In a study of the blue-footed booby, which is native to the Galapagos Islands and Ecuador, they found that when birds had a year off from reproducing, their feet became brighter in color. For female boobies, the more luminous the foot, the more desirable the male.

Generally, young male boobies have the brightest feet and they fade in color over the bird’s lifespan, which can be more than 20 years. Scientists found that a period of abstinence can restore some of the youthful glow, making the birds more desirable in the following mating season.

Previous work has shown that foot color is a strong predictor for reproductive success. Males with brighter feet have more offspring and are cuckolded less by their mates. “For birds with pale feet, we observe the females courting with other males more often and a decrease in copulation,” said Dr Velando.

The coloration of the feet is caused by a mixture of yellow light, reflected by carotenoid pigment, and blue light, reflected by collagen. Carotenoid is an antioxidant and a stimulant in the immune system, making it a good proxy for fitness: only well-nourished males with the fittest immune systems can afford the luxury of expending the chemical on the luminescence of their feet.

Tags: booby, sexual trait, mating, biology, bird,

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