In the first part of this series, we went through bird colours formed by melanosomes and structural properties of their skin and feathers. These mechanisms produce blacks and greys, browns, iridescences, blues and purples. These are also the only colours that can be at the moment partially deciphered from well-preserved fossils.
However, that is not even close to the full rainbow of living birds, and unlikely was that for non-avian dinosaurs either. In this article, we’ll go through other mechanisms living birds use to produce colour, and speculate how likely each of them would be in ancient non-avian species.
Fruity hues – carotenoids
Carotenoids are a huge group of some 1100 pigments, mostly produced by plants, algae, and some fungi and bacteria. The only animals capable of producing their own carotenoids are, oddly, tiny spider mites and aphids, who have hijacked the necessary genes from fungi.
This lack of carotenoid genes does not stop the pigments from being widely used in animals. Dietary carotenoids – yellow, orange, red and purple – are passed up in the food chain and metamorphosed to other carotenoid types within the bodies of animals. They are present in the skin, scales and feathers of numerous bird species, perhaps most famously flamingoes. They are the pigments behind the bright yellows and oranges in the chests of many songbirds, and the colourful feet and beaks of mallards and puffins. They are also widely present in squamates, fish, and even some mammals, like bats and humans.
An analysis published in 2014 estimated that in living clades of birds, carotenoid-pigmented feathers have evolved at least 13 times (Thomas et al. 2014). Given the widespreadness and plasticity of this characteristic, it seems quite unlikely to suggest non-avian dinosaur never used it. It is certainly in the realm of plausible speculation, for both feathered and scaly species.
Carotenoids are not just pigments, but also valuable as antioxidants that protect animal cells from oxidative stress and cancer. This makes them a highly useful sexual ornament, as it necessarily acts as a honest signal: only individuals in good condition can spare carotenoids for display. In some bird species with carotenoid-based display colours, individual variation is indeed unusually high. An Eurasian example is the common rosefinch (Carpodacus erythrinus), breeding males of which vary from almost completely drab brown to mostly bright scarlet. In captivity, these animals need to be fed a special diet, or they lose their colours. Rosefinches, for example, generally turn into pale rose in captivity, while other red finches, like crossbills (Loxia) and redpolls (Acanthis) turn yellow (Stradi et al 2001).
Carotenoids can be mixed with melanins and structural colours much like mixing paint. For example, most green feathers are produced by a mix of yellow carotenoid and a blue-refracting spongy structure. By adding some black eumelanins to the mix, you get olive green. Oddly, the brilliantly blue feet of blue-footed boobies also have a mix of structural colour and carotenoid. A study (Velando et al. 2006) showed that the colour of booby feet is an up-to-date billboard on the animal’s condition: it dulls in just a couple of days if the bird goes hungry and has no lipids and carotenoids to carry to the feet, but brightens again when fed. This kind of systems were quite likely widespread in non-avian dinosaurs too.
These blue-with-carotenoid-hues are probably only possible on integument that has the form suitable for structural colour, which, as discussed in the previous article, is not necessarily the case with hair-like protofeathers.
Should we then conclude there were probably no green furry dinosaurs? Not so fast…
Copper and blood – porphyrins
Another relatively widespread group of pigments in living birds are porphyrins. Unlike carotenoids, they are produced within the bird’s own bodies and are indeed part of the basic animal metabolism: heme, the red iron-bearing pigment in your blood, is a porphyrin.
Relatively few birds are known to use porphyrins to colour their feathers, but they are widely scattered among the bird family tree, and new ones are still being found. Porphyrin feathers are far from being a unique specialty of a single lineage of birds, as is sometimes implied.
The most well known porphyrin-bearers are the green turacos (Tauraco) of Africa. These birds have a true green pigment, turacoverdin. Like heme, it too gets its colour from a metal atom – but copper instead of iron. Yet another porphyrin, turacin, produces the brilliant crimson details in turaco plumages.
Porphyrin-based greens are also known in distantly related birds. These include the crested partridge (Rollulus rouloul), blood pheasant (Ithaginis cruentus), northern jacana (Jacana spinosa), African pygmy goose (Nettapus auritus), and males of eider ducks (Somateria) (Dyck 1992). Not so rare after all!
Green porphyrins, being true pigments, do not have requirements of feather complexity. They could possibly also have been present in dinosaurian skin or scales, though structural blue mixed with carotenoid would also be a possibility there.
