S H O R T C O M M U N I C A T I O N
Detecting pigments from colourful eggshells of extinct birds
Branislav Igic
•
David R. Greenwood
•
David J. Palmer
•
Phillip Cassey
•
Brian J. Gill
•
Tomas Grim
•
Patricia L. R. Brennan
•
Suzanne M. Bassett
•
Phil F. Battley
•
Mark E. Hauber
Received: 10 November 2009 / Accepted: 15 December 2009
Ó Birkha¨user Verlag, Basel/Switzerland 2009
Abstract
The known chemical basis of diverse avian
eggshell coloration is generated by the same two classes of
tetrapyrrole pigments in most living birds. We aimed to
extend the evolutionary scope of these patterns by detect-
ing pigments from extinct birds’ eggs. In our samples
biliverdin was successfully extracted from subfossil shell
fragments of the blue-green egg-laying upland moa
Megalapteryx didinus, while protoporphyrin was extracted
from the beige eggs of two other extinct moa species. Our
data on pigment detection from eggshells of other extant
paleognath birds, together with published information on
other modern lineages, confirm tetrapyrroles as ubiquitous
and conserved pigments contributing to diverse eggshell
colours throughout avian evolution.
Keywords
Egg matrix
Á Pigmentation Á Radiation Á
Ratite
Introduction
Avian eggshells are some of the most diversely coloured
natural materials (Kilner
2006
). Despite analytical advan-
ces in the chemical characterisation of pigments, the
B. Igic
Á D. R. Greenwood Á D. J. Palmer Á
P. F. Battley
Á M. E. Hauber
School of Biological Sciences, University of Auckland,
Auckland, New Zealand
B. Igic
e-mail: bigi001@aucklanduni.ac.nz; brani.igic@gmail.com
D. J. Palmer
e-mail: david.greenwood@auckland.ac.nz;
d.palmer@auckland.ac.nz; p.battley@massey.ac.nz
D. R. Greenwood
Plant and Food Research, University of Auckland,
Mt. Albert Campus, Auckland, New Zealand
e-mail: david.greenwood@auckland.ac.nz
P. Cassey
School of Biosciences, University of Birmingham,
Birmingham, UK
e-mail: p.cassey@bham.ac.uk
B. J. Gill
Auckland War Memorial Museum, Auckland, New Zealand
e-mail: bgill@aucklandmuseum.com
T. Grim
Department of Zoology and Laboratory of Ornithology,
Palacky University, Olomouc, Czech Republic
e-mail: tomas.grim@upol.cz
P. L. R. Brennan
Department of Ecology and Evolutionary Biology,
Peabody Museum, Yale University, New Haven, USA
e-mail: patricia.brennan@yale.edu
S. M. Bassett
Department of Zoology, University of Otago,
Dunedin, New Zealand
e-mail: oralis@ihug.co.nz
P. F. Battley
Ecology Group, Massey University,
Palmerston North, New Zealand
e-mail: p.battley@massey.ac.nz
M. E. Hauber (
&)
Department of Psychology, Hunter College,
CUNY, New York, USA
e-mail: mark.hauber@hunter.cuny.edu
Chemoecology
DOI 10.1007/s00049-009-0038-2
CHEMOECOLOGY
diversity of birds’ egg colours has been solely attributed to
pyrroles (reviewed in Gorchein et al.
2009
). In birds, the
bile pigment biliverdin produces the blue-green human-
visible appearance of eggs while protoporphyrin (Miksik
et al.
1996
) is typically detected from red-brown maculated
eggs. Immaculate eggs of diverse colours can contain either
or both biliverdin and protoporphyrin, while white eggs
may contain one, both, or no detectable pigments at all
(Kennedy and Vevers
1976
).
We aimed to extend the comparative scope of prior
published pigment analyses by examining subfossil frag-
ments from an extinct avian lineage. Amongst paleognath
birds, most species of New Zealand’s moa (Dinornithidae/
Emeidae) laid eggs whose remains appear grey to beige to
human observers (Gill
2007
). In contrast, eggshell frag-
ments attributed to the Upland Moa (Megalapteryx didinus;
Megalapterygidae; Bunce et al.
2009
), are typically pale to
bright green to the human eye (Fig.
1
a). One extraordinary
specimen of this species is a near-complete skeleton of a
female, confirmed by nuclear DNA sequences (Huynen
et al.
2003
), that surrounded an apparently unlaid blue egg
(Gill
2007
).
