Charles Darwin acknowledged the importance of colour in the natural selection of bird plumage. Colour can indicate age, sex, and diet, as well as play roles in camouflage, mating and establishing territories. Feather and integument colour depend on both chemical and structural characteristics and so both structure and chemistry can be used to infer colour and pigment patterns in a range of extant and fossil organisms.
Charles Darwin acknowledged the
importance of color in the natural selection of bird plumage. color can
indicate age, sex, and diet, as well as play roles in camouflage, mating and
establishing territories. Feather and integument color depend on both chemical
and structural characteristics and so both structure and chemistry can be used
to infer color and pigment patterns in a range of extant and fossil organisms.
A specific cell controls
pigment patterns, the melanocytes, during feather growth. There are two main
melanin pigments in animal tissues, eumelanins and pheomelanins. The former are
more prevalent (>75%) and furnish dark black or brown hues in both
invertebrates and vertebrates. A diagnostic and functional component of the
molecular structure of melanin is their carboxyl substituents.
charged end-groups function as cation chelators, selectively binding positively
charged particles, such as free radicals and transition metals. Consequently, melanin
granules in bird feather melanosomes display high concentrations of zinc,
copper, calcium and iron. Birds may have evolved this ability to accumulate
toxins in feathers to avoid the chemotoxic affects of minerals and contaminants
in their diet. The mere presence of trace-metals in melanin may play a key role
in the preferential preservation of feathers. When the black and white feathers
of domestic chickens are exposed to feather-degrading bacteria (Bacillus licheniformes), white feather breakdown
significantly faster than the black melanised feathers. Such studies suggest
plumage color might be an evolutionary response to the presence of
feather-degrading bacteria, with high melanic content being more resistant to
decay. The distribution of metal chelates in soft tissue has been preserved in Archaeopteryx and provides a useful
biomarker for eumelanin patterning.
The biocidal properties and
non-biodegradability suggest that trace-metal distributions may represent the
definitive method for revealing eumelanin pigmentation patterns. Synchrotron-based elemental mapping
and spectroscopy of trace-metal inventories in Archaeopteryx have made it possible to make progress in
understanding pigmentation. Results
from the synchrotron-based imaging show clear evidence for the presence of
Cu-O/Cu-N complexation, indicative of endogenous melanin pigments being preserved
within the exceptionally preserved feathers of this early bird. However, it is
the trace-metal coordinated biochemistry of melanin-type pigments in this
iconic fossil that potentially played a key role, both in the life and preservation
of this organism. Feathers, rich in trace-metals, functioned as a ‘natural-biocide’
protecting these structures from bacterial decay in life, but also delivering the
multitude of selective advantage through color. The very same trace-metals inhibited
the natural processes of decay after death, resulting in rare and remarkable fossils
that display soft tissue structures.