Full of Fat
Monarchs arrive in Mexico with stored fat that will be used as energy to survive winter. A colony of 50 million butterflies could contain around 1,500 pounds of fat. The colony could be a banquet for hungry predators, but there's a problem—monarchs contain poison!
Protected from Predators?
Monarchs eat milkweed when they are caterpillars. The milkweed contains a special chemical that is poisonous to many animals — but not to monarchs. The body of the adult monarch contains the same poison it ate as a caterpillar. Most of the poison is stored in the butterfly's cuticle (skin). Even though the butterflies have built-in protection, some animals can eat monarchs. Who are the monarch’s enemies in Mexico?
Looking for Clues
Dr. Lincoln Brower and his research teams worked for many years to identify the predators in the winter sanctuaries. The scientists discovered three main monarch predators. Each predator has its own method of eating a butterfly, and each predator leaves clues that reveal its identity.
Mice Eat Monarchs
Mice feed at night. They eat butterflies that they find on the ground. The mice will eat living, dying, and recently dead butterflies. There are 4 species of mice in the sanctuary area, but only one species is known to feed heavily on monarchs. For some reason, the Black-eared mouse (Peromyscus melanotis) can eat monarchs without being hurt by the poison. One mouse can eat about 37 monarchs a night. Mice leave a pile of wings on the ground.
Orioles Eat Monarchs
Black-backed orioles prey on monarchs. Orioles avoid the poison by not eating the cuticle where most of the toxins are stored. An oriole uses its sharp beak to slice the cuticle open, and then eats the fat inside. Birds feed on monarchs in the morning and evening, when the butterflies are too cold to fly.
Grosbeaks Eat Monarchs
It's easy to tell when a Black-headed Grosbeak ate a butterfly. The monarch's entire abdomen is missing. Grosbeaks do eat the cuticle. However, they prefer male monarchs who have 30% fewer toxins than do females. There are 37 bird species in the region that eat insects. Yet only the oriole and grosbeak prey on the millions of monarchs that visit Mexico every winter.
Impact of Predators
Millions of monarchs are eaten in Mexico. The ground under a butterfly colony is peppered with wings by the end of the winter. In a typical year, scientists estimate that more than 15% of the entire overwintering population is killed by predators.
Safety in Numbers
Beyond protection from poison, monarchs have other ways to defend themselves. "Forming colonies is clearly an anti-predator strategy," says Dr. Brower. "The probability of any one individual getting killed is lower if you aggregate, or group together."
Quiet and Camouflaged
The monarch's bright orange wings are closed when the butterflies rest in the colony, providing further protection. Thus, the oyamel forest provides sanctuary for overwintering monarchs, but it cannot protect the butterflies from all dangers.