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Monarchs rely on a combination of mechanisms in order to ward off predators. In both caterpillar and butterfly form, the monarch uses a bright display of contrasting colors to warn potential predators of its undesirable taste and poisonous characteristics. This aposematic behavior is common among many insects, amphibians, and mammals alike. Additionally, monarchs are physically similar to the viceroy butterfly, exhibiting a classic case of mimicry.

Aposematism
Monarchs acquire their poisonous nature during their caterpillar stage. By ingesting a large amount of plants in the genus Asclepias, primarily milkweed, monarch caterpillars are able to sequester cardiac glycosides, or more specifically cardenolides, which are steroids that act in heart-arresting ways similar to digitalis. It has been found that monarchs are able to sequester cardenolides most effectively from plants of intermediate cardenolide content rather than those of high or low content.

Additional studies have shown that different species of milkweed have differing effects on growth, virulence, and transmission of parasites. One specific species (Asclepias curassavica) appears to reduce the proportion of monarchs infected by parasites. There are two possible explanations for the positive role of A. curassavica on the monarch caterpillar. The first is that A. curassavica promotes overall monarch health to boost the monarch’s immune system. A second theory is that A. curassavica has a direct negative effect on the parasites.

After the caterpillar becomes a moth, the toxin shift to different parts of the body. Since many birds attack the wings of the butterfly, having three times the cardiac glycosides in the wings leaves predators with a very foul taste, and may prevent them from ever ingesting the body of the butterfly. In order to combat predators that remove the wings only to ingest the abdomen, monarchs keep the most potent cardiac glycosides in their abdomens.

Monarch toxins are pharmacologically similar to digitalis and produce extremely similar results in experimental settings. In the wild, the toxins cause many birds to experience intense discomfort and vomiting. Many birds find monarchs unappetizing and quickly begin recognizing their distinct colors and avoiding them as food sources.

Predators
Another predator of the monarchs is the Chinese mantid (Tenodera sinensis). Chinese mantids have been shown to have adapted to handle monarch caterpillars to reduce poisonous effects. Chinese mantids are capable of identifying monarch larvae and approach them in a different manner than other caterpillars. Upon encountering a monarch caterpillar, Chinese mantis will chew open the integument to let the gut fall out. Once the gut is removed, they will proceed to consume the rest of the body. This is effective as monarch caterpillars have much higher levels of cardenolide content in their guts than in the rest of their bodies. By practicing this method of ingestion, Chinese mantids are able to avoid the gut which contains high levels of milkweed particles. This practice has also been documented in some other predators as well.

Several birds have also adapted various methods that allow them to ingest monarchs without experiencing the ill effects associated with the cardiac glycosides. The oriole is able to eat the monarch through an exaptation of its feeding behavior that gives it the ability to identify cardenolides by taste and reject them. The grosbeak, on the other hand, has adapted the ability an insensitivity to secondary plant poisons which allows it to ingest monarchs without vomiting. As a result, orioles and grosbeaks will periodically have high levels of cardenolides in their bodies, and they will be forced to go on periods of reduced monarch consumption. This cycle of predation effectively reduces the potential predation of monarchs by 50 percent and indicates that monarch aposematism has a legitimate purpose.