Female guppies are turned on by male guppies sporting orange spots.

Bjorn Carey (“FYI: Why Do Ducks Have Orange Feet?,” Popular Science, 3-03-2010) wrote …

“Actually, many species of ducks have feet – and legs! – tinted a bluish green or gray. But for the ducks that do have orange feet, well, it’s all about attracting the ladies. Chicks dig orange.”

Bjorn Carey (same source as above) wrote …

“Bright orange coloring suggests that a male duck, also known as a drake, is getting all his vitamins, particularly carotenoids, such as beta-carotene and vitamin A, antioxidants that can be beneficial to the immune system. ‘This indicates that his behaviors and genes are good enough for him to recognize and eat the right food, or that his immune system is strong enough to produce bright orange legs,’ [Kevin] Omland says. ‘The female sees this as a very attractive trait to pass on to her offspring.’”