Feet Don't Lie

          I have a personal love for the foot coloration and courtship displays of the Blue Footed Booby. As the name suggests, the species is most notable for the brightly colored feet found on both males and females in addition to the infamous “Booby mating dance” in which the males lift their feet as high as possible to display their color to the female. This added to sky pointing and presentation of nesting materials (an artifact of ancestral behaviors as these birds don’t actually make nests any longer) can net a male Booby a mate for the breeding season. But the foot color is key in these displays, and this is because it is an incredibly honest indicator of both how well fed they are as well as their immunological state.

Male Blue Footed Booby lifting feet during courtship display.

The colors of feet range from blue-violet to bright green-blue, with the greener feet belonging to those individuals who are healthiest. One study was able to document the drastic reduction in vibrancy of the pigment in the feet from bright green to a dull blue-violet over the course of 48 hours when deprived of food (Velando, Beamonte-Barrientos, & Torres. 2006). Such a noticeable change makes it quite apparent to females whether their suitors are healthy, and vice versa as the female foot coloration is just an honest an indicator of health as the males. This is where it starts to get interesting in terms of reproduction.

Example of the difference in foot coloration amongst Blue Footed Boobies

Blue Footed Boobies are socially monogamous birds, and actually continue to undergo courtship displays after breeding has occurred. This isn’t the result of sentimentality, or of male blue-footed boobies being overly romantic. It is actually each member of the breeding pair checking up on the health of their partner and deciding whether or not it’s worthwhile to maintain the pairing. Should one of the pair find that their partner is “letting themselves go” in any way the results can actually be pretty dramatic.

Female boobies lay anywhere from 2-3 eggs a season, with the average being 2 laid several weeks apart. The same study on foot color by Velando, Beamonte-Barrientos, & Torres (2006) found that during that intermittent time frame between laying, if she discovers her mate is starting to drop in health she will actually adjust the amount of energy and nutrients she puts into producing offspring to result in an egg that is greatly reduced in size.  Essentially, the female puts in so little effort to egg formation that the offspring is almost assured not to hatch. If the female does not believe that the male can adequately help rear the offspring she simply ceases having them. All based on the color of his feet.

It’s not unheard of that females control their clutch sized based on environmental factors or mate quality, but in this case all the same requirements put upon males by females apply directly to them as well. In a study done by Torres and Velando (2005) it was found that females with duller foot coloration received less post-copulation courtship displays as well as fewer extra-pair copulations. Their own mates were also more likely to seek extra pair copulations with females who have brighter foot coloration.

             Essentially, the blue feet were selected for through sexual selection on behalf of both the male and the female of the species. Which makes sense, as their chicks require a long period of biparental care after hatching to rear them to independence. In many cases with birds only the males are pushed by female preference into large showy plumage, so I find it interesting when in nature you a find a sort of equality in the expectations of both males and females for brood rearing.

References:

Torres, R., Velando, A. 2005, ‘Male preference for female foot colour in the socially monogamous blue-footed booby, Sula nebouxii’. Animal Behavior, vol 69, pp. 59-65.

Velando, A., Beamonte-Barrientos, R., 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, vol 149, pp. 535-542.

Picture Credit

Dance: Personal Picture

Darker feet: http://photography.nationalgeographic.com/photography/photos/life-color-blue/#/blue-

                     feet-laman_1331_600x450.jpg

Lighter Feet: http://www.galapagos-islands.com/what-to-see/blue-footed-booby.html