jouw wetenschapsgids in de hoofdstad

jouw wetenschapsgids in de hoofdstad

Kermit confesses – there are no green animals

If I would ask if you know any green animals, I am sure you could reply instantly with a ton of examples– from the majority of amphibians and reptiles to the brightly coloured green tropical birds, insects, and some exotic fishes. However, there simply are no green animals. Most of the animals who appear green to us, are not actually able to produce green pigment, but they rather have different structures in their skin, scales, fur or feathers that reflect the light in a way that looks green to us.
Much like other objects – most obviously prisms, the cover of an animal absorbs certain wavelengths of the light spectrum and bends the light in ways such that only one (or a combination of several) colours is reflected. These wavelengths and the resulting colour are available for our eyes to see – and this is why objects look like they have a specific colour. So simply put, the perception of colour is not a property of an object but rather of the organism looking at this object and its abilities to see different wavelengths as different colours.

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” simply put, the perception of colour is not a property of an object but rather of the organism looking at this object and its abilities to see different wavelengths as different colours

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But that still doesn’t justify my statement that there are no green animals. Green animals would be animals that are able to produce a green pigment, a substance that reflects the green light an absorbs the light of all the other colours. The truth is, very few animals are able to produce truly green pigments as such – it seems this is a trick reserved for plants only. The few pigments known in the animal kingdom responsible for truly green colour, so-called carotenoproteins, are usually found in marine animals and even then they usually get them from plants. Those plants can be symbionts – organisms which they are very closely associated with; or plants they feed on, after which the pigment simply accumulates on their surface. The most common such protein for green colour is the ovoverdin. All the other green looking animals have ingenious mechanisms to make them look like your average leaf, grassland or algae covered pond.
One of the most fascinating ways animals trick us to believe they are green, is the one employed by butterflies. They use a phenomenon called structural colouration – when indeed structures on the surface of the wings bend light in a way that makes different colours. In butterflies, like the Emerald swallowtail, part of the structures refract the light in blue, and the rest in yellow wavelengths. This way our eye is mischievously tricked into thinking that the butterfly is emerald green (you might remember from primary school that blue plus yellow equals green). A similar mechanism of “colouration” is used by many other species – peacocks (whose feathers are actually brown), the blue-yellow macaws, some squids and insects. 
The only mammal which appears to be green (but also lacks green pigmentation) is the sloth. Mammals, of all animals, are the least creative when it comes to pigmentation and the use of tricks to colour themselves. The only pigments they can really produce give out red to brown colour and in different combinations or amounts attribute for the different colours of mammals. The sloth gets its green colour with age – its fur is extremely coarse and eventually with age it begins to crack. These cracks turn out to be a suitable habitat for green algae, which are the real green deal.
So next time you see a green animal think again if what you are seeing is not yet another trick of nature, just to make life a bit more colourful and interesting!
 

This article originally appeared on ‘Inkubatorium‘, the author’s blog page.