Hiding in plain sight: disruptive coloration

Just above a stony beach on the way to Lena Point, a patch of yellow paintbrush caught my eye. Inspecting one of the blossoms, I found a beautiful little green caterpillar lying in the curve of one of the outer bracts. It had a narrow white longitudinal line down its back. If that caterpillar had rested on a green leaf or petiole, perhaps the conspicuous white line would draw a predator’s attention, distracting it from the whole body of the caterpillar. Furthermore, that white line would have disguised its real shape, visually dividing it into parts that could seem to be unrelated to each other. And thus, a caterpillar-hunting bird might pass it by.

 

Patterns that visually break up the shape of an animal’s body are called “disruptive coloration”, a common anti-predator ploy of many animals. If it is to work as designed, the animal must behave in a way that suits its pattern. However, this little green caterpillar on a yellow bract was not behaving in a way that best utilized its color pattern. (Maybe it would get away with this apparent mistake by being somewhat concealed within the curved bract.)

Disruptive coloration is often combined with some form of color-matching, such that just part of an animal matches the background (if that caterpillar had been on a green stem, for instance). The blotchy browns on the back of an incubating female mallard might generally resemble a heap of dead leaves. The camouflage effect would be enhanced if the blotches obscured the outline of the sitting duck and even more so if some of the blotches matched the area around the nest.

The optical tricks of disruptive coloration take many forms — not just any color, nor any pattern, nor any arrangement of tone will do. First of all, the effect of a disruptive pattern is enhanced when the color of some patches match the background (as noted above for the mallard) and other patches differ markedly. This makes some parts of the animal fade away visually, while others stand out. In addition, the disruptive effect is greatly intensified if contrasting light and dark tones are adjacent to each other; the more conspicuous patches dominate a predator’s vision, letting the true shape of the prey animal fade back. These effects are strongest if the transition from light to dark tones is sharply defined and not gradual.

A further complexity is provided by a false appearance of relief: colored patches appear to be at different levels or perhaps sloped. This can be accomplished by gradations in tone and is more effective when a light patch becomes still lighter and an adjacent dark patch become still darker at the place where the two patches meet. This illusion can make a flat surface look three-dimensional, or a rounded surface look flat, distracting an observer from the real shape.

Breaking up the body outline may sometimes be accomplished in unexpected ways. For example, experiments have suggested that contrasting marks along the edges of butterfly wings may make the wings less recognizable to avian predators, and thus reduce the risk of predation. Sometimes, color patterns seem to tie disparate body parts together —uniting discontinuous parts; in short, this is the opposite of breaking up a continuous surface or outline. For example, the dark bands on the legs of some frogs seem to merge with similar bands on the body when the legs are folded, so a frog becomes a banded lump with no apparent froggy legs.

All of these delusionary optical tricks are well-known to artists. However, I’ve read that those who use camouflage for military or hunting purposes were initially very reluctant to believe them and so required a lot of persuasion. Of course, Mother Nature has been doing these tricks via mutation and natural selection for eons.

That’s a lot of words sparked by seeing one little caterpillar! And I haven’t even touched on the more famous ways of hiding in plain sight, such as looking like a stick or a leaf or a bird dropping or a bit of algae or some dangerous critter or another time, perhaps.


• Mary F. Willson is a retired professor of ecology. Her essays can be found online at www.onthetrailsjuneau.wordpress.com


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