Defense Against Predators
Monarchs are foul-tasting and poisonous due to the presence of cardenolide aglycones in their bodies, which the caterpillars ingest as they feed on milkweed. Both forms advertise their unpalatability with bright colors and areas of high contrast on the skin or wings ina phenomenon known as aposematism.
Monarchs also contain cardiac glycosides in their bodies from the Asclepias plants the caterpillars eat. Overwintering monarchs in Mexico are often preyed upon by black-headed grosbeaks, which are immune to that toxin. Other birds, such as orioles and jays, have learned to eat only the thoracic muscles and abdominal contents because these contain less poison than the rest of the body. Some mice are also able to withstand large doses of the poison. Over time, overwintering adults become less poisonous, thus making them more vulnerable to predators. In Mexico, about 14% of the overwintering monarchs are eaten by birds and mice.
Monarchs share this defense with the similar-appearing viceroy butterfly, in an example of Müllerian mimicry. (Viceroys were at one time believed to be Batesian mimics of monarchs.)
Read more about this topic: Monarch Butterfly
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