Paleoartists sometimes depict ancient animals affected with albinism or melanism – white or black. It’s a nice touch and certainly realistic, as they are among the most common and recognizable types of mutation affecting animal pigmentation. Since they are phylogenetically widespread today, there is no reason to think they wouldn’t have popped up in ancient animal populations too.
In the last few years, I have looked into colour genetics of several species of pets, including rats, horses and parrots, as well as collected photos of aberrant animals in the wild. It has struck me how similar the genetics of colour are across species, and how the same spontaneous mutations crop up again and again – not just albinism and melanism, but others too. Certainly, these same ones would have happened in prehistoric times, and might provide interesting ideas for paleoart. Let’s have a look!
First, some background on colour mutations. Single mutations that have a large effect on appearance (“aberrations”) are usually telltale signs that an important gene is partially or completely broken. These are rarely adaptive and might have effects far beyond just colouration. Much of the variety of domestic animals comes from breeding and inbreeding these “freaks”, which explains some of their frequent health issues. Natural selection in the wild, however, tends to act against this kind of mutations. Instead, evolution usually shapes animal colouration through numerous mutations that each have a relatively small effect.
Colour aberrations are usually (but not always) recessively inherited: the same allele must be passed down from both parents for it to be expressed. Consequently, they tend to be much more common in small populations, because the likelihood of inheriting the same allele from both parents is high when they are closely related. However, even large and healthy populations tend to have a number of these alleles circulating. Even if they are highly damaging to the organism, natural selection rarely wipes them out completely, since individuals carrying one copy are unaffected. They can stay hidden for generations, only to appear out of nowhere when two carriers happen to mate.
Many colour morphs in domestic animals are produced by expression of several recessive alleles simultaneously. These are highly unlikely to happen in the wild, so we’ll be only looking at colourations caused by a single aberration at a time.
The nomenclature of colour mutations is wild and highly confusing, as the same mutations often have multiple names for different species and even different breeds of the same species! Scientific papers too can have conflicting terminologies. I’ll be mostly using the system proposed by Hein van Grouw in his 2021 paper on bird colour aberrations. Finally, mutations are wildly diverse, as you can see by taking a closer look at any pet fancy. This article is just a simplified look at the most common types.
Albino
True albinos completely lack all melanin pigments. It’s one of the more common aberrations across species, appearing in the ballpark of one in a few tens of thousands of births. It’s caused by breaking of the TYR gene, which produces tyrosinase. Tyrosinase is an enzyme required to synthesize both melanin pigments: eumelanin (black, grey, dark brown) and pheomelanin (rufous, yellowish). It can affect almost any animal, from fish to dinosaurs to mammals. In humans, it’s known as albinism type OCA1a. True albinos are hard to tell apart from very light types of incomplete albinism, which we’ll look at later.
Albino animals generally have red eyes, pink skin and white feathers or fur. They often suffer from sunburns. Their eyesight is bad to begin with and is quickly made worse by tissue damage caused by sunlight. Albino birds often survive their nestling period just fine, but die of collisions or get snatched by predators soon after they start flying. Few true albinos of any species live a long life outside human care.
Interestingly, there seem to be no true albino horses. Perhaps there is something about their development that causes albino foals to die before birth. On the other hand, many cave fish are naturally albinos or incomplete albinos, as blindness and sensitivity to light are of no consequence underground.
Non-melanin pigments are unaffected by albinism. Carotenoid-based reds, oranges, yellows and pinks are present and can even look brighter than usual, if they are usually hidden under melanins. For example, albino leopard geckos are bright yellow. Some structural colours are also present, though others are dependent on melanins.
Acromelanism / Siamese / Himalayan
This is a particularly interesting one, caused by a different type of mutation of the TYR gene. The tyrosinase enzyme is structurally abnormal and tends to break down in high temperatures. The skin over most of an endothermic animal’s body is too warm for tyrosinase to survive, making hair or feathers either pure white or a light creamy colour. Extremities are cooler, giving these animals dark “points” to their noses, ears, paws and tails. While best known in cats, rabbits and other mammals (albinism type OCA1b TS in humans), acromelanism occurs in birds too. Acromelanistic birds tend to have whitish bodies with dark foreheads, tail feathers and primaries. Ectothermic animals probably have similar mutations, but due to their different body temperature patterns, they will look unrecognizable. Presumably acromelanism would also be visible in pigmented naked or scaly skin (such as featherless dinosaurs), since the testicles (the only pigmented skin area) of acromelanistic rats are also dark.
Acromelanism is relatively harmless, though like albinism, it can increase the risk of sunburn. It often influences eye pigmentation, which can cause issues with eyesight and light sensitivity.
Leucistic / piebald / harlequin
White markings in animals have many names, but the mechanism behind them is usually the same. They are caused by the failure of pigment cells to migrate into the skin during a set window of time during development. In mammals, markings often occur in typical, repeated locations on the body. White socks, bellies, tail tips, forehead stars and blazes are common. Markings tend to be relatively symmetrical in mammals and crocodylians, but random and asymmetrical in birds, though I don’t know why. White markings can vary from a single tiny spot all the way to entirely white animals.
