Evolution is a treasure trove of wonder. It has led to an unimaginable diversity of adaptations for every type of habitat and ecosystem on Earth. Cool features like camouflage help protect critters from hungry predators. Batesian mimicry is a fascinating adaptation that provides a different sort of clever protection for some species.
You’ve no doubt heard of Charles Darwin. While he is often solely credited for discovering the theory of natural selection, Alfred Russell Wallace is arguably as important. In fact, many scholars believe that Darwin would have waited decades to publish On the Origin of Species had Wallace not independently arrived at the same conclusion and threatened to beat Darwin to the publisher.
Anyways, Wallace and Henry Walter Bates both worked as naturalists in the Brazilian Amazon in the late 1840s. They must have had provoking campfire conversations that seeded the ideas of evolution, zoology, ecology, and novel biological sciences. Wallace left the Amazon after a couple of years to head to Indonesia, but Bates remained in the jungle for much longer. Bates collected tens of thousands of specimens in the deep Amazon for 11 straight years. Those old-school English naturalists sure were dedicated.
After extensively studying butterflies, Bates began to notice that some of his taxidermied butterflies appeared nearly identical. Upon closer inspection, however, he realized that these butterflies that were remarkably similar in appearance were not even in the same families. How did such different butterflies have the same colors, patterns, and shapes on their wings?
Bates proposed mimicry as the mechanism for this type of evolution. Just three years after Darwin published On the Origin of Species, Bates published his book on mimicry. Bates’ mimicry concepts complemented Darwin’s (and Wallace’s) theory of natural selection very well. Bates went on to be part of the prestigious Linnean Society and The Royal Society later in life.
Batesian mimicry is when a harmless species copies the honest warning signals of a dangerous species in order to avoid predation. Let’s break that down.
Batesian mimicry requires three species; a mimic, a model, and a predator. First is the model species. This species has some sort of honest warning signal to deter predators. Sometimes model species have bright colors to indicate that they are poisonous. The warning signal might also be a particular sound that the predator knows to avoid. The important thing here is that the model species is actually poisonous to the predator. Because the model species poses an actual danger, its signal is called an honest warning signal.
The second species is the predator. The predator species has either learned or evolved to avoid hunting the model species. Some predators show innate fear of dangerous model species from birth. Other predators learn to avoid model species after eating the model species a few times and feeling awful afterward. In order for Batesian mimicry to work, the predator must percieve the honest warning signal from the model species and then avoid eating that model species due to the warning signal.
The third species in Batesian mimicry systems is the mimic. This mimetic species evolves to co-opt the same warning signal that the model species evolved to deter predators. However, in the case of the mimic, this warning signal is dishonest. If the predator ate the mimic, it would be just fine! The mimic gets protection from predation by copying the model species’ warning signal. In nature, there are no patents on good adaptations.
“Red on yellow, kill a fellow. Red on black, venom lack.” This playful saying distinguishes between the model species of the eastern coral snake and the mimic species of the scarlet kingsnake. These snakes live in the southeastern United States.
While they both have beautiful color patterns, the coral snake is truly deadly and the kingsnake is not. The coral snake’s colorful banding is its honest warning signal. Those bright colors tell the predator “don’t eat me! I’m nasty!”
Coral snake venom is a systemic neurotoxin that inhibits respiratory muscles and damages the nervous system. Only one human has died from coral snake bites in the recent past, but coral snake’s predators surely aren’t as lucky. You don’t want to mess around with coral snakes.
Mot-mot birds are snake-eating predators in the tropics. While they don’t overlap in range with the eastern coral snake, there are similarly patterned coral snake species in the tropics. Baby mot-mot birds apparently instinctively fear a wooden dowel painted with red and yellow stripes similar to a coral snake. Instead of being a learned behavior, this bird’s fear of coral snakes is instinctive.
Other raptors and snake predators also avoid the coral snake due to its high toxicity and warning colors.
The scarlet kingsnake, like all kingsnakes, is non-venomous. It takes advantage of the avian fear of coral snakes by using the same warning coloration. The kingsnake exhibits imperfect Batesian mimicry. That cutesy rhyme above shows that there are noticeable differences between the banding of the two snakes. However, this difference doesn’t seem to be pronounced enough for predators to mess with the kingsnake. The kingsnake gets all the protection that the coral snake has without having to produce venom.
Interestingly, there are similar Batesian mimicry systems with other coral snake species. The Sonoran coral snake in the deserts of northern Mexico has the Sonoran kingsnake as a mimic. The plains milksnake (which is also a species of kingsnake) mimics the Texas coral snake. These poor coral snakes are being copied left and right!
Batesian mimicry works because the model species is truly dangerous to the predator in some way. Therefore, in self-interest, the predator will avoid eating that model species.
If an avian predator hunted a coral snake, the coral snake would bite the predator and seriously injure the bird, and possibly kill it. That risk of injury or death isn’t worth the single meal the snake would provide for the bird. The bird is better off hunting harmless mice. That’s why the mot-mots won’t hunt coral snakes. The look-alike kingsnakes show a nearly identical warning signal as the coral snakes, so the birds can’t tell the difference. Therefore, the avian predators also avoid eating the kingsnakes.
The mimics are the real winners in these systems. Since the harmless mimics appear so similar to the truly poisonous model species, the predator can’t tell the two apart. The predator determines that the mimic is poisonous, even though it is not. The predator loses out on potential food by falling for the mimic’s phony warning signals.
Poisons and toxins are complex, energy-intensive chemicals to make. The mimicking kingsnakes gain a distinct advantage by getting protection from the threat of using a venomous bite without actually having to produce the costly venom. Mimics can deter predators without the energy cost of making such resource-intensive chemicals.
Batesian mimicry only works well if the model species greatly outnumbers the mimic species. This is because the predator needs to have a high chance of actually being hurt or poisoned if it hunts either species. If only 10% of the model and mimic animals were toxic models, then 90% of them would be palatable mimics. Predators would be more likely to eat both the mimics and models because they would be less likely to experience the negative effects of eating a toxic model. In this situation, Batesian mimicry would fail.
The same researchers also found that the degree of distastefulness of the model species impacts the success of the mimic species. If the model is more toxic to the predator, the mimic is less likely to be predated.
Batesian mimicry is an example of convergent evolution. Convergent evolution is where two separate species or groups independently evolve similar adaptations. Birds and bats are an example of convergent evolution. They both flap wings in order to fly. However, birds and bats evolved flight independently after they diverged in evolution. Natural selection pressured both ancient birds and bats to evolve wings.
Whenever there is a huge competitive advantage in natural selection, there is great pressure on a species to evolve towards that advantage. The evolution of Batesian mimicry provided a huge advantage to the mimic. If an ancient kingsnake was just black and white, it wouldn’t be a Batesian mimic. But imagine a random mutation turned a few bands of scales on that kingsnake into a scarlet color rather than black. That kingsnake’s scarlet phenotype might cause predators a bit of hesitation. The bird might wonder if the mutant kingsnake is edible or not. This decreased chance of predation means the kingsnake with a few red scales has a selective advantage. That mutant kingsnake is more likely to produce offspring. These offspring would then inherit the red scales.
As natural selection works through thousands or even millions of generations, that selective pressure continues to push the kingsnake to appear increasingly similar to the model species. This convergence eventually results in a Batesian mimicry system.
With the advantage of Batesian mimicry being so high for the mimic, it makes sense that natural selection would produce a lot of Batesian mimics on our planet.
Müllerian mimicry is a slightly different form of mimicry from Batesian mimicry. Instead of being harmless, the mimic species is actually poisonous. Therefore, the two species are co-mimics. Both species gain an advantage because predators need to consume fewer individuals from the co-mimic species to learn they are poisonous compared to if they did not mime each other.
For example, let’s imagine a bat needs to eat 4 poisonous moths before learning that type of moth is poisonous. If there are 100 moths and the bat eats 4 then the population is reduced to 96. Imagine a Müllerian mimicry example, though. If there are 100 moths from each species and the bat only needs to eat 4 moths total to learn that both species are harmful, then the final population of moths is 196. This population decrease is only 2% instead of 4% in the non-mimic situation.
That being said, there is a gradient between Batesian and Müllerian mimicry. Sometimes one of the co-mimics is less poisonous than the other co-mimic. The less poisonous co-mimic reaps more of the rewards from the mimicry than the more poisonous co-mimic. How much less poisonous does the co-mimic need to be before it becomes a Batesian mimic? That’s a hard question that doesn’t have concrete answers.
Monarch butterflies (Danaus plexippus) are noxious to predators because they eat tons of milkweed as caterpillars. Milkweed contains compounds that are toxic to many creatures, but not monarchs. As the caterpillars munch away at milkweed, they concentrate these toxins in their bodies. These toxins make them unpalatable to predators. The monarch’s striking black and orange coloration serves as the honest warning signal to potential predators.
The queen butterfly (Danaus gilippus, same genus as the monarch) is less toxic than the monarch but serves as another model species in this system.
The viceroy butterfly (Limenitis archippus) was, for a long time, thought to be the mimic species in a Batesian mimicry system with the monarch and queen butterflies. In the 90s, however, researchers discovered that the viceroy exhibited the same degree of unpalatability as the monarch! More surprisingly, it was actually more toxic than the queen butterfly. These findings prompted the monarch-viceroy mimicry to be reclassified as Müllerian mimicry. Remember, science is never fixed! It is in constant flux.
We still have much to learn when it comes to mimicry. Most of the famous mimicry examples come from aposematic coloration. This means that the model and mimic species appear the same to our eyes. But we humans are visual beings and our perspectives are biased towards the visual realm. Certainly, there must be tons of examples of mimicked odors, sounds, tastes, and textures. Currently, we have proof of only a few non-visual Batesian mimicry systems.
We all know rattlesnakes are highly venomous. The sound of them wiggling their rattle is their honest warning signal. I’ve been warned many times by rattlesnakes and it is very spooky! Rattlesnakes like to lie low in old ground squirrel holes when they aren’t moving around and hunting. If an animal approaches them in that hole, they will use their honest warning signal, the rattle, to tell that animal to bug off.
Burrowing owls are much less terrifying than rattlesnakes. They are fluffy, have goofy long legs, and are itty bitty. They build their nests in old burrows in the ground. Birds eggs are perfect treats for any predator, especially when they are conveniently located just below the ground. Without a venomous bite or huge teeth, how can the burrowing owl defend its nest?
Burrowing owls have learned to mimic the rattlesnake’s rattle as a call. This vocal imitation isn’t perfect, but it’s pretty darn good. With the consequences of being bitten by a rattlesnake so high, would you risk blindly sticking your badger paw into the hole in the ground? Well, maybe would take the chance if you were a roadrunner.
The burrowing owls’ auditory mimicry is a rare confirmed example of aural Batesian mimicry.
As you probably know, bats use echolocation to hunt their prey. Some species of tiger moths are toxic to bats. These species of moths respond to bats’ echolocation with an auditory click that acts as an honest warning signal. The bats then avoid eating those tiger moths. Other non-toxic species of tiger moths have co-opted this clicking response to confuse the bats. The researchers determined that “acoustic mimicry complexes are likely common components of the acoustic landscape.”
Whether it is snakes in the nearby hills or hoverflies in your garden, you can probably find some Batesian mimics near you. But be careful! As we’ve learned, the mimics and models can look so strikingly similar that messing with either of them could be dangerous. Don’t find yourself fooled by the Batesian mimics by picking up a coral snake in the woods!