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Why didn’t green fur evolve among mammals, when there are birds with green plumage and reptiles with green skin?

Herman D’Hondt
Sydney, Australia

Reptiles and birds have three types of colour-sensing cone cells in their retinas that are each sensitive to a specific colour – usually, there are red, green and blue receptors. They also have a fourth type that works in the ultraviolet range. Mantis shrimp can have up to 16 different types of cones: what they use them for is anybody’s guess.

When mammals first evolved from reptiles, they lost one or two of these receptors. This may have been because early mammals were mostly active at night or underground, while dinosaurs controlled the land during the day. Since then, almost all mammals, including marsupials and monotremes, have had only two types. It is only humans and primates native to Africa, Asia and Europe that regained a third type.

Having only two types of receptor gives most mammals red-green colour blindness. As a result, they can’t tell the difference between red and green.

Adding genes to produce green fur would come at an additional cost. Because evolution tends to select the least costly option, that didn’t happen. I guess evolution could have given mammals green fur instead of red, but mutations are random.

Having said all that, it is worth noting that some sloths are green. However, that isn’t because they are growing green fur, but rather because their fur is host to symbiotic green algae. This may serve as camouflage when moving around in trees.

Jonathan Wallace
Newcastle upon Tyne, UK

For any particular trait to evolve, two things need to happen. First, genetic mutation needs to create the code for the trait in question so that individuals enter the population displaying that trait. Secondly, the trait needs to confer a competitive advantage over individuals with other phenotypes such that those with the trait have more young surviving in following generations.

With respect to the first of these, it is important to recognise that mutations occur at random and not as a purposeful response to a design problem. It is possible that a mutation (or combination of mutations) coding for green fur simply hasn’t happened.

With respect to the second, it is conceivable that even if green fur has appeared at some time or another, it failed to give any significant advantage to its bearers (or may have conveyed a counteracting disadvantage). Anyone who has tried to see mammals in the wild will know that many of them hide away extremely effectively, so it isn’t clear that being green would necessarily be an improvement.

Andrew Taubman
Perth, Western Australia

Maybe green fur did evolve, but, it being such excellent camouflage, we haven’t been able to see those creatures?

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