Markings also occur as normal part of coloration in some wild animals. For example, red foxes have white bellies and tail tips, while arboreal chinchilla rats of the genus Cuscomys have white blazes and socks.
Markings are caused by numerous mutations with complex inheritance, but they are usually related to the development of an embryonic structure called the neural crest, from which the pigment cells start their journey towards the skin. Starting late, not all make it to their designated skin areas in time. Other neural crest cells go on to make parts of facial cartilage and bone, smooth muscles and some parts of the nervous system, including neurons of the digestive tract.
Markings are sometimes inherited together with temperament: animals with more babylike (docile / domesticated) behaviour tend to be more likely to have white markings, because the development of their nervous system is delayed. An interesting example is the Siberian domestic fox experiment, in which piebald foxes became increasingly common when tame temperaments were selected.
Usually markings are harmless and adult piebald animals are regularly seen in the wild, as long as they don’t get picked by predators for being too visible or too trusting. Sometimes though, certain markings are associated with neurocristopathies: severe syndromes affecting facial proportions, hearing, cognition, heart, and digestive system. One of them is Waardenburg syndrome, which occurs in humans, dogs, cats, cattle and rats among others.
Progressive greying
Progressive greying has a somewhat confusing name, as it refers to turning white. In animals with this type of mutation, the amount of white hair, feathers or spots starts small or nonexistent and grows with age. It seems to be related to premature aging of the pigment cells, at least in horses, which are the best studied.
This type of mutation is called grey in horses, greying in dogs and husky in rats. Types of it are regularly seen in wild birds, making it plausible for dinosaurs especially. In birds, it often produces a Dalmatian-like pattern of randomly occurring white feathers. People have followed progressively greying individual garden birds over several years, showing that they can live long and successful lives in the wild.
Dilutions, browns, incomplete albinism
This is a massive group of mutations that produce different types of pastel-colored animals with lighter hues and softer contrasts than usual. They range from fairly common to extremely rare. They are known with a massive and confusing mess of names, including blue, Russian blue, fawn, cinnamon, cream, pastel, Isabel, silver, saffron, chestnut etc.
Brown, ino or incomplete albinism are mutations that reduce the amount of melanins and / or cause them to be incompletely oxidized during their synthesis in the pigment cells. They turn blacks into browns and rufous hues creamy. Incompletely oxidized pigments are also easily bleached by sunlight. In extreme cases, these animals can look like albinos, all the way to having red eyes, but they still have some melanins, even if it’s a minuscule amount. In many species, these are called pink eyed dilutions (though in the terminology used here, it’s not a dilution). Their eyesight is far better than true albinos, and they have a higher probability to survive into adulthood. Redheaded people can also be considered a type of incomplete albinism, as their skin and hair lack most eumelanin, but pheomelanin is present.
Dilutions, on the other hand, cause melanin granules to form clumps. Though they have the same amount of pigment as “wild type” animals, having it aggregated into clumps here and there makes the animal appear much lighter. Usually “brown” type mutations produce warm brownish and creamy colours, while dilutions tend to be blue-grey or silvery.
Many of these mutations are harmless, but depending on the specific gene mutated, they can come with neurological problems, skin issues and blood clotting disorders all too familiar to some pet owners.
As with true albinos, dilution and brown leave non-melanin colours unaffected and can even reveal bright hues usually concealed by melanin. This is the kind of mutation exhibited by the most famous of pet fish – the goldfish. Wild goldfish (Crucian carps) are silvery-grey, like most cyprinids. The “golden” mutation eliminates their black pigment cells (ameba-like things called melanophores in fish, amphibians and squamates), leaving xanthophores and erythrophores, cells containing yellow and red pigments. Golden individuals are known from numerous species of bony fish, from lungfish and gar to goldfish and perch. Sharks, on the other hand, don’t seem to have xanthophores. Rare individuals lacking melanin pigments show a whitish skin underneath.
Melanism
A polar opposite of albinism, melanistic animals have extra eumelanin, varying from slightly darker than usual to all black. These mutations are often connected to the agouti signaling protein, which regulates the process in which pigment cells switch between producing eumelanin and pheomelanin. If agouti signaling protein is not present (the gene for it is broken), the default condition is to produce eumelanin all the time, causing the animal to have all black fur or feathers. Ancient DNA research showed that black horses were present in Europe during the early Holocene, making black one of the wild type colours of horses.
Interestingly, the agouti signaling protein is also present in mammalian brains, where it has an important part in temperament and stress response. Non-agouti animals of several species are more docile, less reactive and have naturally lower stress levels than their wild type counterparts.
In birds, subtler kinds of melanism are common, causing black parts of the plumage to be bolder, overrun their usual boundaries and sometimes giving a dark overtone to the entire bird, but without completely covering the original colours. These might be interesting to depict on non-avian dinosaurs or pterosaurs.
I hope you have found these mutations and their implications to prehistoric life as interesting as I do!
Humming Dinosaurs and Nature's Other Small Wonders 
Really good writeup of the topic, and welcome back!
Welcome back! I missed your content so much!