Book T of C
Chap T of C
This is the 2007 version. Click here for the 2017 chapter 08 table of contents.
Pigment on a butterfly wing
Sandved (1981) came across a butterfly in Thailand with a repeated pattern of pigment that bears a remarkable resemblance to a small monkey face (right). It is only about as big as a pencil eraser, and it is repeated along the edge of the butterfly wing. Monkeys are predators of birds in Thailand, so this might be an effective way to scare off birds that might otherwise eat the butterfly.
How does such an accurate "picture" evolve?
A monkey face is also found on the chrysalis of the butterfly spalgis epius or "apefly," shown next.
Chrysalis of the spalgis epius butterfly
Presumably the first occurrence of the pattern was much cruder than this, a lucky accident that helped the chrysalis avoid being eaten by birds. With succeeding generations, the pattern was refined, as chrysali with a cruder (less face-like) patterns were eaten by birds. The more face-like the pattern, the more likely the butterfly was to survive and reproduce. Eventually the artwork became quite accurate.
This example is so dramatic that it could easily provoke skepticism. After all, it is a sketch, not a photograph. Might the unknown artist have taken some liberties? However, the spalgis epius has the nickname apefly, so the resemblance has been known and remarked-upon for a long time. A second illustration of the apefly, below left, comes from an 1892 article in the journal Psyche. A photograph can be found at the Wikipedia public domain art project (bottom right). Comparing the three samples shows that the "artwork" has a lot of variability. That is helpful in explaining how it could be shaped up by variable predation rates.
Two more examples of the apefly chrysalis
Earlier we made the point that evolution favors any adaptation that encourages successful reproduction of the species. We also made the point that birds respond to highly specific visual stimuli of evolutionary significance. Now we see how these factors come together. One species (the potential prey) evolves a visual pattern (eye spots or a monkey face) that acts as a significant sign stimulus triggering avoidance in other species (the bird predator).
Why are humans likely to be impressed by these examples?
These are all examples of visual mimicry or imitation. Humans are highly visual animals, like birds. Like birds, we cannot help but see a face in a few blotches of pigment on the spalgis epius chrysalis. Our brains respond to significant visual patterns automatically. Other species respond more to mimicry of odors, textures, and sounds. Odor and sound mimicry are also widespread in nature.
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Copyright © 2007-2011 Russ Dewey