Here we used modern chemical techniques to determine
whether biliverdin and protoporphyrin could be detected in
colourful subfossil moa eggs. Although all taxa of the
diverse moa radiation have been extinct for over 500 years,
their mating systems, size dimorphism patterns, and
plumage colours have been the subject of several recent
Ratite taxon:
F
Struthio camelus
D
Euryapteryx curtus
C
Dinornithidae/Emeidae spp
40
60
80
B
Megalapteryx didinus (AV10049)
A
Megalapteryx didinus (AV7477)
E
Dromaius novaehollandiae
0
20
Reflectance (%)
G
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 1 a
–f Representative
eggshell fragments (white scale
bar 1 cm for a–d and 2 cm for
e
, f) and g reflectance spectra of
extinct and extant ratites
included in our analysis
B. Igic et al.
discoveries using ancient DNA and microscopy-based
techniques (e.g. Bunce et al.
2003
, Turvey et al.
2005
,
Rawlence et al.
2009
). This continued interest in moa
renders them a highly desirable focal lineage for new
analyses of the chemical basis of egg coloration. However,
as eggshells’ appearances both change over time, even
when specimens are stored in museums (Starling et al.
2006
), and vary with environmental conditions at the time
of laying (Aviles et al.
2007
), in the absence of a known
time series of moa eggs and their colours, our tests nec-
essarily remain exploratory.
Methods
Sample description
Three moa eggshell samples were kindly provided by
Otago
Museum,
New
Zealand:
(1)
green
eggshell
(AV7477), attributed to Megalapteryx didinus, from Chatto
Creek, Central Otago, South Island; (2) blue eggshell
(AV10049) of the unlaid M. didinus egg from Mount
Aspiring National Park, South Island; (3) beige moa egg-
shell (AV7371, species unidentified) from Central Otago,
South Island. Upland Moa remains are known only from
the South Island (Tennyson and Martinson
2006
), and so
we sourced a fourth moa eggshell fragment from the North
Island of New Zealand (Auckland Museum LB12048),
beige, probably of Euryapteryx curtus, from Tokerau
Beach, Northland.
For a comparison with extant paleognath birds, we
extracted pigments from eggshell fragments from extant
ratites, including the following samples: (1) a fresh farmed
ostrich egg Struthio camelus; (2) a fresh farmed emu egg
Dromaius novaehollandiae; (3) two North Island Brown
Kiwi Apteryx mantelli eggs hatched successfully in cap-
tivity at Rainbow Springs, Rotorua, (1)–(3) all sourced in
New Zealand; (4) a farmed Greater Rhea Rhea americana
egg purchased from a store in Berkeley, USA; (5) a fresh
captive Chilean Tinamou Nothoprocta perdicaria egg from
British Columbia, Canada, and (6) a fresh Great Tinamou
egg Tinamus major from a nest at La Selva, Costa Rica
(Brennan
2009
).
Colour analyses
All eggshell fragments were washed with 70% ethanol and
air dried. Each sample was photographed with a digital
camera and, using Image J 1.40 (National Institute of
Health, USA) to calculate surface areas. We collated
published information on the human-perceived colours of
each species’ eggs (Walters
1994
; Gill
2007
). To record
physical measures of appearance, prior to destructive
chemical analysis, the reflectance of eggshells for extinct
moa and two extant ratites was also documented following
Cassey et al. (
2009
) (Fig.
1
).
Pigment analysis
We followed the extraction protocol for eggshell pigments
given in Kennedy and Vevers (
1976
). Each eggshell sam-
ple of measured area (maximum * 1 cm
2
) was dissolved
in fresh 5% sulphuric acid in methanol and steeped for
1–2 days (no longer than 2) before filtering through 1 mL
barrier pipette tips (Axygen Biosciences) under slight
pressure. The acidified methanolic filtrate was extracted
into dichloromethane/methanol/water (1:2:1 v/v/v) three
times recovering the lower phase of the organic layer each
time, then once in 10% sodium chloride solution and twice
with water, ensuring the pH of the final water wash was
above 5. The organic solution was evaporated to dryness
under a stream of nitrogen and dissolved in 1 ml of
methanol. An aliquot was measured in an Agilent 8453
diode array spectrophotometer for its absorption spectrum
from 220 to 1,000 nm versus methanol (as a blank).
