The Bluer, the Better: The Hue of a Booby’s Feet Reveals His Age

A blue-footed booby, mid-dance. Photo: Pete, via Wikimedia Commons

It’s quite the show: First, he lifts the left, oh so earnestly. Then, he lifts the right, serious as can be. He sways like so, slowly, and after awhile might invert his wings toward the sky, in a pose both graceful and awkward, while giving a plaintive whistle.

This, you might know, is the dance of the male blue-footed booby, a fish-eating seabird found in the Galapagos, Peru, and Mexico. Of course, it's performed to impress a female, which will join him in dance if it's seductive enough. But the bluer those feet, the better—and the younger, it turns out.

Recently, a Spanish-Mexican research team used 30 years of data to study one particular booby population. What they found was that male germ lines—the DNA sequence passed from one generation to the next—were increasingly deteriorated in older birds. Though we've learned of late that children of men over 50 or 60 may be more likely to have genetic illnesses because of gametic mutations, it’s long been thought that biological old age was unique to humans and the animals we keep; that, in nature, animals were eaten or undone by parasites before, genetically speaking, they were no longer quite themselves. But boobies have proved us wrong: They don't just live a long time, they reach reproductive senescence, too.

Fascinatingly, blue’s the external clue. The intensity of those famous feet is a worthy indication of age, thus “mirroring” the risk that a male might pass on a genetic mutation to its chicks. Since female boobies are less attractive to lighter, older hues (who wants wispy blue, when you can have cerulean?), it suggests that those feet may be a measure of fitness in an additional sense, allowing females to avoid producing young with congenital illnesses (that is, abnormalities present at birth).

"The study provides us with a new way of looking at what lies behind sexual signals, pointing to the importance of sexual selection in eliminating genetic mutations," says Alberto Velando, a researcher at the University of Vigo and lead author of the study, published in the Journal of Evolutionary Biology.

Sorry, old boy boobies. But dance on, dance on—it's for the good of the population.