Turacoverdin is a uniquecopperuroporphyrinpigment responsible for the bright green coloration of severalbirds of the familyMusophagidae, most notably the turaco. It is chemically related toturacin, a red pigment also found almost exclusively in turacos.[1] Turacoverdin is one of the only true green pigments found in birds, as the coloration that appears in most greenfeathers is due to the unique properties of bluestructural coloration in combination with yellowcarotenoids.[2] Turacoverdin and turacin were the first ever chemically characterized feather pigments, and turacoverdin was first isolated and described in 1882 by Dr. C.F.W. Krukenberg.[3]
Few studies into the chemical nature of turacoverdin have been performed to date. Research by R.E. Moreau in the 1950s showed it to be lesssoluble inbasicsolutions than its chemical cousin turacin. While originally thought to contain little copper by its discoverer, who instead believed it to beiron-based, laterspectroscopic analysis demonstrated high copper (and low iron) content in pigment from the green feathers of theKnysna turaco and theSchalow's turaco. Moreau also demonstrated that the green coloration of turacos might actually be due to the combined effect of two different turacoverdin pigments that differ slightly inpolarity.[4]
When extracted and exposed to light, oxygen, or strong bases, turacin has been shown to take on a green hue. This has caused several researchers to suggest that turacoverdin may be anoxidizedmetabolite of turacin.[4][5] This has been supported by data comparing the absorption bands of the "altered turacin" with those of turacoverdin, which are shown to be very similar to one another.[6] Several researchers have noted the chemical similarities between turacin and turacoverdin. This relationship has been supported by spectral properties, the fact that both pigments contain copper, their similar microscopic arrangement in feather cells, and the co-occurrence of the pigments: turacin and turacoverdin are always found together in the same species, and in many cases are also found in the sameplumage locations. The green appearance of turacoverdin can be derived from itsabsorbance curve, which peaks at blue wavelengths and in the long-wave range above yellow. Turacoverdin shows little UV reflectance.[4]
Turacin and turacoverdin are both found in four of the six genera of turacos. The subfamily Criniferinae is typically regarded as being devoid of the brighter pigments, although the genusCorythaeola does have a small stripe of turacoverdin on its breast. The remaining turaco genera are placed in the subfamily Musophaginae and are referred to as the turacin-bearing turacos. More than half of turaco species belong to the genusTauraco, and are all notable for being mostly a vivid green.[7]
Recentspectrophotometric evidence suggests that turacoverdin may be closely related or identical to green pigments in the feathers of thenorthern jacana (Jacana spinosa), theblood pheasant (Ithaginis cruentus), and thecrested wood-partridge (Rollulus rouloul).[4] AsIthaginis andRollulus are members of the orderGalliformes, this has led some researchers to assume support for a turacoverdin-bearing common ancestor of Musophagidae and Galliformes, making the presence of the pigment asymplesiomorphy for these groups. The pigment data specifically suggests that turacos evolved from a group of galliform species, which is represented by the extant generaIthaginis andRollolus. Based on the appearance of its green feathers, researcher Jan Dyck speculates thatRollolus is closer to a possible ancestor thanIthaginis.[6]
The northern jacana, on the other hand, is a member of the orderCharadriiformes, a group clearly not closely related to either Musophagidae or Galliformes. This makes it highly unlikely that turacoverdin inJacana reflects common ancestry with either turacos or galliforms. If the green pigment inJacana is truly turacoverdin, then the pigment must have evolved independently in this order. This is further supported by the fact that the pigment inJacana is located only in theremiges, while in all musophagid and galliform species, the pigment is found mainly in body feathers.[6]
Turacin and turacoverdin, being copper-based pigments, require large quantities of copper in order to be manufactured. As turacos are primarilyarboreal species, they are able to accumulate copper through a diet rich infruits,flowers,buds, and other plant matter. Church and Moreau estimated that 3 months' worth of fruit intake contribute to producing the pigment present in the newly grown plumage of the turaco speciesT. corythaix (the Knysna turaco).[4] It has also been observed that it takes young turacos around a year to acquire their colorful adult plumage, and some authors have speculated that they probably need that long to acquire the necessary copper. It has also been noted that turacos all live acrossCentral Africa, which corresponds geographically with one of the world's richestcopperbelts.[8] It is unknown whether turaco diets are especially rich in copper as compared to the diets of other birds, or whether turacos are especially effective at extracting copper from their foods. It is also unknown whether turaco species lacking turacin and turacoverdin-induced pigmentation have a comparatively copper-deficient diet, absorb less copper from their diet, or lack theenzymes required to synthesize the pigments.[4]
Although no formal tests have been performed investigating the functional significance of turacoverdin coloration, speculation abounds. Moreau in 1958 observed that turaco species inhabiting forests are more likely to be green in color than species inhabiting other environments, which may offer concealment from predators.[9] In fact, it has been observed that the greener and denser a turaco's forest habitat, the deeper green its plumage, while non-forest-dwelling turaco species tend to be devoid of the green pigment.[10] This claim has not been studied rigorously from a biochemical orphylogenetic perspective, however, and awaits further research. Turacos may employ their unique green coloration for sexual or social advertisement, but again no spectrophotometric or biochemical studies have been conducted to test forsex differences in coloration, and to limited human perception there appears to be none.[4]
Other authors speculate that turacos and other birds employing the use of turacoverdin may derive a physiological and biochemical benefit from synthesizing the pigment. Copper, like porphyrins, can be damaging to birds when accumulated at high concentrations.[11] Turacos may detoxify the high levels of copper ingested in a diet rich with porphyrins, thereby advertising the protection they've granted themselves by depositing the copper-rich pigments in their feathers.[12] It also may be of some biological significance that turacos all seem to be pigmented with turacin and turacoverdin in exactly the same regions of the wing feathers.[4]