Soaring high above the mountains from Europe to China and to Africa, the bearded vulture (Gypaetus barbatus) commands attention at any lunch party. It likes to gatecrash into the frenzy around carrion, pushing other scavengers aside with wings that can stretch the height of Michael Jordan ... only to ignore the carrion’s juicy flesh or brains and fly away with a large piece of bone in its talons.
The bearded vulture is one of the largest birds of prey, yet it eats mostly bone marrow. It is also the only bird known to decorate itself.
Adult bearded vultures sport a coat of snow-white feathers on their necks, shoulders and chests. On this white feathery canvas, the vultures paint a shade of rusty red by bathing in soils or water rich in red iron oxide deposits.
For years, scientists questioned the origins of the vultures’ red paint. Field studies, including an intensive three-year radio-tracking study, failed to uncover the origins of the red colouration. Researchers suggested that the red stains might have been caused by the birds randomly resting near iron deposits. Yet the colouring seemed intentional, as captive birds that were given access to damp red soil quickly pounced on it, dusting their bellies and necks red like their wild kin. The birds would use the beak and talons to spread the red mud from their chest to their shoulders and upper back.
Bearded vultures clearly like to put on a shade of red.
Finally, in 1995 and then 1998, wild bearded vultures were seen bathing in pools thick with iron deposits in the French and Spanish Pyrenees. We now know that these birds deliberately dust themselves red. But why?
“We now know that these birds deliberately dust themselves red. But why?”
Camouflage is unlikely – vultures sit atop the food chain so there's no need to hide from predators, and since they eat bones, there's no creeping up on prey either. The iron oxides also do not seem to improve feather durability, as coloured and white feathers wear down just the same. So scientists settled on two other possible functions ... and they just cannot agree which is correct: are the iron oxides cosmetics or prophylactics?
In 1999, Juan Jose Negro, ecologist at the Doñana Biological Station in Spain, reasoned that bearded vultures might use iron oxides to advertise their strength to other vultures. Suitable iron oxide deposits are rare (as suggested by the infrequent observations of wild vultures bathing in them), so finding this red treasure would cost the birds energy and time – making iron oxides a cosmetic that only a strong or healthy bird could afford. If so, sporting a red coat would be no less a stamp of strength than a Rolls Royce would be a mark of wealth.
Advertising strength could be used to establish dominance: bearded vultures fighting over bones (and the fatty marrow within) would stare down their challengers and swing their heads while the neck and head feathers stand up like sentinels. Colouring seems to intensify with age, size and dominance. Females, often larger than males, also sport more intense iron oxide colours and dominate matings; among males, paler suitors reportedly mate less frequently.
Three years after Negro’s publication, Raphael Arlettaz, ecologist at University of Bern in Switzerland, offered an alternative hypothesis to explain the vultures' red colouration: the iron oxides are not mere cosmetics, but prophylactics instead. Chlorine dioxide and ozone kills bacteria, prompting Arlettaz to suggest that iron oxides may also do the same. Bearded vultures probably come into contact with lots of bacteria in their 'professional' lives as scavengers, and these bacteria threaten the vultures’ nestlings and eggs. So Arlettaz suggested the iron oxides could be a weapon against the bacterial onslaught the vulture parents bring home.
Like most other animals, bearded vultures cannot produce carotenoids, antioxidant compounds that protect cells from free radicals in the body. Many bird species acquire them through their diets ... but the bearded vultures' marrow meal plan (though fatty and delicious) is not exactly rich in carotenoids. So Arlettaz speculated that bearded vultures use iron oxides in place of carotenoids. Citing his colleague’s observation that captive vultures would return to their nests after a bath in iron oxide-rich water and "rub their feathers impregnated with pigments on to their eggs or offspring", Arlettaz suggested that bearded vulture parents may pass the iron oxides on to their progeny. That might also explain why females tend to sport a brighter hue than males: they have a greater need to protect their offspring with iron oxides.
Negro responded to this ‘prophylactic hypothesis' with several counter-arguments. He noted the lack of evidence that iron oxides kill bacteria (on the contrary, many microbes fight for iron). The theory also doesn't explain why other vultures with carotenoid-poor diets have not evolved similar feather-decorating habits, or why juvenile vultures start staining themselves with iron oxides years before they breed.
The debate on the matter seems to have died down since, and both Negro and Arlettaz have moved on to other projects. Until it resurfaces, the exact reasons for the vultures' passion for decoration remain a mystery.
Top header image: jayhem, Flickr
Sources:
Negro, J.J., Margalida, A., Hiraldo, F., and Heredia, R. (1999). The function of the cosmetic coloration of beraded vultures: when art imitates life. Animal Behaviour 58, F14–F17.
Arlettaz, R., Christe, P., Surai, P.F., and Pape Møller, A. (2002). Deliberate rusty staining of plumage in the bearded vulture: does function precede art? Animal Behaviour 64, F1–F3.
Negro, J.J., Margalida, A., Torres, M.J., Grande, J.M., Hiraldo, F., and Heredia, R. (2002). Iron oxides in the plumage of bearded vultures. Medicine or cosmetics? Animal Behaviour 64, F5–F7.
Zooborns
Success Stories
Innovation
