Why are there no Green Grazers ?

Colours and camouflage

High above New Zealand’s North Island’s pastures, comfortably seated in an aeroplane and looking down at the world below through your window, you can still see the landscape dotted with specks of white: sheep. Were they green, you would not notice them at all. Likewise deer in Europe, grazing on the lush turf of a forest clearing in the morning sun: brown shades against a green background. Were they green like some grasshoppers, stick insects, or many caterpillars, they’d be far less visible.

So, how come that grazing mammals are not green while foliage-feeding insects are? It’s impossible to answer that for sure, but the main predators of insects like spiders and birds (and predatory insects as well) can see and differentiate colours, mammalian predators like tigers, lions, cats and dogs cannot. For them anything that moves and is small enough to be overpowered is considered possible prey. Furthermore, the grazing mammals themselves are usually colour-blind.

Evolution in grazing mammals seems to have favoured either earthy coloration as in rabbits and hares or has banked on disruptive patterns consisting of stripes (e.g., zebra) and patches (e.g., panda, giraffe, etc.). Evidently, to make a green pigment and produce a green pelt has been evolutionarily difficult and the only mammal with at least a touch of greenish fur area species of South American sloths. These remarkable vegetarians, hanging upside down from the branches of tree whose leaves they devour, possess modified hairs in their coat with microscopic grooves and ridges specially adapted to allow tiny green algae to grow in. The green colour of a sloth is therefore not due to a colour produced by the sloth itself, but stems from a plant/animal association, in which the animal provides nothing else but the substrate for the plant to grow and the plant in turn, because of its green colour, gives the sloth an improved camouflage. An association such as this, in which both partners “win”, is termed symbiosis, something I shall have more to say about sometime in a future essay.

However, not all forms of coloration are designed to let an individual with its environment: zebras and some other species possess disruptive patterns, seemingly breaking up the image of an individual. The Hungarian researcher Professor Gabor Horvath, moreover, has demonstrated with horse-sized coloured and striped dummies in the field that horses with black and white stripes would be much less attacked by biting flies than horses of uniform coloration. In the African environment where horses and disease-transmitting biting flies occur, a stripy coat could therefore have been of significant survival value.

Repelling and warning colours are, of course, the exact opposite of coloration to disguise an animal, but both, i.e. those helping to make an organism invisible and those helping to make it conspicuous, can be useful to humans. Apparently in Iceland authorities are considering painting roads in colours that would reduce Arctic terns (a migratory bird that annually flies from the Arctic to the Antarctic and back) to be run over by cars. Imagine you would then look down from an aeroplane onto a network of coloured roads! Something not even the Bulgarian outdoor avant-garde artist Christo had thought of.

More colours over there : “It’s a colorful world!”

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2016.
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