Female ducks not only use the colour of a mallard’s bill to select a mate, but also to determine the sexual health of that male.
Scientists have reported that markings on the bill show evidence of sexually transmitted diseases (STDs) and other sperm damaging infections in the male, which reduce the chances of successful mating. They report that males with more colourful bills have semen with superior bacteria-killing ability, so not only can the females identify males with stronger sperm, but close attention to bill colour may protect them from STDs too.
This may open the market for duck cosmetics!
