Biological mimicry is an evolutionary strategy where one species evolves to share the appearance of another. For butterflies, this deception is primarily visual, relying on wing patterns and colors to manipulate the judgment of potential predators. This adaptation significantly increases an individual’s survival chance by confusing animals that hunt by sight.
The Survival Strategy of Deception
The driving force behind butterfly mimicry is the intense predation pressure from visually hunting animals, especially insectivorous birds. To counteract this threat, many species have evolved a strategy known as aposematism, or warning coloration. This involves the display of bright, contrasting patterns, such as vibrant reds, yellows, and blacks, which advertise the butterfly’s unprofitability.
These conspicuous signals are not meant for camouflage but rather to be highly memorable, indicating the insect contains toxins or tastes foul. The chemical defenses, often sequestered from host plants like milkweed or willows, provide the actual deterrent. The bright colors advertise this chemical defense, maximizing the chance a predator will remember the unpleasant experience.
The Bluff: Batesian Mimicry
Batesian mimicry represents a deceptive, one-sided relationship where a harmless or palatable species, the mimic, copies the warning signals of a defended species, the model. The mimic gains protection by exploiting the predator’s learned aversion to the model’s appearance, effectively running a bluff. This strategy is only successful if the predator is successfully tricked into avoiding the palatable mimic.
This system requires that the mimics remain at a relatively low density compared to the models. If mimics become too common, predators will encounter them too frequently and learn that the warning signal is often false, causing the deception to fail. The frequency-dependent nature of this relationship maintains the balance within the ecosystem.
The classic example involves the Monarch butterfly as the model and the Viceroy butterfly as the mimic. The Monarch acquires foul-tasting cardenolide toxins by feeding on milkweed plants, making its distinct orange and black pattern an honest signal of unpalatability. The Viceroy, historically considered palatable, has a nearly identical appearance, differing only by a thin black line across its hindwing. By closely resembling the Monarch, the Viceroy gains protection.
Shared Warning: Müllerian Mimicry
In contrast to the Batesian bluff, Müllerian mimicry involves a mutually beneficial system where two or more unpalatable species evolve to share a similar warning pattern. In this scenario, every species acts as both a model and a mimic, and the signal is honest because all participants possess chemical defenses, such as cyanogenic glycosides. The shared signal is a collective advertisement of toxicity.
The benefit arises because predators only need to learn one pattern to avoid multiple species, which shares the cost of predator education and lowers mortality for all co-mimics. This collective advertising accelerates the learning process for predators, reinforcing avoidance behavior more quickly than if each defended species maintained a unique pattern.
The tropical Heliconius genus illustrates this strategy, forming “mimicry rings” across the Amazon rainforest. Species such as Heliconius erato and Heliconius melpomene, despite being distant relatives, have evolved nearly identical, brightly colored wing patterns in the same geographical areas. The genetic basis for this convergence is controlled by a small number of genes, such as optix and WntA, which govern the arrangement of colored scales.
How Predators Learn and React
The success of any mimicry system rests entirely on the cognitive abilities of the predator, primarily insectivorous birds. Birds possess high visual acuity, allowing them to perceive the fine detail and specific color wavelengths of butterfly wing patterns. This visual capability is necessary for the predator to distinguish and remember the aposematic signal accurately.
Studies on avian predators suggest they maintain a much higher resolving power—upwards of 18 to 25 cycles-per-degree—compared to the butterflies themselves, which have a mean acuity around 0.5 cycles-per-degree. This sharp vision allows the predator to quickly form an association between the warning coloration and the resulting negative experience, such as nausea or foul taste.
Predator education occurs when a naive predator learns to avoid a specific pattern after a few unpleasant encounters. This learning requires the sacrifice of a few individuals, known as the “training sample,” which teaches the predator to generalize avoidance to all individuals sharing that aposematic signal. The memory formed by the predator maintains the selective pressure.