By Ella Davies
Earth News reporter
A drake's bill colour communicates information about its sexual health
Ducks use bill colour to determine a potential mates' sexual health, according to scientists.
A study has revealed that ducks' semen can destroy bacteria such as E. coli, which can infect and damage sperm.
The same study found that males with more colourful bills had more effective antibacterial sperm.
Researchers suggest that females probably use bill colour as a visual cue when choosing a mate, to avoid sexually transmitted diseases (STDs).
Their findings were published in the Journal of the Royal Society Biology Letters.
Dr Melissah Rowe from the University of Oslo, who led the research, said: "This is the first time anyone has shown that avian semen has antibacterial activity".
"Previously, [it] has been shown from mammals, crabs and insects, but never birds."
DUCK SEX FACTS
Both male and female ducks have unusual genitalia: males have corkscrew penises while females have labyrinthine vaginal tracts
Mallards pair off in the autumn until the females lay around 12 eggs in the spring
After eggs are laid, males remain potent and often mate forcibly with unpaired females
Researchers found that mallards with more colourful bills containing more yellow pigments had superior bacteria-fighting semen.
"Sexual colouration may reflect a male's ability to kill ejaculate-borne bacteria and thus defend sperm from bacteria-induced damage," said Dr Rowe.
It was already understood that females select their mates based on the colour of their bills.
But the new evidence shows how a brighter, more colourful bill communicates specific and important information about a potential partner's sexual health.
Not only can the females identify males with stronger sperm, but close attention to bill colour may protect them from sexually transmitted diseases.
"Females might be able to identify males with high ejaculate antibacterial activity," said Dr Rowe.
"By doing so they may be able to avoid sexual transmission of pathogenic bacteria and copulate with males whose sperm suffer less bacteria-induced damage."