Monarch butterflies, with their orange and black patterns, are often perceived as immune to predation. Their vibrant coloration acts as a warning signal, suggesting they are not a palatable meal. Despite this defense, various animals successfully prey on monarchs at different life stages. Monarch predation is more complex than their bright colors suggest, revealing a dynamic interplay between defense and adaptation.
The Monarch’s Defense Strategy
Monarch butterflies possess a defense mechanism rooted in their diet during the larval stage. As caterpillars, monarchs exclusively feed on milkweed plants, which contain a class of toxins called cardiac glycosides, or cardenolides. The caterpillars ingest these compounds and sequester them within their body tissues. This accumulated toxicity persists through their pupal and adult stages, making monarchs unpalatable to most vertebrates.
The bright orange and black coloration of monarch butterflies is a visual advertisement of this internal toxicity, known as aposematic coloration. This warning signal communicates to predators that consuming a monarch will result in an unpleasant experience, often leading to sickness or vomiting. Predators that have encountered a monarch and experienced its ill effects learn to avoid individuals displaying these colors. This learned avoidance benefits the monarch population, as predators are deterred by the general warning.
Who Preys on Monarchs
Despite their chemical defenses, monarchs face predation across all life stages from various opportunistic animals. Eggs and young larvae are vulnerable before significant toxins accumulate. Ants, spiders, and the larvae of ladybugs and lacewings prey on monarch eggs. Small milkweed bugs also consume eggs and young caterpillars.
As monarchs grow into larger caterpillars, they become targets for predatory insects like lacewings, stink bugs, and praying mantises. Paper wasps prey on monarch caterpillars, sometimes attacking them to feed their young. Spiders continue to pose a threat, capturing caterpillars in their webs. Rodents like mice and squirrels eat monarch caterpillars.
The pupal stage, or chrysalis, also faces predators. Mice and shrews consume pupae, particularly if accessible on the ground. Wasps and hedgehogs also prey on chrysalises.
Adult monarch butterflies, while protected by their toxicity, are still preyed upon by certain birds, dragonflies, and praying mantises. Spiders can trap adult butterflies in their webs, and some lizards consume them.
Predators Unfazed by Toxicity
Some predators have evolved adaptations that allow them to consume monarchs despite their chemical defenses. These adaptations can be behavioral or physiological. Two bird species, the black-backed oriole and the black-headed grosbeak, prey on monarchs, particularly in their overwintering grounds in Mexico.
Black-backed orioles use their beaks to slit open the monarch’s abdomen and consume only the fat-rich internal tissues, discarding the toxin-laden cuticle and wings. Black-headed grosbeaks, by contrast, consume the entire abdomen, indicating a higher physiological tolerance for cardenolides. Grosbeaks possess genetic mutations in their sodium pump genes, similar to those in monarchs, which grant them resistance to these toxins. They also selectively eat male monarchs, which contain lower toxin concentrations than females.
Rodents also exhibit adaptations to overcome monarch toxicity. The black-eared mouse (Peromyscus melanotis), common in Mexican overwintering sites, can consume many monarchs per night. Other mouse species, such as the eastern deer mouse and western harvest mouse, also consume monarchs. These mice often favor body parts like the abdomen and thorax, which may contain fewer toxins. Like some birds, many rodent species possess genetic changes in their sodium pump, allowing them to tolerate cardenolides.