The monarch butterfly's journey

There is a butterfly on the wing right now, across the northern United States and Canada, that will fly two thousand miles or more this autumn to mountain forests in central Mexico that it has never seen. It will arrive at the same mountains its ancestors arrived at, within the same weeks of the year. No one taught it. Nothing in its lifetime has pointed it there.

The monarch butterfly (Danaus plexippus) has always been described as miraculous. It is more specific than that.

The monarch overwintering in Mexico this November is not, in any useful sense, the same individual that flew north from Mexico last March. It is three to five generations on from that butterfly, most often four. A spring butterfly leaves Mexico in March, flies north to Texas, lays eggs, and dies. Its offspring carry the journey north through late April and May. Two more generations live and die across the breeding grounds through June and July, each lasting two to five weeks as an adult. The butterflies that emerged from mid-August onward are different. They do not breed. This generation lives six to nine months. It will not reproduce until late winter, when the colonies break up and it mates in Mexico before flying north. It will build the fat reserves that have to carry it through the winter. The same genetics; a completely different life.

The destination requires solving a navigation problem. In 2009, Christine Merlin, Robert Gegear, and Steven Reppert at the University of Massachusetts Medical School published an experiment in Science that found the clock the compass runs on. The compass itself sits in the central complex, a structure in the middle of the brain. The clock that tells it the time sits in the antennae.

The monarch uses a time-compensated sun compass: it tracks the position of the sun to maintain a consistent southwestward bearing through the day. Because the sun moves across the sky, the compass needs a clock to compensate, "toward the sun" means a different direction at 9am than at 3pm. The clock that does this compensation is in the antennae. Merlin and colleagues flew tethered migrants in an outdoor flight simulator. Butterflies with their antennae painted black held a bearing as strongly as untouched ones, but held the wrong one, flying north-northwest where the controls flew south-southeast. Butterflies with their antennae painted clear, a coating that blocks smell but lets light through, oriented correctly. Butterflies with their antennae removed altogether did not orient as a group at all. The light reaching the antennae resets the clock. The clock calibrates the compass. The compass points south.

None of this was visible in the caterpillar. It looked like every other caterpillar, banded yellow and black and white, working methodically through a milkweed leaf. But in its cells were instructions for a life of six to nine months, for reproductive delay and fat accumulation, and for a clock in the antennae that will keep a compass honest across two thousand miles, toward a few mountains in Michoacán and the State of México, high in the Trans-Mexican Volcanic Belt, among the oyamel firs at ten thousand feet.

Millions of butterflies will crowd those trees in November. Their weight will bow the branches down. Every one of them will have arrived there for the first time.

The butterflies moving over the eastern United States this month are that generation. Not summer butterflies drifting between flowers, but animals on a heading. At this latitude the passage peaks in the middle of September.

September is not a winding down. It is the middle of the crossing.