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Mimicry: “Red on yellow kills a fellow; red on black, friend of Jack!”

Posted by: Editor on 2/15/2012
“Red on yellow kills a fellow; red on black, friend of Jack!”  That’s one of several old folk rhymes used to help distinguish between the poisonous coral snake and several non-venemous mimic species of kingsnake and millk snake (though this only works in North America, as coral snakes in other parts of the world may not follow this rule of thumb precisely!)  We’ve always found mimicry fascinating in the animal kingdom.  How and why does mimicry arise?  How do mimics choose their models?  What’s the difference between Batesian mimicry and Mullerian mimicry?  Read on if interested in exploring the issue in more detail.

Mimicry emerges from the evolutionary process of variation and selection.  In general, though not in all cases, there are three primary players in the game: 1) the mimic, 2) the model, and 3) the agent who receives the false signal from the mimic and thereby is “duped” into actions beneficial to the survival of the mimic.  Imagine, for example, two butterflies that live in the same general area.  The first, Butterfly A, is bright red and contains a poison that makes birds in the area sick if they eat them.  The Bird is the agent, in this case, and has learned over time to avoid bright red butterflies.  The second butterfly, Butterfly B, is greenish-brown in color, and tastes very good to the birds in the area.  As a result, the birds actively hunt and eat Butterfly B.  

Every now and again, however, random variation in Butterfly B produces a butterfly that is more reddish-brown than greenish brown.  When the birds in the area come across this variation in Butterfly B, they aren’t quite sure which butterfly it is - the bright red poisonous one or the tasty greenish butterfly.  As a result they leave it alone.  In this way, the reddish-brown variation of Butterfly B is able to pass it’s reddish coloration down to the next generation, while the greenish version of Butterfly B faces continued predation pressure from the birds in the area.  

Over time, selection pressure from the birds leads to the elimination of the the greenish version of Butterfly B, and causes the reddish version to proliferate in terms of both number and intensity of redness.  Eventually a situation arises where all of the examples of Butterfly B become bright red, just like the poisonous Butterfly A, and consequently the birds in the area begin to avoid both Butterfly A and Butterfly B, even though Butterfly B would still be a very tasty meal!

You can see that in this situation, Butterfly A is the model, Butterfly B is the mimic, and the birds are the “dupes.”  In terms of the impact on each of the actors, Butterfly A, the model, is no worse or better off than before.  Butterfly B, however, is now protected from predation by mimicking Butterfly A’s color - a clear benefit to Butterfly B.  And the birds, the “dupes,” are worse off because they have lost access to a viable food source staring them right in the face!  

The above is only a simple hypothetical example of how variation and selection, the bread and butter of evolution,  can lead to mimicry in the real world.  In practice, there can be many different forms of mimicry, ranging from visual (color and appearance, behaviour) to olfactory (scent) and auditory (sound) and anything and everything in between.  There are also many different categories of mimicry, with perhaps Batesian mimicry and Mullerian mimicry being the two most important.

Batesian mimicry is the type of mimicry described by Butterfly B above and is named after British naturalist Henry Walter Bates (1825-1892).  In the case of Batesian mimicry, a harmless mimic adopts certain attributes from a dangerous model, therefore advertising the dangerous attribute to predators without actually having the dangerous attribute in question.  The “advertising” attribute taken on by the mimic can range from warning coloration (known as aposematism) to behavior, general appearance, and scent among others.

Mullerian mimicry, on the other hand, is a situation where two dangerous species over time come to resemble each other.  This type of mimicry was put forward by German biologist Fritz Muller (1821-1897), who noticed that certain different species of non-palatable creatures often resembled themselves.  In this case it’s a little hard to delineate who is the mimic and who is the model, but what you can say is that all three actors in the evolutionary drama benefit: the mimic and model both benefit because in advertising their dangerous attributes, they avoid predation, while the “dupe” benefits as well because he/she can avoid a toxic meal by heeding the signal (as opposed to the “dupe” in Batesian mimicry who misses out on a nourishing meal!)

In practice, the selection pressures and variations that lead to mimic-model relationships in the wild can involve significant subtlety and complexity.  There are, however, many clear examples in the reptile world that we intend to explore in the next post, including the coral snake and it’s milksnake and kingsnake mimics, the cobra and false cobra, the rattlesnake and the many harmless snakes that also rattle their tails, and Asian pit vipers among others.  Hopefully we can find a few examples of both Batesian and Mullerian mimicry.  Until then!

HVH
Categories: Biology |
Tags: Mimicry | Evolution | Biology |
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