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Melanosomes in modern birds

In modern bird feathers, melanosomes provide a broad range of colours, from black, grey, and white (dependent on the density and presence or absence of eumelanosomes) to browns and reddish yellows (produced by phaeomelanosomes). Melanosomes can also produce iridescent effects, which may show as bright blues, according top their packing and reflection of light.

Other colours are produced by carotenoids and porphyrins. Carotenoids are colouring agents that birds acquire when they eat certain plants. Carotenoids are responsible for bright yellow colours, and they can interact with melanins to produce colors like olive-green. Porphyrins are produced by modifying amino acids, and they all fluoresce bright red when exposed to ultraviolet light. Porphyrins produce a range of feather colours, including pinks, browns, reds, and greens.

As part of our study of melanosomes in fossil birds and dinosaurs from the world-famous Early Cretaceous Jehol fossil beds of NE China, we had to demonstrate that the organelles in the fossil feathers really were melanosomes, and not bacteria, or some other decay product. These studies of the fossils required care and ingenuity in collection and study.

Melanosomes in zebra finch feathers

Rationale. We chose to study (Zhang et al. 2010) the ultrastructure of feathers of the zebra finch, Taeniopygia guttata castanotis (Fig. 1a), the subspecies that occurs over much of Australia. These birds are commonly kept as pets. The coloration of flank feathers in this bird is based on melanin (McGraw & Wakamatsu 2004). Breast feathers contain predominantly eumelanin, and flank feathers very high concentrations of phaeomelanin. Breast feathers are either uniformly black or show black and white striping (Fig. 1a); the former were used here (Fig. 1b). The distal half of flank feathers is brown-coloured with two white, unpigmented, spots; otherwise, the barbs and barbules are pigmented evenly (Fig. 1c).

Method. Feathers were placed in an ultrasonic bath in distilled water for 10 minutes to remove surface detritus, and then air-dried on a 50oC hotplate. The feathers were cut in two transversely and the proximal part discarded; the pigmented parts of samples from flank feathers thus exhibited only brown colours. The rachis was separated from the rest of the feather using a scalpel, and discarded. To fracture the barbs and barbules, the remainder was mounted between two glass slides and ground lightly by manually rotating one slide past the other. Samples were then mounted on a pin stub, gold-coated, and examined using the scanning electron microscope (SEM).

Figure 1. Melanosomes in the feathers of the male zebra finch. (a) Optical photographs of lateral (top) and ventral views of the specimen sampled. (b, c) Optical photographs of breast feather (b) and flank feather (c). (d-i) SEM images of melanosomes inside barbs and barbules. (d-e) Eumelanosomes (at arrows in (e) in the barb of a breast feather. (f-g) Phaeomelanosomes (at arrows in g) in the barb of a flank feather. (h-i) Barbule of a flank feather in which the superficial layer has peeled away to expose aligned phaeomelanosomes (at arrows in i). Abbreviations: b, barb; bl, barbules; s, superficial layer. Scale bars: a, 10 mm; b, 5 mm; c, 5 mm; d, 100 Ám; e, 5 Ám; f, 30 Ám; g, 3 Ám; h, 10 Ám; I, 2 Ám. Click the image above for a high- resolution version.

Results. Only rod-shaped eumelanosomes were identified in breast feathers (Fig. 1b, d-e). The brown-coloured distal part of the flank feathers (Fig. 1c) revealed only sub-spherical melanosomes (Fig. 1f-i). The sub-spherical melanosomes are interpreted as phaeomelanosomes, as previous biochemical analyses have recorded very high abundances of phaeomelanin in flank feathers (McGraw & Wakamatsu 2004). It is likely that both phaeomelanosomes and eumelanosomes occur within a single feather (eumelanin occurs in low concentrations in flank feathers). However, the colour of the feather clearly corresponds to the most abundant type of melanosome. There is thus a correlation between the shape of the melanosome and feather colour. This, which has been recorded previously (McGraw 2006), forms the basis for our interpretations of colour in the fossil birds and theropods.

Further reading

Read all about zebra finches here, and about bird coloration here.

Cited references:

  • McGraw, K.J. & Wakamatsu, K. 2004. Melanin basis of ornamental feather colors in male zebra finches. Condor 106, 686-690.
  • McGraw, K. J. 2006. The mechanics of melanin coloration. In Bird Coloration. 1. Mechanisms and Measurements (eds Hill, G.E. & McGraw, K.J.), pp. 243-294 (Harvard University Press, Cambridge).
  • Zhang, F., Kearns, S.L, Orr, P.J., Benton, M.J., Zhou, Z., Johnson, D., Xu, X., and Wang, X. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463, 1075-1078 (doi:nature08740.3d). pdf.

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