Expanding the eggshell colour gamut: uroerythrin and bilirubin from tinamou (Tinamidae) eggshells

Randy Hamchand,Daniel Hanley,Christian Brückner,Richard O. Prum

doi:10.1038/s41598-020-68070-7

Abstract

To date, only two pigments have been identified in avian eggshells: rusty-brown protoporphyrin IX and blue-green biliverdin IXα. Most avian eggshell colours can be produced by a mixture of these two tetrapyrrolic pigments. However, tinamou (Tinamidae) eggshells display colours not easily rationalised by combination of these two pigments alone, suggesting the presence of other pigments. Here, through extraction, derivatization, spectroscopy, chromatography, and mass spectrometry, we identify two novel eggshell pigments: yellow–brown tetrapyrrolic bilirubin from the guacamole-green eggshells of Eudromia elegans, and red–orange tripyrrolic uroerythrin from the purplish-brown eggshells of Nothura maculosa. Both pigments are known porphyrin catabolites and are found in the eggshells in conjunction with biliverdin IXα. A colour mixing model using the new pigments and biliverdin reproduces the respective eggshell colours. These discoveries expand our understanding of how eggshell colour diversity is achieved. We suggest that the ability of these pigments to photo-degrade may have an adaptive value for the tinamous.

Highlights

Birds’ eggs are found in an expansive variety of shapes, sizes, and c olourings[1]
The compound proved to be identical in retention time, composition (C35H39N4O6 for [M + H]+ as per electrospray high-resolution mass spectrometry in the positive ion mode (ESI + HR-MS)), and molecular ion fragmentation pattern to biliverdin dimethyl ester 27 or emu eggshells (2Me) extracted from emu (Dromaius novae‐ hollandiae) eggshells[28] and of commercial samples
Employing colour mixing models[35], we find that combinations of biliverdin 2Me with uroerythrin 3Me or bilirubin 4Me could generate approximate colour matches to the reflectance spectra of the surfaces of the N. maculosa and E. elegans eggshells, respectively

Summary

Introduction

Birds’ eggs are found in an expansive variety of shapes, sizes, and c olourings[1]. The diverse array of appearances found across Aves is achieved—in large part—through a combination of structural features, solid or patterned colorations, the use of two different dyes, and differential pigment deposition. Eggshell pigments are embedded within the white calcium carbonate matrix of the egg and within a thin outer proteinaceous layer called the cuticle[2,3,4] These pigments are believed to play a key role in crypsis[5,6], other, possibly d ynamic[7,8], roles in inter- and intra-species signalling[5,9,10,11,12] are possible. Tinamou eggs exhibit a diversity of bright colours, ranging from blues and greens to exotic greys and deep purplish-browns Their eggshell surfaces have a distinctive glossy, porcelain-like appearance generated by nanostructured surface calcite and calcium phosphate c rystals[3]. We re-examined these tinamou eggshells, extracting and identifying their pigments, and analysed their contributions to the observed eggshell coloration

Methods

Results

Discussion

Conclusion