In addition to these permanent pigments, surprisingly many birds also have more ephemeral porphyrins, of kinds that are quickly broken by sunlight. The undersides of wing feathers in many owls, for example, have a porphyrin that is colourless to human eyes, but has a strong bright pink fluorescence under UV light. It is still unknown whether the owls or their prey can actually see this pink in natural light. It could be that they use it for intraspecies communication, or that the fluorescence helps mask them against the sky in the eyes of rodents. For ornithologists, it’s highly useful that this colour fades in a predictable manner over months, and can be used to age the owls.
Even more ephemeral are salmon-pink details in the white fluffy feathers of bustards (Otididae). These huge birds have a lekking system, where males display to impress females, fluffing up their feathers. When the males fluff themselves for the very first time that season, the pink lower parts of their feathers are exposed for the first time, and will fade to white in just minutes. It was suggested (Galván et al. 2016) that this functions as a sort of ‘virginity signal’, informing the females that this particular male has just arrived. That might be an important piece of information if, for example, they are looking for males that have not yet depleted their sperm stores.
Some birds seem to have insisted on reinventing the wheel. Parrots, for example, are famous for their bright colours, but despite having a diet rich in carotenoids, they don’t have any in their feathers. Instead, they synthesize a unique class of pigments known as psittacofulvins (McGraw & Nogare 2004). These have much the same hues as carotenoids (except the reds are deeper) and are also used to produce green in combination with blue structural colour (Tinbergen 2013).
While evolution often comes up with the same innovations multiple times, giving non-avian dinosaurs macaw-like psittacofulvin colours seems far-fetched. This might be the one class of pigments that really probably wasn’t present.
Another ‘unique’ pigment is trickier. Only in 2007 it was revealed (McGraw et al. 2007) that the bright yellows present on the plumage of many penguins are actually not carotenoids, but a formerly unknown type of pigment named spheniscin. Unfortunately, the same name is also used for a peptide found in penguin stomachs, which makes Googling for information somewhat inconvenient.
According to the paper, these pigments probably belong to a class called pterins, named after where they were originally discovered, that is, in butterfly wings (pteron = wing). These yellow, orange and red pigments are widespread in animals, including insects, fish, amphibians, and reptiles. They aren’t completely unknown in non-penguin birds either: the eyes of many birds get their colour from pterins.
It appears that the lineage leading to birds (or perhaps to all archosaurs, since crocodylians also lack them) has lost the pterins in their skins and feathers. However, they kept synthetizing pterins for their eyes. Penguins have then independently reintroduced it for their feathers for whatever weird reason, since their diet is certainly not lacking in carotenoids.
If dinosaurs have had the potential to produce pterins all along, it’s entirely possible they have transferred it into their feathers or scales multiple times. However, speculation with pterins has little to offer to paleoart: they are essentially the same colour as carotenoids.
The take home message? There is a wide palette of plausible colour for non-avian dinosaurs, and no need to stick with mammalian grey mouse colours, since dinosaurs aren’t and weren’t partially colour blind like us.
However, not every species needs to, or can afford to, be a macaw or a peacock. In the next part, we’ll take a look at the ecology of colour: how natural selection shapes animal colour.
Natural History Magazine: Dark moon traveler. Photos of the UV fluorescence of owl wings.
Cornell Lab of Ornithology: How birds make colorful feathers?
Dyck 1992: Reflectance spectra of plumage areas colored by green feather pigments. The Auk. (pdf)
Galván et al. 2016: Porphyrins produce uniquely ephemeral animal colouration: a possible signal of virginity. Scientific Reports.
McGraw & Nogare 2004: Carotenoid pigments and the selectivity of psittacofulvin-based coloration systems in parrots. Comparative Biochemistry and Physiology – Part B: Biochemistry & Molecular Biology.
McGraw et al. 2007: A description of unique fluorescent yellow pigments in penguin feathers. Pigment Cell Research.
ScienceDaily 25.2.2009: Carotenoids Are Cornerstone Of Bird’s Vitality.
Stradi et al. 2001: Carotenoids in bird plumage: the complement of red pigments in the plumage of wild and captive bullfinch (Pyrrhula pyrrhula). Comparative Biochemistry and Physiology – Part B: Biochemistry & Molecular Biology.
Thomas et al. 2014: Ancient origins and multiple appearances of carotenoid-pigmented feathers in birds. Proceedings of the Royal Society B.
Tinbergen et al. 2013: Spectral tuning of Amazon parrot feather coloration by psittacofulvin pigments and spongy structures. Journal of Experimental Biology.