Monarch Butterfly

Danaus plexippus, the monarch butterfly, performs the most extraordinary insect migration on Earth. Each autumn, monarchs born in southern Canada and the northern United States fly up to 4,800 km south to a small set of overwintering groves of oyamel fir trees in central Mexico — a destination none of them have ever seen. The migration takes one generation. In spring, the same individuals fly partway north, lay eggs, and die; the next generation continues, and so on for three to four generations until the population reaches Canada in summer. The fall migration is encoded genetically: the great-great-grandchildren of the previous winter's overwinterers find the same Mexican forest. Monarch caterpillars feed exclusively on milkweed (Asclepias spp.), accumulating cardiac glycosides that make them and the adult butterflies poisonous to most predators. Their orange-and-black wing pattern is a textbook example of aposematic warning coloration. Monarch populations have declined dramatically — over 80% in the past two decades — primarily due to habitat loss, milkweed eradication, pesticides, and climate disruption.