What if you could tell a potential partner's STD status just by looking at them? That's the evolutionary advantage that ducks appear to have.
It turns out mallard ducks have a problem with the males passing dangerous bacteria on to females during the process of reproduction. The obvious answer? Select for partners with antibacterial sperm. New research shows that not only do some drakes have ejaculate that can kill off Escherichia coli and Staphylococcus aureus, but this trait seems to be linked with how colorful their bills are. It's an external indicator of sperm quality, and allows for females to pick mates that are less likely to give them STDs.