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In 1983, the writer Alan Moore decided to take a break from groundbreaking graphic novels like Watchmen and V for Vendetta. Under the pseudonym Translucia Baboon, he wrote a song called . In it of, among other things, being “Nasty and small, undeserving of life”, on the grounds that “They smirk at your hairstyle and sleep with your wife”.

The more we find out about ducks, especially mallards, the more it seems Moore was as ahead of his time with regard to waterfowl as he was in comics. The males will do almost anything for sex, so their lives are a riot of corkscrew penises and forced copulations.

But male mallards are, at least, honest birds. Males with colourful beaks are more attractive to females, and it now seems that beak colour tells the females one of the males’ secrets: how good their semen is at killing bacteria.

Quack quack

The mallard is the species from which all domestic ducks are descended, and they have colonised most of the planet. In autumn they form up into mating pairs, which stick together until the female lays a clutch of eggs the following spring. The male will while she is fertile – obviously, this is beneficial for him, as it helps ensure that any offspring are his own, but there is some evidence that .

That’s because other males are bad news for her. , they will often try to force her to mate with them. Rather than , these males will chase her violently and may injure her. Fertile females are , and one study found that . Males whose partners have been “forced” in this way will often attack the intruder, but are also likely to .

Males have evolved long, corkscrew-shaped penises to make it easier to force themselves on females. In response, the females have evolved elaborate reproductive tracts with cul-de-sacs that trap unwelcome sperm. They also twist in the opposite direction to the males’ penises. Even if the uninvited male’s sperm has its way, his genes may not: offspring from unwelcome males and .

Towards the end of the breeding season, unpaired males sometimes gather in groups to pursue females. Essentially ganging up on lone females, they scramble to mate with her and often trap her underwater for minutes at a time. Around 10 per cent of female mallard deaths can be attributed to forced copulations.

Check out my beak

The battle over females does not stop after mating. Sperm from different males must race up the female’s reproductive tract to fertilise her eggs. To win this race, they must be in prime condition.

of the University of Oslo in Norway knew that human sperm can be harmed by bacteria in semen, and she wondered if the same thing happened in mallards. She reasoned that if it was a major issue, males would have evolved antibacterial defences in their semen to protect their sperm – and if that were so, females could avoid infections if they pick males with high levels of these defences.

She collected samples of ejaculate from 11 captive male mallards. This was tricky: a male’s phallus is normally tucked away inside his cloaca, and only emerges when he is aroused. Rowe and her colleagues had to massage each male’s cloaca, pushing lymphatic fluid up the length of the phallus until it popped out of the cloaca and he ejaculated.

It turned out that the ejaculate could inhibit the growth of Escherichia coli bacteria, and that this effect was stronger if the ejaculate came from males that had more colourful bills. She thinks the bills act as a signal, essentially telling females that certain males have strong antibacterial activity in their ejaculate.

The females may actually get a double benefit from choosing these colourful males. Not only do they receive sperm that is less likely to carry an infection, it is of higher quality. However, Rowe cautions that we don’t yet know whether E. coli affects duck sperm in the same way that it affects human sperm, and it’s also not clear if it can be transmitted sexually.

Journal reference:

Read previous Zoologger columns: The African eel that travels light, The only primate that eats its dinner twice, Biofuel powers biggest flying marsupial, Cryo-frog survives deep freeze, Megamouth, the shark that has to suck it up, The hairy beast with seven fuzzy sexes, Australia’s truly glamorous camper, Jet-propelled living fossil with a problem, The sharpest mind in the farmyard, Invasion of the crazy incestuous ants, The fish with no stomach for its prey.