Carotenoids need structural colours to shine

Abstract

The bright colours of feathers are among the most striking displays in nature and are frequently used as sexual signals. Feathers can be coloured by pigments or by ordered tissue, and these mechanisms have traditionally been treated as distinct modes of display. Here we show that some yellow plumage colour is created both by reflection of light from white structural tissue and absorption of light by carotenoids. Thus, structural components of feathers contribute substantially to yellow 'carotenoid' displays, but the effect of variation in structural components on variation in colour displays is, to our knowledge, unstudied. The presence of structural colour in some carotenoid-based colour displays will have to be considered in studies of colour signalling.

Figure 1

Spectral reflectance curves (±1s.e.) of (a) white breast feathers from chickens and (b) yellow breast feathers from American Goldfinches before and after carotenoid extraction with acidified pyridine. The untreated feathers had reflectance curves typical of carotenoid-pigmented barbs whereas pyridine-treated feathers had typical reflectance curves for white feathers (b). n=5 in all cases.

Figure 2

Spectral reflectance curves (±1s.e.) of cresol-treated breast feathers of American Goldfinches (a) against a black background after being chemically de-pigmented (b) against a black background intact, and (c) against a white background intact. Panel A shows that de-pigmented feathers became transparent after cresol treatment, as their reflectance matched that of the black background after being treated with cresol, except for a small increase in overall reflectance. Panel B shows that cresol-treated yellow feathers against a black background are nearly transparent with a faint yellow cast, as indicated by the small increase in brightness from 500 to 700 nm. c shows that cresol-treated yellow feathers against a white background are once again bright yellow, as carotenoids absorb light from the white background. n=5 for all feather reflectance curves. Because all feathers were measured on the same piece of background paper, n=1 for these curves.

Figure 3

(a) TEM micrograph (1900×) of a yellow American goldfinch (Carduelis tristis) feather barb. P, carotenoid pigments; K, keratin substrate; and V, air-filled vacuole. Scale bar, 1 μm. (b) Light micrograph (1000×) of a yellow American goldfinch (C. tristis) feather barb. V, Vacuole. Scale bar, 2 μm.