duck with huge cock

In September 2001, the scientific journal Nature published a picture of a small duck with a penis longer than its body. What the Argentinian lake duck Oxyura vittata uses this extraordinary appendage for is a mystery, but we can make some informed guesses.

In almost every instance of extreme genitalia, in males and females, the explanation lies in sexual conflict. In most animals, females typically mate with several males during each sexual cycle. Species where females do this are said to be ‘polyandrous’, and the consequence of multiple sexual partners is sperm competition – the competition between the sperm of the different partners to fertilise the female’s eggs. It may benefit a female to be polyandrous, but it doesn’t pay males to allow their partner to be fertilised by someone else’s sperm – hence the conflict.

The cowpea weevil Callosobruchus sp., for example, has a very spiky penis. In common with many other insects, male and female cowpea weevils disagree over the duration of mating, and towards the end of copulation, the female starts to kick at her partner in an attempt to remove him – and with good reason, for the longer the male mates, the more damage he inflicts on her. Worse still, the longer a female spends copulating, the shorter her lifespan, suggesting that male-inflicted damage is the root of the problem.

By making copulation an unpleasant and damaging experience, a male may deter his partner from copulating with another male. In other words, copulatory-damage is a form of mate-guarding by males.

Females of some other insects appear to have ‘fought back’. The South American leaf beetle Chelymorpha sp. has a huge penis, which, like the South American duck’s, is several times longer than his body. Again it appears that sperm competition is what has caused this elongation; females copulate with several different males, and the male who can place his sperm in the best position for getting fertilisations ‘wins’.

Males have evolved ever longer penises to position their sperm closer and closer to the site of fertilisation. But the female wants to retain some control over which male fathers her offspring, and so she has co-evolved a longer and longer reproductive tract – keeping the male’s penis at arms length, so to speak. In fact, her reproductive system is one of the most elongated and convoluted of any insect found so far.

The Argentinian lake duck is an almost unknown species as far as its sexual behaviour is concerned. I’m convinced that the male’s penis has evolved in response to intense sperm competition, but I’m also sure that we won’t know what advantage it confers on males until we look at the female side of things.