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Birds, bugs, fish and more come in every color of the rainbow, meanwhile mammals seem to offer more generic pallets, and dinosaurs might be to blame.

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Nature is full of color! Birds, beetles, fish, flowers, and more come in every color of the rainbow.

But mammals, not so much. We and our fuzzy relatives don't tend to have the vivid colors of other animals. Think of a tiger or a calico cat -- most of the time, that's about as vibrant as it gets.

The evolutionary reason for mammals' subdued coloration is more complex than you might think -- and it takes us all the way back to the Age of Dinosaurs. Most of the colors you see in plants and animals come from molecules called pigments, which absorb certain wavelengths of light and reflect back others. These are stored in their skin, feathers, fur, or scales to produce all kinds of bright reds, pinks, yellows, and more.

Us mammals tend to be less colorful than, say, birds, lizards, or insects, in part because we don't have the same range of pigments they do. Most mammals can only make one category of pigments, called melanin. Melanin comes in two forms: eumelanin, which can produce black or brown coloration, and pheomelanin, which produces yellow or reddish-brown colors.

Different amounts of melanin in different parts of the body can create a variety of patterns, from the black and white stripes of zebras to the brownish and yellowish spots of giraffes. As for other pigments, we don't really have the genes to make them. But mammals /can/ be more colorful.

Mandrills are a type of monkey that exhibit striking blue and red coloration on their faces … and also around their genitals. The red comes from blood vessels showing through the skin. And the blue is a result of structural coloration, in which the skin scatters and reflects light in such a way that blue wavelengths are directed back at our eyes.

In fact, most blues you see in animals are structural colors. And lots of animals don't even make their pigments themselves. Flamingos, for example, pick up their pink from their diet.

And there doesn't really seem to be any specific reason mammals haven't evolved to do something similar. Which leads to a conundrum of sorts. We know mammals have the /capacity/ to be more colorful.

But they usually aren't. So why not? The most likely explanation is that mandrills, birds, and butterflies all share an ability that most mammals don't have:.

They have excellent color vision. You see, inside your eyes are specialized cells called cone cells, which grant you the power of color vision. Many fish, reptiles, and birds have four types of cone cells, each with light receptors tuned to a different wavelength of light.

Seeing all these different wavelengths together creates a complex, multicolored picture. [DYE-kro-matt-ik] But most mammals only have two types of cones, making them dichromatic. And since their cone cells can only pick up two main wavelengths of light, they miss out on a lot of color information. Usually, these cones are tuned to the green and blue end of the spectrum, leaving them less able to clearly discern reds and yellows.

In human terms, they're partially color blind. Primates are an exception to this. Most primates – including ourselves – are trichromatic, with three types of cones for seeing reds, greens, and blues.

Since primates can see a variety of colorss, it makes much more sense for them to use bright colors for communication. There's less point in flashing fancy colors if the other members of your species can't see them. Even with our fancy third set of cones, though, our color vision isn't as good as many of the non-mammals in the world.

If you're feeling a bit cheated, try blaming the dinosaurs. During the Mesozoic Era, roughly 250 to 66 million years ago, the world was ruled by reptiles. But there were mammals back then, too, and although they were mostly small and scarce compared to today, they were pretty successful in a variety of niches around the world.

With dinosaurs dominating most land ecosystems, mammals needed to find strategies to avoid competing with them. And being nocturnal could have been a great tactic. It's hypothesized that many Mesozoic dinosaurs were active during the day, so only coming out at /night/ would have been a way for early mammals to avoid either competing with them for food, or becoming lunch themselves.

This has become known as the nocturnal bottleneck hypothesis. In evolution, a bottleneck occurs when a population's genetic diversity becomes reduced -- in this case, as a result of adapting to life in the dark. In studying the genomes of modern mammals, scientists have determined that not only do we have reduced vision, we also tend to be missing certain genes that protect our skin and eyes from damaging UV radiation.

Color vision and UV protection are both really helpful if you spend a lot of time in the sun, but less important if you're active at night. In this case, all or most mammals are thought to have lost these daytime traits during the. Mesozoic.

So scientists suspect our mammalian ancestors may have spent most of the Mesozoic in the dark. Then, when the dinosaurs' reign ended, and mammals finally got their time in the sun, they were working with limited genetic tools for adapting to daylight. Some of the genes that enabled daytime activities had been selected out and were long gone.

So even though many mammals are active in the daytime now, their DNA is still sort of stuck in nighttime mode. Only certain mammals – like primates – have separately evolved more complex color vision, and with it, more colors on their bodies. Our mammal eyes may never be able to see colors as vivid and varied as birds' eyes can see, but that's the trade-off that allowed /our/r ancestors to survive living alongside /their/ ancestors.

Fortunately, we humans can always add a splash of color with a favorite sweater.

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