An indicator of the pigments present was evident from
these spectra and then confirmed by mass spectrometry. To
assign the presence or absence of biliverdin and proto-
porphyrin we used, high-resolution accurate mass infusion
mass spectrometry on a ThermoFinnigan LTQ FTMS ion
cyclotron resonance (ICR) mass spectrometer operating in
electrospray mode. We employed tandem mass spectrom-
etry (MS
n
) analysis with helium as the collision-induced
dissociation gas on diluted methanolic samples using sev-
eral diagnostic ions in the ion-trap (see below) followed by
accurate mass using a resolution of 100,000 (at m/z 400) in
the ICR cell on these ions when present in sufficient con-
centration. Several orders of magnitude of concentration
(*10
6
) were discernable through this approach, confirming
the identity of the pigments though their diagnostic daughter
ion fragments and atomic composition with accurate mass
values less than 2 ppm.
A quantitative assessment of both biliverdin and proto-
porphyrin was also conducted for a separate subset of the
available shells using the same pigment extraction protocol
followed by analysis on an ion-trap mass spectrometer.
Samples were analysed by flow injection analysis using
an Agilent 1100 series capillary HPLC, delivering
95% methanol with 0.1% formic acid at a flow rate of
20 lL/min and coupled to an Agilent ion-trap mass
spectrometer model SL with an electrospray ionization
interface. Biliverdin IXa dimethyl ester and protoporphyrin
IX dimethyl ester were quantified simultaneously by mul-
tiple reaction monitoring; simply protonated biliverdin IXa
dimethyl ester was isolated at m/z 611.4 and was quantified
using the fragment at m/z 311.1, fragments at m/z 209.1 and
Detecting pigments from colourful eggshells of extinct birds
m/z 283.2 were used as qualifier ions, simply protonated
protoporphyrin IX dimethyl ester was isolated at m/z 591.3
and was quantified using the fragment of m/z 513.3, frag-
ments at m/z 485.3 and m/z 445.3 were used as qualifier
ions. Biliverdin IXa dimethyl ester and protoporphyrin IX
dimethyl ester obtained from Frontier Scientific Inc.
(Logan, Utah) were used as standards and they gave a
linear response over the range 8 fmol to 2.4 pmol.
For the quantitative assessment measurements, we
standardised the detected pigment concentrations to egg-
shell fragment size used in extractions by dividing
measurements by eggshell sample area to calculate pig-
ment concentration as pmol/cm
2
. We consider this
appropriate because pigments are predominantly found
within the eggshell cuticle (Miksik et al.
2007
). Presence of
compounds is reported only for samples where detected
concentrations were elevated relative to internal controls.
To describe quantitative data, we compared the findings
from moa explicitly with our result from eggs of emu (a
known source of biliverdin, but not protoporphyrin) and
ostrich (a known source of protoporphyrin but not bili-
verdin) (Kennedy and Vevers
1976
).
Results
Detection of biliverdin
The result of our two detection techniques allowed us to
identify the presence of biliverdin from shell samples of
two different Upland Moa eggs, but not of the other two
moa taxa in our sample (Table
1
). The concentration
of recovered biliverdin from the eggshell samples was
0.21 pmol/cm
2
from Upland Moa AV10049, which was
two magnitudes lower than that recovered from the emu
(19.8 pmol/cm
2
) and 20 times higher than that from the
ostrich (0.001 pmol/cm
2
).
Detection of protoporphyrin
We successfully detected protoporphyrin from the beige
eggshells of both the South Island moa (0.30 pmol/cm
2
)
and the North Island moa (0.37 pmol/cm
2
) samples. These
concentrations were similar in magnitude to that of proto-
porphyrin detected from the ostrich eggshell (0.10 pmol/
cm
2
). We did not detect protoporphyrin above instrumental
threshold in either the green or the blue shells of Upland
Moa or the emu sample.
Discussion
Despite the small number of taxa and shell fragments
available to us, a necessary corollary of our methods to
destructively sample museum specimens of extinct species,
we detected the pigments biliverdin and protoporphyrin
from different subfossil eggshells of extinct birds. All of
the moa fragments were collected from different localities
and had been stored in different storage units or facilities
prior to analysis and we positively detected both classes of
pigments from different specimens. Nonetheless, it remains
possible that the reported patterns of pigment detection are
due to environmental contaminants or chemical and phys-
ical changes to the pigments on and inside the eggshell
matrix that occurred during the unknown period between
egg formation and laying and the time of pigment extrac-
tion in this study. For example, there was disagreement
in the positive detection of biliverdin between our two
Table 1
Outputs of two analytical detection protocols of biliverdin from extinct and extant paleognath bird eggshell samples available for
destructive sampling
Taxon
High-resolution accurate mass
infusion mass spectrometry
Liquid chromatography-
mass spectrometry
Combined
detection
Human-perceived
eggshell colour
Dinornithidae/Emeidae spp.
