Somewhere over Texas each October, a butterfly weighing less than a paperclip is flying south towards a patch of forest it has never seen, in mountains it could not possibly know, guided by a map no one taught it. It will arrive at the same few hectares of Mexican woodland where, in a sense, its story began — even though not one of its ancestors for three or four generations has been there either. The monarch migration is among the most improbable feats of navigation in the animal kingdom, and it is stranger than a simple bird's round trip, because no single butterfly makes the whole loop.
The relay race of generations
Eastern North American monarchs spend the winter clustered in a small number of oyamel fir forests in the mountains of central Mexico, tens of millions of them packed onto the trees in orange drifts so dense the branches sag. When spring comes they fly north, breed, and die in the southern United States. Their offspring continue north and breed in turn. Over the summer, monarchs live fast — an adult butterfly survives only a few weeks — so it takes a relay of perhaps three or four short-lived generations to populate the breeding range as far as Canada.
Then, as the days shorten in late summer, something changes. The final generation of the year is born different. These butterflies do not breed straight away; their reproductive development is paused, and they live not for weeks but for months — six, seven, even eight. This is the so-called super-generation or Methuselah generation, and it is the one that flies the entire way back to Mexico, sometimes more than three thousand kilometres, and overwinters there before starting the northward journey again in spring. One extraordinary generation does the work of the return trip that the others, living and dying in relay, never attempt.
Reading the sun
How does a butterfly navigate to a place it has never been? The core of the answer is a sun compass. Monarchs use the position of the sun to hold a steady southward bearing. But the sun moves across the sky through the day, so a fixed reference to it would send the butterfly in a curving, useless path. To correct for this, the monarch runs an internal clock — a circadian rhythm housed partly in its antennae — that compensates for the sun's movement, allowing it to keep a true heading from morning to afternoon. Experiments in which antennae were painted over or removed disrupted the butterflies' orientation, revealing the antennal clock's role. There is good evidence they can also detect the Earth's magnetic field as a backup cue for cloudy days.
What draws them to those particular Mexican groves is subtler still and not fully understood, though the high-altitude fir forests offer a narrow band of conditions — cool but rarely freezing, humid, sheltered — that keeps the butterflies alive through winter without burning through their fat reserves.
The overwintering sites were a mystery to science for decades. Everyone knew monarchs vanished south each autumn, but where they went was unknown until January 1975, when Canadian researchers Fred and Norah Urquhart, working with a network of volunteers and local guides in Mexico, finally located the colonies. It was one of the great natural-history discoveries of the century, hidden in plain sight on mountainsides local people had known about all along.
The migration is now under threat. Loss of milkweed — the only plant on which monarchs lay eggs and the only food of their caterpillars — across intensively farmed North America, along with logging pressure at the wintering sites and climate stress, has driven the overwintering population down sharply from its historic levels. Conservationists measure the population not by counting individuals, which is impossible, but by the area of forest the clusters occupy.
There is a kind of poetry in the arrangement. Most animals that migrate carry their own memories of the route. The monarch carries none. The knowledge of how to reach a specific forest a continent away is written entirely in its genes and read out fresh by each super-generation, an inheritance with no teacher. Every autumn, millions of butterflies that have never seen Mexico fly unerringly to the same trees their great-grandparents left — a journey completed by insects that will never know they finished it.
Quiz nuggets
- Eastern North American monarchs overwinter in oyamel fir forests in the mountains of central Mexico.
- No single monarch makes the full round trip; it takes about three to four short-lived summer generations to travel north.
- The autumn "super-generation" lives around six to eight months — far longer than the few weeks of summer monarchs — and flies the whole way back to Mexico.
- Monarchs navigate with a sun compass corrected by a circadian clock housed partly in their antennae, plus a magnetic backup.
- The Mexican overwintering sites were located by science in January 1975, through the work of Fred and Norah Urquhart and their volunteers.