10 Elusively Blue Animals: The Rarest Critters of Them All

Two blue morpho butterflies resting on a green leaf

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Blue is the most popular color in the world, with a plurality of people picking blue as their favorite color when surveyed. However, blue is also one of the rarest occurring pigments found in nature. Sure, the sky and the ocean are blue, but while there are an abundance of green, yellow, and red animals, almost no blue animals exist.

The main reason why blue is so elusive is because of the relatively narrow range of pigments that cause coloration in animals. Some colors are common among animals due to those animals' abilities to either produce pigments of those colors or absorb them from the food they eat. For example, melanin is one of the most common pigments produced by animals and is responsible for the brown or black colors of most mammals' hair or fur and some birds' feathers. Meanwhile, red and orange pigments are produced by carotenoids in plants and algae, which are then consumed by animals like shrimp and lobsters, giving them their distinct pink and red colors. Flamingos also gain their pink coloration from carotenoids found in the shrimp that they eat.

While some 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 in animals are typically the result of structural effects, such as iridescence and selective reflection.

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Blue Jay

A blue jay perched on a branch of a tree

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The blue jay (Cyanocitta cristata) produces melanin, a black pigment, meaning that its feathers should appear black. However, tiny air sacs in the bird's feathers scatter light, making them appear blue to our eyes. This scattering of light within the blue jay's feathers is very similar to Rayleigh scattering, the phenomenon responsible for the answer to the age-old "why is the sky blue?" question.

Thus, since the distinctive blue color of the blue jay's feathers is not caused by pigments, it is possible to change the color of the bird's feathers back to black by altering their structure. In fact, damaged blue jay feathers do appear black as all traces of blue disappear when the light scattering is disrupted.

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Blue Iguana

A blue iguana standing against a background of rocky terrain

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The blue iguana (Cyclura lewisi), endemic to the island of Grand Cayman, has one of the longest lifespans of any lizard, living up to 69 years. When the lizards are born, they are intricately patterned but barely blue, with only some parts of their bodies maintaining a pale blueish-gray color. As they mature, they become bluer in color. However, mature lizards have the ability to change color and usually make themselves gray in order to blend in with the rocks that are found throughout their habitat.

A blue iguana will only make itself blue when it comes in contact with other members of its species either to communicate with them or to establish its territory. Males of the species also tend to have a more pronounced blue color than females do.

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Glaucus atlanticus

A blue Glaucus atlanticus swimming through the water over sand and aquatic plants

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Glaucus atlanticus is a bizarre-looking species of nudibranch, and just like many other nudibranchs, it is notable for its bright coloration. The species floats upside down in the water and feeds on the dangerous Portuguese man o' war (Physalia physalis), which is infamous for its venomous stingers that can kill fish and sometimes even humans. The blue color of Glaucus atlanticus serves as a form of camouflage, allowing the sea slug to blend in with the blue of the ocean and making it difficult for predators like seabirds flying over the water to spot it.

If its blue coloration wasn't protection enough, this sea slug is also able to absorb the stingers from the man o' war that it eats and use them itself either for defense or to hunt its prey.

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Mandarin Dragonet

A blue and orange mandarin dragonet swimming past coral underwater

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The mandarin dragonet (Synchiropus splendidus) is a brightly-colored fish from the Pacific Ocean that is one of only two vertebrates whose blue coloration is a result of cellular pigment rather than structural coloration. The only other vertebrate with blue cellular pigment is the picturesque dragonet (Synchiropus picturatus) from the same genus. The mandarin dragonet's skin contains cells known as cyanophores that contain organelles called cyanosomes that produce blue pigments. The cyanophores are not the only pigment-producing cells in the fish's skin, however, which explains the orange stripes that decorate their bodies. Due to their bright, colorful patterns, mandarin dragonets are popular fish for aquariums.

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Blue Poison Dart Frog

A blue poison dart frog resting on a green leaf

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The blue poison dart frog (Dendrobates tinctorius "azureus") is found in the forests of southern Suriname and northern Brazil in South America. The frog's blue coloration warns predators that it is poisonous, a phenomenon known as aposematism, and is caused by the structure of its skin cells. Frog skin has a layer of cells called xanthophores, which produce yellow pigments and rest on top of a layer of cells called iridophores. When light hits a frog's skin, it passes through the layer of xanthophores to the layer of iridophores, which then scatter the blue light back through the xanthophores.

Since the xanthophores produce yellow pigments, the yellow mixes with the blue light scattered by the iridophores, making frogs appear green. However, the blue poison dart frog has reduced xanthophores, meaning that almost no yellow pigment is produced in its skin. Thus, the blue light scattered by the iridophores never mixes with yellow pigment, making the frog appear blue.

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Blue Morpho

A blue morpho butterfly resting on a green leaf

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Butterflies in the genus Morpho, commonly called blue morphos, are notable for their beautiful blue wings. The butterfly's blue coloration is caused by the structure of its wings, which contain microscopic scales that have ridges shaped like Christmas trees with alternating thin layers known as lamellae. The nanostructure of these scales scatter the light that strikes the butterfly's wings, making them appear blue.

Since these structures are only present on the dorsal side of a blue morpho's wings, the ventral side of the butterfly's wings are actually brown. Furthermore, for many species of morphos, males tend to be more blue than females, and for several species, only the male butterflies are blue while the females are brown or yellow.

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Sinai Agama

A blue Sinai agama resting on a rock

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The Sinai agama (Pseudotrapelus sinaitus) is a species of lizard that is found in deserts throughout the Middle East. The lizard's skin is usually brown, allowing it to blend in with its environment. However, males become bright blue in color during the lizard's breeding season in an effort to attract females, making the Sinai agama one of only a few blue reptiles. During this time, females remain brown but may also have some red markings on their sides.

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Linckia laevigata

A blue Linckia laevigata resting on top of gray coral

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Linckia laevigata is a species of sea star that is found throughout the tropical waters of the Indo-Pacific. The sea star is notable for its blue coloration, which ranges from a light blue to a dark blue depending on the individual. Occasionally, individuals may be other colors as well, such as orange or pink. Linckia laevigata is one of the few blue animals whose coloration is caused by a pigment rather than by structural coloration. The species produces a carotenoprotein known as linckiacyanin, which is made up of multiple different carotenoids, giving the sea star its distinctive blue color.

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Carpathian Blue Slug

A Carpathian blue slug resting on gravel

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The Carpathian blue slug (Bielzia coerulans) is found in the Carpathian Mountains in Eastern Europe. While the species is best known for its dark blue coloration, the slug is not always blue. As juveniles, these slugs are actually yellow-brown in color. As they mature, they become blue, and adults range in color from bluish-green to fully blue or even black.

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Indian Peafowl

A blue and green peacock standing in a forest and displaying its tail feathers

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The Indian peafowl (Pavo cristatus) is an iconic bird endemic to the Indian subcontinent that is famous for its intricate, brightly colored feathers. Only male peafowls, known as peacocks, possess such bright blue and green feathers. Female peafowls, known as peahens, have only a few green feathers on their necks and are mostly dull brown in color. Peahens also lack the massive, colorful train of tail feathers that males possess. The bright coloration of the males is likely a result of sexual selection, as brightly colored peacocks are more attractive to peahens and are thus more likely to find mates. Peacocks also engage in elaborate courting displays during which they display and shake their large trains to attract peahens.

Like those of blue jays, peacock feathers contain the black pigment melanin, and their blue coloration is derived from their structure. Peacock feathers contain a crystalline lattice of microscopic rods that reflect light, causing them to appear blue. Their green feathers receive their coloration from a similar structure.