Why is blue the rarest colour in nature?

Kai Kupferschmidt, the author of the 2021 book, Blue: In Search of Nature’s Rarest Color says that the colour blue is rare in birds, reptiles, insects, crustaceans, molluscs, and fish, but even rarer in mammals.

Bluespotted Ribbontail Ray

Animals with scales, shields, or feathers are more likely to contain iridescent blue, perhaps to attract a mate or as a defence mechanism to ward off predators. 

For example, the flash of blue feathers of the Blue Jay bird (Cyanocitta cristata), the fish scales of the Powder Blue Tang (Acanthurus leucosternon), and the flashing rings of the venomous mollusc Blue-Ringed Octopus (Hapalochlaena maculosa) are reflected blue colours.

Powder Blue Tang
Blue Linckia
Blue Coral

Kai Kupferschmidt says that the blue colour in animals is a result of physics and not chemical pigments, because it is the colour that is not absorbed, and is therefore reflected outwards which is seen through the lens of the eye. He says that when you see a blue feather, you see it because it absorbs the red part of the colour spectrum and rejects blue. In the visible colour spectrum, red has long wavelengths, meaning that it is very low-energy compared to other colours. For an animal to appear blue, Kai Kupferschmidt says ‘it needs to be able to produce a molecule that can absorb very small amounts of energy’ to absorb the red part of the spectrum. Generating such molecules, which are large and complex, is difficult for animals to do, which is why blue is a rare colour in animals – and in plants too.  

Mammals are rarely blue. Some marine (sea) mammals, like dolphins and whales, such as the Blue Whale (Balaenoptera musculus), have bluish skin. Some terrestrial (land) mammals with fur have parts of their body with blue skin. For example, the Golden Snub-Nosed Monkey (Rhinopithecus roxellana) has a blue-skinned face, and the Mandrill (Mandrillus sphinx) has a blue face and blue rear. The Platypus (Ornithorhynchus anatinus) has fur that glows blue and green when exposed to ultraviolet (UV) light, says Live Science.

The adult Blue Sheep (Pseudois nayaur) has blue-grey fur, but it is not a bright blue. The Blue Monkey (Cercopithecus mitis albogularis), which is also called the Sykes’ Monkey, does not look blue, but its face has a blue tinge. Its body is grey with white ears and a white collar. 

Blue Sheep
Blue Monkey

The trait of fur on terrestrial mammals is never naturally bright blue (in visible light) and it is therefore the rarest for all blue-coloured animals. 

Blue and Yellow Macaw
Blue Crane
Long-Tailed Starling
Blue-Crowned Laughing Thrush
Red-Billed Blue Magpie
Blue Tree Monitor Lizard
Blue Poison Dart Frog
Meleager’s Blue Butterfly
Dark Blue Pansy Butterfly
Blue Fly

Photographer: Martina Nicolls


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