Bates reasoned that there must be some survival advantage for two unrelated species to look alike. He also noted that species which looked alike but were not closely related were often brightly colored. He reasoned that one of the species was distasteful to predators and that the usefulness of the bright coloring was to signal this distastefulness to the predator and thus increase its chance of survival. The other, similarly colored, species will also be avoided by predators and does not need to be distasteful. The distasteful species is called the model and the similar, but not distasteful, species is called the mimic. The predator is called the signal-receiver.
In most biology textbooks, the Monarch is described as being distasteful (from feeding on milkweed) and is avoided by avian predators and the Viceroy, not closely related to the Monarch, is is described as not being distasteful but is also avoided by predators. This seems like a classic case of Batesian mimicry. The Monarch would be the model and the Viceroy the mimic. But, work done in the last ten years indicates that something else may be going on. Brower showed that both the Monarch and the Viceroy are unpalatable. This would indicate that a different kind of mimicry - Mullerian mimicry - might be taking place. the follow quote is from an article in The New Brunswick Naturalist by Christopher Majka:
"Now, however, standing conventional wisdom on its head, come new findings which suggest that this whole scenario, and all that we once believed about mimicry 'ain't necessarily so.' In a definitively and provocatively titled paper -- The viceroy butterfly is not a batesian mimic -- David Ritland and Lincoln Brower [Nature 350:497-498 (1991)] studied the palatability of Monarchs, Viceroys and Queens (Danaus gilippus, a close southern relative of the Monarch and another species which, it was assumed, the Viceroy mimicked). In their work they presented Red-winged Blackbirds with abdomens of all three species (as well as of non-toxic control butterflies). They gave the birds only abdomens to prevent them from seeing or learning the aposematic patterns. Thus they could respond only to the taste of the butterfly. To their surprise they found that the Viceroy and Monarch were both equally unpalatable to the birds (only about 40% eaten, versus 98% of the controls) and both were more distasteful to birds than Queens (approximately 70% eaten). This was borne out by several other indicies which they used to measure the birds' response (how long and the way in which the birds handled the butterfly, etc.). In one fell swoop the basis for supporting Batesian mimicry between these species disappeared."|
There are many examples of Batesian mimicry in the insects. Many insects are bee mimics. Obviously, the bee is "distasteful" - Try eating one sometime and see if you can get it down without being stung! (Just kidding - don't try this!!!) Notice that the bee is a good example of a model, being brightly colored, often with stripes on the abdomen. Here is an example of a beetle, the locust borer, a bee mimic (Robert C. Madden). Notice the prominent stripes:
The locust borer even mimics the sound of a bee when picked up, just to make certain that predators become convinced!
The following are a bee mimic and a bee, specimens courtesy of Prof. Keith Waddington of the Department of Biology. The bee mimic on the left is a fly, a member of the Asilidae. The bee on the right is a member of the genus Bombus, a true bee. The fly has no stinger and is harmless but is colored and patterned just like a bee. The fly is actually a predatory insect but notice how the legs which make it such a good predator look just like those of a bee.
If we examine the data obtained by Brower and others on the Monarch and the Viceroy, it now seems that they may be Mullerian mimics of each other, and not a case of Batesian mimicry.
What do you see in this exhibit? Batesian or Mullerian mimicry? Or both? (Remember, you can click on this image to see more or larger, just as with most of the figures outlined in a colored border.)
And ... two pictures of Costa Rican snakes, courtesy of Prof. Jay Savage (now retired) of the Department of Biology. The snake on the left is Scaphiodontophis venustissimus from Limon province and the snake on the right is Micrarus nigrocinctus from La Selva biological station in Heredia province. Can you figure out which is the poisonous snake and which is the mimic?
Often, people in the U.S., including Florida, where the Coral Snake is found, remember which is the poisonous snake by noting that the Coral Snake has red and yellow bands which touch. The Kingsnake and Milk Snakes have red and yellow bands which are separated by a black band. But, if one goes to Mexico and Central America, this means of identification does not work. Look at this striking diagram of coral snakes and mimics (from: Coral Snake Mimicry: Does It Occur by Harry W. Green and Roy W. McDiarmid, Science 213, pp. 1207-1212, 1981): (Make sure that you click on the image to view the entire figure!)
The right snake in each pair (and central snake in diagram E) are rear-fanged snakes of the genus Pliocerus while the left snake in each pair (and the right and left snakes in diagram E) are front-fanged poisonous snakes of the genus Micrurus. Pliocerus (Colubridae) can still give a bite but the Coral Snakes (Micrurus, Elapidae) are the really dangerous ones. Notice the difference in head shape.
S. Smith (1975, 1977) did some really interesting experiments on color patterns. Predators on snakes in Central America include motmots and kiskadees. Motmots are funny looking birds which often have a long tail ending in a little tassel-like feather. Smith painted wooden dowels with color bands and solid colors and watched to see which were attacked and avoided by the avian predators. These birds attacked dowels with solid colors and ones which were banded with a neutral color. The birds pecked slightly at dowels with red and yellow longitudinal stripes but fled abruptly at the sight of tricolor bands similar to those on coral snakes. The response to pattern and banding was quite specific to those types of snakes living in the same range as the birds. And, even a small region of patterning, such as would be found on the head, was enough to elicit the avoidance response.
So, Mertens proposes that the moderately poisonous snakes are the model and both the poisonous and non-poisonous snakes are the mimics. This situation is termed Mertensian mimicry
There are not many examples of mimicry in birds or mammals but most of them that do exist fall into the category of aggressive mimicry. Wickler gives the example described by Willis of the rare zone-tailed hawk (Buteo albanotatus). This hawk, which is black, glides in the company of vultures. The wings are long and black, much like a vulture's, and it glides like a vulture. (Peterson's Field Guide to Western Birds says, "Might be mistaken for soaring Turkey Vulture because of proportions and 2-toned wing effect".) It does not hover like a hawk. Thus, the hawk mimic looks like a vulture (the model) and tricks the small animal prey into not being fearful of its approach. Clearly, the hawk does not want to be left alone but "wants something" from its mimicry. Look at the pictures below - the zone-tailed hawk is on the left and a black vulture on the right:
As another example of aggressive mimicry, "Science Frontiers" describes the results of a 1987 paper in the journal "Science": "Field studies have revealed that bolas spiders can mimic the odor of female moths, thus attracting for consumption the male moths. More specifically, the hunting adult female spider, Mastophora cornigera, releases volatile substances containing three moth sex pheromone compounds." Question: Imagine how this mimicry evolved. What good is a little mimicry?
Finally, note the mimicry of Prof. Jeff Prince, who has taken on the appearance of a bee in order to forage for sweets. Is he an aggressive mimic or something else?