Mammals generally do not display the vibrant colors found in birds, lizards, and fish. This difference is attributed to a combination of biological and evolutionary factors.
Color Expression Mechanisms
Animals primarily express color through pigments or structural coloration. Pigments are molecules within the animal's body that absorb and reflect light. Structural coloration involves nanoscale structures that interact with light to produce iridescent hues.
Mammals possess only one type of pigment, melanin, which is responsible for colors such as browns, blacks, and whites. The absence of melanin results in white areas, as seen in zebras and pandas. Unlike feathers, scales, or skin, mammalian hair lacks the complex structure necessary to create structural coloration. For instance, the bright red and blue on mandrills appear on fur-less skin, while sloths' green color comes from algae growing on their fur, not from the hair itself.
Mammals possess only one type of pigment, melanin, which is responsible for colors such as browns, blacks, and whites.
Evolutionary History
One hypothesis suggests that the lack of vibrant color in most mammals stems from their evolutionary past. For over 100 million years, during the age of dinosaurs, early mammals were primarily nocturnal prey animals. A 2025 study in the journal Science compared melanosomes in modern and Jurassic/Cretaceous mammal fossils, finding that prehistoric mammals were shades of brown or gray.
These darker colors provided camouflage in the dark, and bright colors would have been disadvantageous and selected against. Despite the extinction of nonavian dinosaurs 66 million years ago and the subsequent diversification of mammals, most species have retained their predominantly brown, gray, and black coloration.
Vision and Signaling
Another contributing factor is the vision of most mammals. Researchers speculate that mammals sacrificed some color vision for improved night vision during the dinosaur era. Most mammals have dichromatic vision, meaning they have only two types of cones for color perception, limiting their ability to see colors like red, orange, and purple with the saturation that trichromats (like humans and some primates) experience.
The primary purposes of color in the animal kingdom—attracting mates, camouflage, and signaling danger—are less effective if the target audience cannot perceive the colors.
The primary purposes of color in the animal kingdom—attracting mates, camouflage, and signaling danger—are less effective if the target audience cannot perceive the colors. Consequently, many mammals utilize patterns and contrasting colors (e.g., black and white, brown and yellow) for communication and camouflage. Examples include skunks and polecats signaling danger with black and white markings, African wild dogs using white tails for signaling during hunts, and Indian giant squirrels using high-contrast patterns for camouflage against predators.
Recent Discoveries
Recent research has uncovered some exceptions to the typical mammalian color palette. Many mammals exhibit fluorescence under ultraviolet light, which is detectable by some other mammals. Additionally, iridescence has been observed in the fur of several tropical rat species, a feature not previously known to be widespread in mammals. While the evolutionary purpose of this iridescence is currently unknown, these discoveries suggest that mammals may possess more varied color expressions than previously recognized.