It’s a highlight of any Galapagos Islands cruise experience — standing just steps from comical male blue-footed boobies as the honk, sway, whistle and dance their way to a mate. But it turns out that dancing ability may not be all it tales to impress a female.

According to a group of Spanish and Mexican researchers, the intense blue hue of the male bird’s feet is actually an external indicator of age and fitness for breeding.

After studying 30 years of data on one particular booby population, researchers found that male germ lines — the DNA sequence passed from one generation to the next — were increasingly deteriorated in older birds. And since the intensity of those famous feet is a worthy indication of age, females have a visual cue that there is a risk that a male might pass on a genetic mutation to its chicks. Over time, females of the species have evolved to become less attracted to males with lighter, older hues.

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