The Grand Cayman iguana comes by its beautiful hue naturally. (Photo: Matt Reinbold [CC BY-SA 2.0]/Flickr)
Blue has long been considered the world's most popular favorite color, even though it's the rarest occurring 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 pictured 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:
Little Blue Heron
Photo: Charles "Skip" Martin/500px
Photo: Angell Williams [CC BY 2.0]/Flickr
Photo: Pablo Antonio Vincente Casillas/500px
Photo: Randen Pederson [CC BY-SA 2.0]/Flickr
Augochloropsis sumptuosa, Also Known as the Sweat Bee
Photo: USGS Bee Inventory and Monitoring Lab [public domain]/Flickr
Photo: Johnathan Nightingale [CC BY-SA 2.0]/Flickr
Blue Poison Dart Frog
Photo: Philipp Weimer/500px
Glaucus atlanticus, Also Known as a Blue Dragon
Photo: Imtorn [CC BY-SA 3.0]/Wikimedia Commons
Blue Morpho Butterfly
Photo: William Warby [CC BY 2.0]/Flickr
Arizona Bark Scorpions, as Seen Under a Black Light
Photo: Matt Reinbold [CC BY-SA 2.0]/Flickr