Blue animals: Iguana

Photo: Matt Reinbold/Flickr

Blue has long been considered the world's most popular favorite color, even though it's the rarest occuring pigment found in nature. Sure, we can see blue quite easily in the sky and water, but its tendency to appear in animals, like the endangered Grand Cayman blue iguana above, is fairly uncommon.

The main reason why blue is so elusive has to do with the relatively narrow range of pigments that cause coloration in animals. Red and orange pigments are produced by carotenoids, brown and black pigments are produced by eumelanins and yellow pigments are produced by pteridine compounds.

While plants can produce blue pigments thanks to anthocyanins, most creatures in the animal kingdom are unable to make blue pigments. Any instances of blue coloration you come across in animals are typically the result of structural effects, such as iridescence and selective reflection.

Take, for example, the bluejay. This little bird produces melanin, meaning it should technically appear almost black. However, tiny air sacs in the bird's feathers scatter light, making it appear blue to our eyes. This is called Rayleigh scattering, a phenomenon that is also responsible for the age-old "why is the sky blue?" question.

Even though these critters are rare, they do, in fact, exist. Here's a look at just a few animals with exquisite blue coloring:

 
Blue animals: Little Blue Heron

Photo: Charles "Skip" Martin/500px

Little blue heron

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Blue animals: Blue-ringed octopus

Photo: Angell Williams/Flickr

Blue-ringed octopus

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Blue animals: Blue-eyed kitten

Photo: Pablo Antonio Vincente Casillas/500px

Blue-eyed kitten

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Blue animals: Bluejay

Photo: Randen Pederson/Flickr

Bluejay

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Blue animals: Metallic blue bee

Photo: USGS Bee Inventory and Monitoring Lab/Flickr

Augochloropsis sumptuosa, also known as the sweat bee

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Blue animals: Moon jellies

Photo: Johnathan Nightingale/Flickr

Moon jellyfish

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Blue animals: Frog

Photo: Philipp Weimer/500px

Blue poison dart frog

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Blue animals: Glaucus atlanticus

Photo: Imtorn/Wikimedia

Glaucus atlanticus, also known as a blue dragon

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Blue animals: Morpho butterfly

Photo: William Warby/Flickr

Blue morpho butterfly

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Blue animals: Arizona bark scorpions under black light

Photo: Matt Reinbold/Getty Images

Arizona bark scorpions, as seen under a black light

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Blue animals: Peacock

Photo: Soren Kleen/500px

Peacock

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Catie Leary is a photo editor at Mother Nature Network. Follow her on Twitter and Google+.