Butterflies, with their intricate patterns and graceful flight, possess a defense mechanism: toxicity. These “poisonous” butterflies are not venomous, meaning they do not inject toxins. Instead, they are toxic if ingested, acting as a deterrent against predators. This adaptation allows them to survive.
How Butterflies Become Toxic
Butterflies do not produce their own toxins. They acquire these compounds from the plants their larvae, or caterpillars, consume. Certain plants produce toxic chemicals as a defense against herbivores. Caterpillars of specific butterfly species ingest these plant toxins without harm, a process known as sequestration.
These sequestered toxins are stored within the caterpillar’s body tissues. As the caterpillar undergoes metamorphosis, the toxins are retained and transferred to the adult butterfly. The adult butterfly remains toxic to potential predators, even with a diet change.
Examples of Toxic Butterflies
The Monarch butterfly (Danaus plexippus) gains its defense from milkweed plants (Asclepias spp.). Monarch caterpillars feed exclusively on milkweed leaves, which contain cardiac glycosides, or cardenolides. These compounds can cause vomiting and heart problems in vertebrates, including birds, making the Monarch unpalatable. The Monarch’s toxicity varies with the specific milkweed species consumed by the caterpillar.
The Pipevine Swallowtail (Battus philenor) acquires its toxicity from pipevine plants (Aristolochia spp.). Its caterpillars consume these plants, sequestering aristolochic acids. These toxins remain in the butterfly throughout its life cycle, making both the caterpillar and adult unappetizing to predators. The Queen butterfly (Danaus gilippus), a close relative of the Monarch, also sequesters cardenolides from milkweed.
How Toxic Butterflies Protect Themselves
Toxic butterflies use aposematism, or warning coloration, to advertise their unpalatability to predators. They display bright, contrasting colors such as orange, black, yellow, and red. These conspicuous patterns signal to predators that the butterfly is not a desirable meal.
Predators, such as birds, learn to associate these vibrant colors with an unpleasant experience after an initial encounter. If a bird attempts to eat a toxic butterfly and becomes ill, it will avoid similarly colored butterflies in the future. This learned avoidance benefits the individual toxic butterfly and other species that share similar warning patterns through mimicry.
Batesian mimicry occurs when a non-toxic species evolves to resemble a toxic one, gaining protection. Müllerian mimicry involves multiple toxic species displaying similar warning patterns, reinforcing the predator’s learned aversion.