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Journal of Experimental Biology partnership with Dryad

Structural colouration of mammalian skin: convergent evolution of coherently scattering dermal collagen arrays
Richard O. Prum, Rodolfo H. Torres
  1. Fig. 1.

    Structurally coloured skin of mammals examined. (A) Male mandrill (Mandrillus sphinx) blue facial skin (reproduced with permission from Corbis Photo). (B) Male mandrill blue rump skin (reproduced with permission from Jay Peterson). (C) Male vervet monkey (Cercopithecus aethiops pygerythrus) with vividly blue scrotum (reproduced with permission from Roland Kays). (D) Close-up of the blue scrotum of a vervet monkey (C. a. pygerythrus; reproduced with permission from Dr Kenneth Soike). (E) Male mouse opossum (Marmosa robinsoni) with blue scrotum (reproduced with permission from Louise Emmons). Photo depicts a species that is closely related to the species examined (M. mexicana).

  2. Fig. 2.

    Light micrographs of structurally coloured skin of (A) male mandrill (Mandrillus sphinx) facial skin; (B) male mandrill rump skin; (C) vervet monkey (Cercopithecus aethiops) scrotum and (D) wooly opossum (Caluromys derbianus) scrotum (epidermis detached and missing). All specimens were stained with Masson's trichrome, which stains collagen blue. All scale bars are 100 μm. Abbreviations: c, collagen; e, epidermis; m, melanin.

  3. Fig. 7.

    Fourier predictions of the reflectance spectra (black; right axes) for different structurally coloured mammal tissues in comparison with their respective reflectance spectra (grey; left axes). (A,B) Male mandrill (Mandrillus sphinx) facial skin. (C,D) Male mandrill rump skin. (E,F) Female mandrill facial skin. (G) Vervet monkey (Cercopithecus aethiops sabaeus) `pearly' light blue scrotum. (H) Vervet monkey (Cercopithecus aethiops pygerythrus) vividly blue scrotum with reflectance data from Findlay (1970). Samples A and C, and B and D were from the same individual males, respectively.

  4. Fig. 5.

    Two-dimensional Fourier power spectra from transmission electron micrographs of colour-producing collagen arrays from: (A) male mandrill (Mandrillus sphinx) facial skin; (B) male mandrill rump skin; (C) female mandrill facial skin and (D) vervet monkey (Cercopithecus aethiops) scrotum. Colour scale (from blue to red) indicates the relative magnitude of the squared Fourier components, which are dimensionless quantities. Direction from the origin indicates the direction of the 2-D component waves in the image, and the distance from the origin indicates the spatial frequency (cycles nm–1) of each Fourier component. The ring-shaped distribution of Fourier power demonstrates a predominant periodicity at intermediate spatial frequencies that is equivalent in all directions. The diameter of the ring is inversely proportional to the wavelength of the coherently scattered colour, so the power spectrum ring of the (A) light blue mandrill facial skin (Fig. 2A) has a smaller diameter than the (B) dark blue mandrill rump skin (Fig. 2C).

  5. Fig. 6.

    Radial averages of power spectra from (A) male mandrill (Mandrillus sphinx) facial skin; (B) male mandrill rump skin; (C) female mandrill facial skin and (D) vervet monkey (Cercopithecus aethiops) scrotum. The shaded zone shows the range of spatial frequencies that are likely to produce mammal-visible coherent reflections, assuming a visible spectrum of 400–700 nm and an average refractive index in the dermis of 1.4. Peaks in the grey zone demonstrate that the predominant spatial periodicity of the collagen arrays is appropriate for the production of visible colours by coherent scattering. Sample sizes for each analysis are shown in Table 1.

  6. Fig. 3.

    Reflectance spectra of fixed samples of structurally coloured mammal skin: (A,B) male mandrill (Mandrillus sphinx) facial skin; (C,D) male mandrill rump skin; (E,F) female mandrill facial skin; (G) vervet monkey (Cercopithecus aethiops) scrotum. Samples A and C, and B and D were from the same individual males, respectively.

  7. Fig. 4.

    Transmission electron micrographs of collagen arrays from structurally coloured mammal skin. (A) Female mandrill (Mandrillus sphinx) facial skin at 7500×. Scale bar, 500 nm. (B) Male mandrill rump skin at 25 000×. Scale bar, 1000 nm. (C) Male mandrill facial skin at 40 000×. Scale bar, 250 nm. (D) Male mandrill rump skin at 40 000×. Scale bar, 250 nm. (E) Vervet monkey (Cercopithecus aethiops) scrotum at 25 000×. Scale bar, 500 nm. (F) Mouse opossum (Marmosa mexicana) at 50 000×. Scale bar, 100 nm.