Why are poisonous bugs brightly colored?

Dear Straight Dope: While watching a nature show once, I heard that some predatory animals know not to eat certain creatures of a specific colour (the colour signifying that it is poisonous or foul-tasting). This is obviously something that is learned or taught but what about the creature not getting eaten? How did it come to be that it being a particular colour tells predators to back off? So my question is: How do creatures evolve special colours or colour patterns? Is genetic information aimed at achieving a specific colour passed down? Or is it simply a matter of trial and error and the best shade wins? JL, Toronto, Ontario, Canada

SDStaff Doug replies:

I know you Canadians spell it “colours,” JL, but in the U.S. we’re trying to conserve the electrons, so it’s “colors” from here on out. Hope there’s no hard feelings. God save the Queen.

The story behind aposematic coloration (the scientific term for “warning colors”) is pretty simple. They are always bright, highly-contrasting patterns. Black and yellow is the most common, but black and orange, black and red, and black and white are also encountered. There are also a smattering of color combinations in very specific small groups of butterfly and moth species that involve other colors (like green and orange, blue and red, black and green), but always the rule of high contrast applies.

These are organisms that want to be seen. It’s to the advantage of a species if predators only have to eat (or attempt to eat) a small number (ideally only ONE) of individuals before they learn not to try it again. The more unique and memorable the appearance of something, the easier it is to learn not to mess with it. The result is that if you have a species which is losing, say, 5 out of every 10 individuals each generation to predators, and a brightly-colored mutant arises which loses only 1 out of every 100, the mutant will have a 50-fold advantage, and that mutant coloration gene will quickly replace the original version by natural selection.

Naturally, in order for this to work properly, the critter(s) in question has to be poisonous or foul-tasting. If there are multiple species that all taste bad, they often will converge on the same color pattern so the predators learn even faster (called Mullerian mimicry). Once a system like this evolves, however, then other, non-toxic species will evolve to exploit it, which is called Batesian mimicry. If a mutant arises, say, in an edible butterfly species and it thereby happens to resemble an inedible one, then that mutant will spread because predators will avoid it, giving it a survival advantage. But it can only spread so long as the inedible originals greatly outnumber the edible mimics. If the mimics become too common, the predators will learn that the color pattern now signifies edibility, and the system will collapse.

One final wrinkle is that this has been going on for so long now (several million years) that in many predators, the avoidance of warning-colored prey is instinctive — in other words, it is not always learned or taught. Why do you think people use black and yellow signs to indicate danger? Why are cartoon honeybees black and yellow when real honeybees are actually brown?

SDStaff Doug, Straight Dope Science Advisory Board

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Why are poisonous bugs brightly colored?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD