A while ago I wrote about the phenomenon of mimicry, and talked a bit about the earliest descriptions of mimicry. In Batesian Mimicry, two species have converged on the same conspicuous “warning” phenotype, but only one is giving an honest signal. In the case of butterflies this phenotype is usually warning coloration, or aposematism. The honest signal indicates that the animal is toxic, and the animal who is giving this signal is called the model. Predators that encounter the model have a bad reaction, associate it with the aposematic signal, and avoid prey with that phenotype in the future. Mimicry comes in when another species, the mimic, is perfectly harmless but benefits from sharing a similar phenotype with the model. Predators either can’t tell the difference between the two species, or aren’t willing to take a chance, and therefore avoid the harmless mimics. Mimics get protection from predation by virtue of their false warning coloration, plus they don’t have to pay the metabolic cost of sequestering toxins in their bodies. Henry Walter Bates first described this type of mimicry while studying butterflies in the Amazon.
In 1878, Fritz Müller proposed another type of mimicry after studying some of the same butterflies in Brazil that Bates had investigated. These specific cases had puzzled Bates, but Müller realized that mimicry doesn’t always involve a dishonest signal. Like Batesian mimicry, this type of mimicry was eventually named after the naturalist who first described it. In a Müllerian mimicry system, two or more species strongly resemble each other and are poisonous to common predators. The burden of educating a predator to avoid a specific color pattern is shared between both species, and some biologists consider it a form of mutualism.
Monarch butterflies were originally believed to be Batesian models to Viceroy butterflies after a pioneering study performed by Jane Van Zandt Brower in 1958 indicated that naive Blue Jays would reject Viceroys after exposure to Monarchs. The story wasn’t that simple, though, as David Ritland and Lincoln Brower showed later that Viceroy toxicity varies geographically (and, actually, so does Monarch toxicity). In some areas, Viceroys are not poisonous, and if they coexist with Monarchs, they are Batesian mimics of Monarchs. In other areas, though, Viceroys sequester toxins from their caterpillar host plants (often salicylic acids from willow trees) and function as Müllerian mimics. Even more interesting, Viceroys also coexist with Queens in some places. Queens and Monarchs are closely related, both sequester toxins from milkweed plants, and both are toxic. This means there are many combinations of species pairs and mimicry types possible just within this one group: Monarch + Viceroy can be Batesian or Müllerian mimicry; Monarch + Queen is Müllerian mimicry; and Queen + Viceroy can be Batesian or Müllerian mimicry.
Today some of the strongest research on Müllerian mimicry comes from researchers studying butterflies of the genus Heliconius. Heliconians occur in the tropics of Central and South America, and there are about 39 recognized species within the genus. They occur in many different species combinations and a variety of color pattern phenotypes. They take in amino acids from feeding on pollens, and these compounds allow them to make cyanic compounds- cyanide. Published studies on Heliconius mimicry have exploded in recent years, and have led to an understanding of the genetic mechanisms controlling color evolution, the physiological processes that protect the butterflies from the poisons they are carrying, the behavioral interactions that allow butterfly species to tell each other apart and find suitable mates, and many other things.