Iron oxides in the plumage of bearded vultures. Medicine or cosmetics?

We have hypothesized (Negro et al. 1999) that bearded vultures, Gypaetus barbatus, deliberately stain their plumage with iron oxides to signal dominance status. The conceptual frame to which we ascribe our hypothesis is the handicap principle (Zahavi & Zahavi 1997). For a signal to convey reliable information, and thus to evolve, it has to be costly (Zahavi 1975), and we proposed that the associated cost of the red colour signal for bearded vultures is that adequate ferruginous springs are a limited resource. Arlettaz et al. (2002) propose alternative explanations, namely that iron oxides have two functions that help bearded vultures in reproduction. One function (hypothesis 1) is that iron oxides have pro-oxidant effects that may kill bacteria, which in turn are detrimental for embryo and nestling development. The second function (hypothesis 2) is that iron oxides might increase mobilization of vitamin A or scavenge free radicals (a typical job for antioxidants such as the carotenoids, Surai et al. 2001), and this would enhance embryo and chick survival. Previous functional hypotheses for cosmetic coloration in bearded vultures (see review in Negro et al. 1999) include protection against feather wear or protection against mallophaga (Brown & Bruton 1991). However, Frey & Roth-Callies (1994) found no experimental support for any of them. That iron oxides may be toxic to feather ectoparasites is essentially the same idea as in hypothesis 1 in Arlettaz et al. However, these authors state that ectoparasite avoidance is unlikely because it remains unclear why iron oxides would be detrimental for ectoparasites (and we agree with this). Effects of Iron on Bacteria