2. Mechanistic Considerations
- Melanins are the most ancient pigments in living organisms and the most widespread in animals. They appear from bacteria to humans in virtually all extant organisms . For instance, except for albino mutants, all birds seem capable of synthesizing melanins . The possibility exists that pigments different from those present in extant species evolved and disappeared in the past. If, as an example, all extant parrots (Order Psittaciformes) and turacos (Order Musophagiformes) vanished for any reason, the pigments that have evolved only in these birds, i.e., psittacofulvins and turacins , would vanish forever along with these lineages. This means that the complete disappearance of pigment classes should be accompanied by extinctions of entire lineages, which is highly improbable unless the given lineages are small and phylogenetically restricted. If this was the case, the pigments would not be very representative of the coloration of the groups: for example, the exceptionally vivid colors displayed by parrots and turacos do not represent the color patterns commonly observed among all extant birds.
- Although extinct animals must have displayed conventional colors produced by melanins, an open question that still should be addressed is the exact color hues and the patterns they created. This is because, as it has recently been claimed [9,10], color patterning may provide clues on the ecology of animals. We have, however, two reservations about such claims. The first is that deciphering the ecology of extinct animals through their color is still out of reach. The second concern is that fossilized melanin granules provide, at best, information on the presence of the two main chemical forms of melanins (i.e., eumelanin and pheomelanin; e.g., ), but this is notenough to determine the exact color patterning of extinct species. A long-standing assumption is that eumelanin is a dark polymer that gives rise to colors from black to grey, while pheomelanin produces lighter, rufous colorations, and this has been embraced by most studies on preserved melanin granules to make claims about our capacity to infer the color of extinct animals [1,4]. This is, however, insufficient to infer color patterns with enough detail, and in the best of cases it may only serve to determine whether animals displayed contrasted patterns of dark and light color patches. Thus, fossilized melanin granules can be used to determine relative dark and light body patterns (in the case that the chemical nature of melanins can be obtained; see below), but not the actual colors exhibited by these body regions.
- An additional concern is that, assuming that differentiating eumelanin and pheomelanin granules in the fossil record was enough to infer color patterns (which is not the case, as explained above), such differentiation should be made properly and consistently. However, eumelanin and pheomelanin identification has largely been made on the basis of the morphology of fossil melanin granules. The term melanosome is frequently used in paleo-color studies but not correctly, as it actually refers to a functional organelle in a melanocyte and not to melanin bodies that are pumped out of the organelles to be deposited in integumentary structures . The morphology of melanin granules is not a reliable indicator of melanin composition. Indeed, several studies have based their conclusions on the assumption that eumelanin granules are rod-shaped structures while pheomelanin granules are spherical, which has thus constituted the basis of a large number of inferences made in the field of paleo-color [1,2,8,32,33]. Such an idea is based on previous studies [34,35]. However, Zi et al.  did not provide any chemical evidence that the rod-like granules that they found in the barbules of peacock Pavo cristatus feathers were actually composed of eumelanin, and Liu et al.  used a rather aggressive procedure to extract melanins from a hair matrix. As pheomelanin granules are less resistant to mechanical stress than eumelanin granules, such treatments might break pheomelanin granules making them appear spherical in contrast to unaltered eumelanin granules . In fact, Liu et al.  already noted that both spherical and rod-shaped granules were observed in pheomelanin-colored hairs. This contradicts the idea that pheomelanin granules are spherical, an assumption later embraced by paleo-color studies.
3. Functional Considerations
Conflicts of Interest
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|Protection||Ultraviolet (UV)-protection. UV light is damaging to biological tissues.|
Abrasion-resistance. Sand and dust transported in the wind damages fur, feathers, and skin.
Thermoregulation. Dark colors absorb more radiant energy than light colors.
|Concealment||Crypsis. Blending in the environment by coloration and pattern.|
Countershading. Darker color on top compared to a lighter ventral area.
Disruptive coloration. Irregular color patches to disrupt the shape of an animal in a variable environment.
|Advertising||Unprofitable prey. Conspicuous colors in prey inform potential predators the prey is hard to get or distasteful.|
Allurement to prey. For instance, the red crest of kingbirds (Tyrannus tyrannus) attracting the bees they eat.
Pursuit deterrence. Warning the presence of predators to conspecifics (e.g., white rumps or tails).
Cohesion and coordination of the group. Black or white marks in flocking or bunching animal species, such as shorebirds, savannah gazelles, or fish schools.
Startle or flash markings. Sudden appearance of conspicuous color patches in cryptic individuals to confuse predators and gain time to escape.
|Deception||Resemblance to objects. This particular form of crypsis takes advantage of behavioural adaptations, such as slow motion in sloths, swaying movements in stick insects, and resting owls and frogmouths resembling tree bark or broken branches.|
False eyes (ocelli) or false faces. As in the wings of some butterflies or the nape of small owls and falcons.
Directive marks. Intimidating markings, such as bright-colored irides, to scare off enemies or to obtain prey.
Mimicry. Animals resembling others of a different species. The mimic is generally less poisonous or less powerful than the model.
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