a
n/c
No
No
Beige
Megalapteryx didinus (AV10049)
a
n/c
Yes
Yes
Blue
Megalapteryx didinus (AV7477)
a
Yes
No
Yes
Green
Euryapteryx curtus
a
n/c
No
No
Beige
Struthio camelus
No
No
No
Beige
Apteryx mantelli
No
n/c
No
White/blue
Dromaius novaehollandiae
Yes
Yes
Yes
Green
Rhea americana
Yes
n/c
Yes
White/blue
Nothoprocta perdicaria
Yes
n/c
Yes
Brown
Tinamus major
Yes
n/c
Yes
Blue
n/c refers to analyses not conducted
a
Extinct taxa
B. Igic et al.
analytical methods when destructively sampling two dif-
ferent fragments of the green Upland Moa egg (Table
1
),
implying a sensitivity to sample identity of the detection
protocols.
We cannot be certain of either the original coloration of
the extinct birds’ eggs, nor the interpretation of negative
chemical detection results in our analyses here. Nonethe-
less, following the directional predictions of published
results on patterns of human-assessed coloration and bili-
verdin detection in the eggshells of modern bird species
(Kennedy and Vevers
1976
; Moreno et al.
2006
), we also
found a consistent association of human-perceived blue-
green eggshell colours with the presence of biliverdin from
the combined ratite and tinamou eggs in our samples at the
species level (G
2
= 2.8, p = 0.045, 1-tailed; Table
1
).
It is unknown whether moa, or their competitors, para-
sites and predators (Cassey et al.
2008
), had evolved to
perceive and behaviourally discriminate between diverse
eggshell colours (Reynolds et al.
2009
). Irrespective of any
potential adaptive function of diverse egg colours, paleo-
neurological data already suggest that visual perception
was more important for moa compared to their nocturnal
kiwi relatives in New Zealand (Corfield et al.
2008
), per-
haps in the context of their diverse plumage colours
(Rawlence et al.
2009
) and/or herbivorous foraging tactics
(Fadzly et al.
2009
) of the different moa species. Our
research thus contributes to recent advances in research on
the behavioural and sensory ecology of extinct animal
lineages, including genetic and microscopic analyses of
sexual dimorphism in growth (Bunce et al.
2003
), behav-
ioural reconstruction of nesting patterns (Varricchio et al.
2008
), and neuroanatomical correlates of sensory functions
(Zelenitsky et al.
2009
; Patek and Oakley
2003
).
The discovery of biliverdin and protoporphyrin in
moa eggshells is consistent with a ubiquitous role of tet-
rapyrroles generating eggshell colours throughout avian
evolution (Kennedy and Vevers
1976
; Gorchein et al.
2009
). Physiologically and biochemically, the evolution-
arily ubiquitous and conserved role of pyrroles contributing
to eggshell coloration is mechanistically feasible because
both classes of the dominant tetrapyrrole pigments of
eggshells, are involved in the general synthesis and
catabolism of vertebrate haem, and are likely deposited as
de novo metabolites in the avian shell gland (Wang et al.
2009
).
There are repeated suggestions, based on phylogenetic
reconstruction and comparison with reptiles, that the
ancestral avian eggshell was white and immaculate
(reviewed in Kilner
2006
), in parallel with the pigment-free
white coloration of the calcium carbonate matrix of croc-
odilian eggs (our own unpublished data). Nevertheless, our
results from extinct bird eggs expand on the scope of this
conclusion as it seems likely that pyrrole eggshell pigments
are both ancient in origin and highly conserved throughout
the diverse radiations of birds. It remains to be shown what
the structural, chemical, and biological mechanisms are
that result in pigmented eggshell types and what role these
pigments play in the protein network of the eggshell matrix
(Sharp and Silyn-Roberts
1984
).
Acknowledgments
We are grateful to the School of Biological
Sciences at the University of Auckland for major support. We thank
D. Dearborn, M. Hyland, C. Moskat, H. Silyn-Roberts, The Univer-
sity of Auckland Vice-Chancellor’s Development Fund and the
Human Frontier Science Program (to P.C., T.G. and M.E.H.) for
assistance, discussions, and funding.
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Document Outline - Detecting pigments from colourful eggshells of extinct birds
- Abstract
- Introduction
- Methods
- Results
- Detection of biliverdin
- Detection of protoporphyrin
- Discussion
- Acknowledgments
- References
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