sexual conflict (2007). These undesired and actively resisted extra pair copulation’s (EPCs) are deemed forced copulations, and are the subject of debate in the field of animal behavior (McKinney 1983). This chapter will outline the mechanisms and specific reproductive interests at stake for each sex. Then, theories for the co-evolution of behaviors and sexual organs will be presented. At the end, some peculiar and intriguing alternative sexual practices of mallards will be given for the reader’s edification.

The male intromittent organ

Male waterfowl are some of the few birds that have well-developed intromittent organs (IO’s), external organs that deliver sperm during copulation (Briskie 1997), which have been termed phalluses. These phalluses grow to full size in the mating season and regress afterwards until the next season (Johnson 1961). The phallus is kept inside the drake until mating begins, see Figure 2. Once the drake has secured his position upon the hen and the two are made contact with their genitalia, the phallus rapidly everts in a counter-clockwise spiral inside the female, allowing ejaculate to run along its length inside the vagina. Eversion can be likened to pulling one’s finger out of a glove turning it inside out.

Some phylogenetic evidence shows that IO’s were lost and redeveloped several times throughout evolutionary history. This is supported by the fact that IO’s are mostly found in primitive birds, and there is evidence of IO’s in some bird embryos that are reabsorbed before hatching (Briskie 1997). Several hypotheses exist as to why this occurred, but this chapter will focus on one reason mallards may have retained them, which is sexual conflict. To fully understand this, it is necessary to examine the female genitalia too.

The female genitalia

Surprisingly, it was only recently discovered that the female waterfowl genitalia were even more complex than the male genitalia and subsequently researched. Waterfowl have the only known coevolved, elaborate female genitalia in the avian family. In 2007, Brennan et al. published their findings: that many female waterfowl have vaginas that twist clockwise, opposite to the direction that male phalluses do, and frequently include dead-end pockets that trap semen (see [link] & [link] ). How could the female have sexual organs that are so elaborately designed to hinder insemination by males? It seems counterintuitive that female sexual organs would not be adapted to best fit male reproductive organs, but that kind of thinking is based on human anatomy.