How does a monarch make a trip no single butterfly finishes?

A monarch cannot make the round trip alone, so it takes a relay of generations. Each autumn, the eastern monarchs of North America leave southern Canada and the northern United States and travel thousands of kilometres, almost directly south, to overwintering sites in central Mexico. The journeys exceed the normal monarch lifespan, which is under two months for butterflies born in early summer. Four generations are involved in the annual cycle.

The trick is that the fall butterfly is special. The generation undertaking the southbound migration lives eight times longer than its parents and grandparents, because of a regulatory hormone. In the following spring the first generation leaving the overwintering sites only flies as far north as Texas and Oklahoma. The second, third and fourth generations return to the northern breeding areas of the United States and Canada.

How does a butterfly keep its bearing? Monarchs fly by day and use a circadian clock, together with the position of the Sun, as a time-compensated compass. Researchers who put monarchs in flight simulators and shifted their clocks by six hours saw them change direction, because the compass was disrupted. They also use a magnetic compass to guide the southern migration.

The story of how scientists found the winter roosts is about people. When the researcher Fred Urquhart advertised in the Mexican press for help locating the roosts, Catalina Trail and Kenneth Brugger responded, and in January 1975 they led him to one of the major overwintering sites. Local residents had been quite aware of the overwintering monarchs long before 1975, and their arrival is closely tied to the traditional Dia de los Muertos celebrations.