Two little ducks. One incredible evolutionary arms race…

Think ‘duck’ and what comes to mind? Fond childhood memories of feeding the duckies at a pond? Dodging one of the feathered beasts whilst rowing bleary-eyed at the crack of dawn? Shredded with a plum sauce and pancakes? Regardless of your current perception of the inconspicuous mallard, it seems research into sexual selection has revealed that below the surface lies a much darker and (literally) twisted tale.

It all began with a post-doctoral researcher from Yale University called Patricia Brennan, who was interested in why the duck genitalia was as complex and unusual as it is. Certain species of male duck seemed to have overly extravagant penises; cork-screwed monstrosities that grow to ridiculous lengths when needed. To the researchers it all seemed like such a pointless waste of energy – why bother evolving such complex and bizarre genitalia? Brennan and her team did a variety of tests to show the exact lengths to which male ducks will go to ensure fertilisation, and the anatomical methods females have developed to avoid it. They videoed the eversion of several species of duck penis, if your eyes (and stomach) can take it, check out one of their videos. You’ll never look at Donald Duck in the same way again…

Disgusting bit over, why do the ducks bother developing such complicated and costly genitalia? It all comes down to conflicts of sexual selection. This arises when male and female individuals of a species have differing ideas of what they want out of their ‘relationship’. For ducks, the male simply wants as many offspring as possible. Following the ideas of W.D. Hamilton’s theory of Inclusive Fitness, the more ducklings, the more of Daddy Duck’s genes get passed on to the next generation. Though females want to have plenty of offspring too, the breeding season can bring many an unwelcome Donald to a Daisy duck’s bed chamber and many females find themselves sexually harassed and raped by other males. In fact it is estimated that up to a third of all duck matings are forced; yet only 3% of ducklings produced come from such forced matings.

A coincidence? Of course not. For a female, living in a world with forced matings means having to find a way to avoid bearing the children of these unwanted visitors who are likely to be substandard to her mating criterion. Such strategies are known as ‘cryptic female choice’, a way the female can control the usage of sperm post fertilisation, thereby avoid becoming fertilised by an unattractive male. For ducks, this entails a maze-like oviduct in a complex cork-screw shape, often ornamented with many out-pockets along the way. Such complex anatomy means that, even if a female is forced to mate with a sub-par male, she doesn’t have to be fertilised by him.
However, as evolution has shown many a time, two can play at that game. Whilst females have developed oviducts of increasing complexity, males have responded with longer and more elaborate genitalia themselves, driven by the selection pressure to impregnate the females despite their internal defence mechanisms. The result is a matching set, both driven by the individuals desire to maximise their inclusive fitness through reproduction (or avoiding it with unfit males, in the female’s case). Though the battle of the sexes rages on, examples such as the duck show just how powerful sexual selection can be. Plus just how much weird stuff you can see on youtube…

For those who fancy a look at the study find it here.