Monarch Butterfly

Monarch caterpillars feed only on milkweed, which contains toxic compounds called cardiac glycosides (cardenolides). Instead of being harmed, they store these compounds in their bodies and keep them through metamorphosis. The wings hold about three times the body’s average concentration, and the abdomen keeps the most potent ones. This makes monarchs toxic and foul-tasting to vertebrates, and their bold orange and black pattern, a strategy called aposematism, warns predators away. The defense is not perfect. Black-backed orioles can taste and reject the toxins, black-headed grosbeaks tolerate them, and some mice eat up to 40 monarchs in a night. Caterpillars also sequester the toxins best from milkweeds with intermediate levels.

Eastern monarchs fly up to about 4,500 km (2,800 mi) from the northeastern US and southeastern Canada to oyamel fir forests in central Mexico. The butterflies making the southward trip have never been there, because the generation that left Mexico the previous spring is long gone. Unlike the northward return, which takes about four generations, the fall generation completes the journey south in a single lifetime. Monarchs steer with a time-compensated sun compass, with circadian clocks in their antennae helping them hold course and time their travel. The behavior is described as instinctive, but researchers are still working out exactly how monarchs find a destination none of them has seen.

The fall migratory generation, sometimes nicknamed the “Methuselah generation,” lives up to 8 or 9 months, compared with about 2 to 5 weeks for summer monarchs. These butterflies emerge in reproductive diapause, a state of suspended development triggered by shortening days, day-night temperature changes, and the age of the milkweed they ate as caterpillars. Instead of mating, they put their energy into migrating. They spend the winter in a torpor similar to hibernation, in Mexico for eastern monarchs and along the California coast for western ones. Diapause ends in late February and March, when lengthening days signal them to become active and reproductive again, setting up the next generation.

Monarchs are mostly independent during the breeding season, but eastern monarchs gather by the millions in winter. They cluster on oyamel fir trees at about 3,000 m (9,800 ft) in the mountains of central Mexico, usually on south-southwest slopes near water. Scientists estimate 10 to 50 million monarchs per hectare (2.5 acres) of occupied forest. The monarchs stay fairly inactive, conserving fat reserves because there is little food. The sites must be cool enough that they do not burn through that energy, yet warm enough that they do not freeze, with temperatures typically just above freezing. On warm days, they fly a little and warm their wings in the sun.

Male monarchs have a black spot on a hindwing vein, made up of scales that produce pheromones, and females do not. Courtship relies on pheromones less than in other butterflies of its genus, though. A male typically pursues a female in the air, brings her down, and mates on the ground, transferring a package of sperm called a spermatophore. Copulation can last up to 16 hours. Overwintering monarchs mate in spring, before leaving their roosts, and become ready to breed in late February and March as they begin moving north. This timing allows females to find new milkweed along the way, where they lay the eggs of the first generation of the year.

Female monarchs lay eggs one at a time, usually on the underside of young milkweed leaves, rather than in clusters. This spacing probably reduces competition among hatching caterpillars for limited leaf material and may keep predators or parasites from wiping out a whole clutch. A female typically lays 300 to 500 eggs over two to five weeks, though some have laid up to 1,180. Because caterpillars eat only milkweed, females must search for suitable plants, spreading their eggs across many of them. That wide distribution is well suited to a species whose only host plant can be patchily distributed.