Nice feet! | Derek Keats - CC BY 2.0

On sexy blue feet

Looks help with being famous in the animal world, too. Thanks to its blue feet, this goofy-looking sea bird is arguably one of the superstar species of the Galápagos Islands.

Its name is blue-footed booby (Sula nebouxii), and it can be found on the western coast of the Americas, ranging from north-west Mexico and Panama to north Peru.1 They are capable divers and swimmers who mainly eat small fish hunted in the ocean. Roughly half of the blue-foot population is found on the Galápagos Islands off the coast of Ecuador, notable for their incredible biodiversity and widely recognised thanks to one exceptional scientist visitor: Charles Darwin.

The only bird in the world with such bright turquoise appendages, the boobies are quite proud of the feet themselves. Males strut around in a comical mating dance showing their feet off to the females. The resulting clumsy escapade is reminiscent of a human trying to walk around in long diving-flippers on a beach. Hilarious.

When blue-foots are profiled by bird enthusiasts, their remarkable feet usually get a mention as a “sexually selected trait” without much explanation otherwise. To Darwin sexual selection was a significant part of his natural selection theory. Simply put, a particular characteristic of a male animal, if it is favoured by the lady, will lead to more offspring with that lady, and the offspring will then carry that characteristic onwards.

 “the [sexual] struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.”2

So why are blue feet so sexy?

First of all, the blue-footed boobies are mostly monogamous, and they breed opportunistically, which means that usually there is no particular season they prefer for egg laying, and instead do it when environmental conditions are good. In such a reproductive setting, the females will want to have the best possible information on the health condition of their male at any given time, so that they can optimise the parenting outcome for their offspring.

It’s been observed that male boobies usually have brighter feet in general, and particularly so when they are young – more fertile, thus fitter for reproduction and favoured by females.

WHAT?! | By Richard Duerksen
WHAT?! | Photo by Richard Duerksen

But the feet also tell whether the blue-foot is healthy and has had a good dinner. Because the blueness comes from carotenoids (organic pigments) available to birds only through their diet, food deprived boobies’ feet are duller. So a male bird who’s had a rough time getting enough fish, is sick or otherwise just not doing so well, reliably signals this to his female with the brightness of his feet.

What can the girl bird do with this information? Well, blue-foots usually lay two eggs with the mean interval of five days. Some researchers did an experiment in which they covered the male feet in make-up to make them look duller after the female had laid the first egg. The females, noticing the apparent deterioration of their partners’ condition, allocated fewer resources to the second egg and it came out smaller, with less of the yolk hormones that are crucial for chick survival.3

The size of the two eggs in a brood comes into play because of the sibling rivalry that boobie chicks naturally engage in. If food is scarce, the older chick will oust or kill the weaker one. Now, if the female suspects, via the brightness her partner’s feet, that food is likely to be scarce, she uses that knowledge to spare resources on the second egg, because the two siblings will fight it out between themselves anyway:

“Females lay heavier second eggs compared to first eggs when ecological conditions are good, possibly as a mechanism to compensate for the asymmetries among brood siblings, but lay lighter second eggs when the food is limited, facilitating the brood reduction.” 4

So, not only do the blue-footed boobies make for an adorable Google image search (seriously, try it), they are also the source of a fascinating evolutionary lesson.

Bonus: watch their clowny mating dance!

Show 4 footnotes

  2. Darwin, C. (1871) The Descent of Man and Selection in Relation to Sex
  3. Velando A., Beamonte-Barrientos R. and Torres R. (2006). “Pigment-based Skin Colour in the Blue-footed Booby: an Honest Signal of Current Condition Used by Females to Adjust Reproductive Investment”. Oecologia 149: 535–542. PDF
  4. Velando et al.
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