Attack of the see-through Fokkers

Red Wings in the Sunset is a fascinating essay by the late Stephen Jay Gould. It's about the American artist and naturalist Abbott H. Thayer and his theory of protective colouration in animals. Thayer was one of the first people to systematically examine how camouflage is used by animals (either by tasty ones, to avoid being seen and becoming lunch for something else, or by the hungry predators to avoid being spotted sneaking up on the tasty ones).

The obvious way for animals to be inconspicuous is simply to match the usual background colour of the place where they live - a sandy colour is good for hiding desert-dwelling creatures, being white helps to hide polar bears creeping up on seals and so on. Thayer also noticed that there were two other, more subtle, things going on with animal camouflage:

1) Countershading. As a painter, Thayer knew all about representing three dimensional objects on a flat surface. One of the painter's tricks is to use highlights and shading to reproduce the effects of light and shadow on solid objects in a picture. Generally, light comes from above and falls on the top of a solid body whilst the underside of the object is in shadow - a skillful representational artist can reproduce highlights and shade to create the illusion that you're looking at a solid object, rather than an area of pigment on a flat surface.

Thayer looked at animal colouration and realised that evolution had given animals colur schemes which reversed the optical illusion used by artists to create the illusion of solidity. The artist tries to make a flat painting appear solid by introducing highlights, mainly on top, and shadows, mainly below, where the observer would expect to see them in a solid object. Many animals have dark colouration on top, where the observer would expect to see bright highlights and are lighter underneath, where the observer would expect to see dark shadows. The effect is to fool the eye of another creature, whether predator or prey, making the countershaded animal seem to blend into the general background of a scene, rather than standing our as a solid object with highlights and shade. This type of protective colouration is called countershading (countershading is also sometimes known as Thayer's Law).

The pale bellies of these countershaded ibex make them appear flat and inconspicuous, especially compared with the jumble of rocks casting shadows all around. The shark in this picture is also countershaded, with a dark back, paler flanks and a pale underside. The shark also demonstrates how, especially among fish and birds, colouration can simultaneously achieve countershading, by counteracting shadows and making the animal's profile appear flat and match the animal's background. When viewed from above, the shark is as dark as the dark waters below it; from below it is very pale, to reduce the contrast with the brighter, sunlit waters above it.


2) Patterns to break up the animal's shape - spots, stripes, irregular blotches of contrasting colour. Tiger stripes, leopard spots, mackerel stripes all confuse the edge detection algorithms the brain uses in vision processing, breaking up a the appearance of continuous object and creating "false edges". The coat of the ocelot is a good example as it the skin of the common european adder. Thayer called these markings "ruptive" - nowadays the phrase is "disruptive" patterns.

Stephen Jay Gould's essay is interesting in both giving Thayer full credit for his observational skills in identifying how animals used camouflage, but also giving the story of how Thayer, having had a brilliant insight, took his theories too far. Having shown how animals could use colouration to hide, Thayer then went off on a wild goose chase by trying to demonstrate that all animal colouration was there to disguise the creature. Even the bright, showy tail of the male peacock, Thayer imagined, might serve to disguise the creature when standing in the dappled sunlight of an Indian jungle, whilst, bizarrely, he thought that the pink colouration of flamingos served to hide them from crepuscular predators, helping them to blend in with a pink sunset. Later naturalists would demonstrate that not all colours are intended to disguise - the peacock's tail looks showy because it is intended for show, to impress a mate, not to hide in a dappled forest clearing. And a flamingo seen in front of a pink sunset does not blend in - it appears, like any other solid object viewed with the light coming from behind it as a dark silhouette. The first picture I could find of flamingos at sunset is a bit cheesy, but illustrates the point.

Humans have adopted some of these techniques, as an aid to hunting other animals and, in the case of military camouflage, to refine our worryingly large repertoire of techniques for killing other humans. Thayer himself was aware of the military potential of his discoveries, patenting a countershaded colour scheme for warships in 1902. In time, the principles outlined by Thayer were adopted for military purposes. This Grumman Avenger (the type of plane flown, incidentally, by the first President George Bush during the Second World War), is painted in a shark-like countershaded scheme - dark on top, lighter sides and pale on the bottom. Here's a CF-116 jet (a Canadian-built Northrop F-5), displaying a disruptive pattern of wavy stripes.

There is another animal camouflage technique which Thayer didn't examine. It's pretty specialised, and doesn't generally apply to big animals, but it's an alternative to countershading or disruptive patterns. It's called being transparent. Here are some see-through animals (ignore the transparent zebrafish which was bred artificially, and the transparent-head fish, where the transparency is to do with how the animal sees, not how it hides). Now humans have copied countershading and disruptive patterns for their own nefarious purposes, but becoming transparent? Nah, that wouldn't work...

Well, actually, even this has been tried. In First World War, when aeroplanes were generally constructed of doped canvas stretched over a wood (or wood and metal) framework, some bright spark in Germany hit on the idea of making German warplanes invisible, or at least a lot harder to spot. The plan was to cover them, not with canvas, but with cellon, a cellophane-like transparent material. And here is the result - a see-through Fokker Eindecker, coated in cellon.

Via an interesting post by Ian Dunn on the QI Talk Forum I found out a little more about the curious history of see through aeroplanes. Apparently the Germans would have like d to use cellon to render much larger planes than the Fokker Eindecker as near to invisible as they could manage - the idea was to make their giant Riesenflugzeug or R-plane bombers inconspicuous by the magic of cellon.

The scheme eventually came to nothing, because cellon proved to have some major drawbacks for an aircraft covering:

a) It was highly inflammable
b) In dry conditions it shrank and warped the aircraft's structure, when it was damp, the cellon expanded and sagged.
c) Exposure to ultraviolet light in sunlight caused cellon to become yellow and brittle.
d) Cellon was shiny as well as transparent - reflected light actually made the cellon-covered aircraft easier to spot, not harder.

In the end the project failed, so becoming see-through is one animal camouflage technique that the human race never successfully adopted ... so far....