The Scientific Method Applied to Understanding the Evolution of Coral Snake Coloration.

The observation that provides a basis for this study is that coral snakes are both highly venomous and brightly colored, with black, red, and yellow stripes, as shown here.

The question the study attempts to answer is why? Why did bright coloration evolve in this highly venomous snake?

Two hypotheses are proposed; either could explain why coral snakes evolved bright coloration. These are:

1. Aposematic coloration. Bright color warns away potential predators.
2. Cryptic coloration. On natural background, the black, red, and yellow pattern is actually hard for potential predators to see (even though it looks bright to us, on a light background.)

Predictions can be developed from these two hypotheses. First, we could ask what they predict about the degree of natural predation on coral snakes as compared to other snakes. BOTH hypotheses predict that coral snakes would experience relative low predation -- the first because predators are warned away from them, the second because predators can't see them. This is not a useful prediction for testing between the hypotheses since both predict the same thing. It illustrates the fact that different hypotheses can make the same prediction (remember, this is why one can never prove a hypothesis to be true -- there might always be some other hypothesis that makes the same prediction.)

Brodie did an experiment for which the two hypotheses predict DIFFERENT things. He made model snakes out of a soft plastic. When birds (the main potential predators on coral snakes) bit these models, the imprints of their beaks were left on the models, so Brodie could count attempts at predation on these models. He made some models that were brown and others that looked like coral snakes. He placed these models on white backgrounds, and placed a large number of them out in the natural habitat (in this case, a rainforest in Costa Rica) of the coral snakes. The snakes on white backgrounds looked something like this:

For this experiment, the two hypotheses make different predictions:

Hypothesis 1 (aposematic coloration) predicts more predation attempts on brown models than on coral models, because predators should be warned away by the bright color of the coral models, but not by the brown models.

Hypothesis 2 (cryptic coloration) predicts equal predation attempts on brown and coral models, because the white background makes both kinds of snake obvious -- neither is cryptic (camouflaged, hard to see) against a white background.

Brodie's results are shown in the following figure:

There were many more attacks on brown models than on coral models. Thus, the aposematic color hypothesis, which predicted this result, is supported. The cryptic coloration model, which did not predict this result, is not supported.

Here are some final points to note about this example:

• The aposematic color hypothesis is supported, but NOT proven (remember that we can never prove a hypothesis) because it is possible that some other hypothesis that hasn't been thought of makes the same prediction. Can you think of another hypothesis that would predict more attacks on coral models than on brown models?
• Note that both hypothesis are ultimate -- they are potential explanations of why bright color has evolved -- and that both are based on natural selection. That is, both are reasons why the bright color pattern of coral snakes would have higher fitness than other color patterns, would therefore be reproduced more than other color patterns, and would evolve within these snakes.

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