Monarch butterflies reach Mexican mountains their great-grandparents left
Every autumn, monarchs from southern Canada and the northern United States fly thousands of kilometres to fir-clad volcanic peaks in central Mexico. None of them has made the trip before. The generation heading south lives eight times longer than its parents, and it takes several short-lived generations to get back north again.
The eastern North American monarchs set off in September and October and arrive in November at roosts on oyamel firs high on volcanic mountains in Michoacán and México state. They stay until March, then head north. Four generations make up the yearly cycle. The first to leave the roosts gets only as far as Texas and Oklahoma before laying eggs; its descendants complete the trip to the northern breeding grounds. Early-summer butterflies live less than two months, while the migrating generation, held in a hormonal pause called diapause, stays active and nonreproductive for the whole winter.
To manage it, the migrants are built differently. Shortening days and cooling temperatures suppress juvenile hormone, shutting down mating and egg development. The butterflies bulk up their fat stores, which can reach 34 percent, those bound for Mexico more so than the ones headed for California, and they start forming social nectaring groups and dense night-time clusters that cut water loss. Cues for departure include the sun's angle, ageing milkweed and falling temperatures, and they refuel on autumn flowers along the way.
Astonishingly, scientists did not locate the Mexican sites until 1975. As late as 1951 monarchs were wrongly thought to overwinter as adults or pupae further north. Fred Urquhart, a Toronto-trained entomologist, began plotting routes in 1937 and ran the first tagging programme to return data, then advertised in the Mexican press for help. Catalina Trail and Kenneth Brugger answered and led him to a major roost in January 1975. Local people, of course, had always known; the Purépecha and Otomi once linked the corn harvest to the butterflies' arrival.
West of the Rockies, a separate population winters along the California coast. Twelve roosting sites have been identified in Mexico since 1976, and most monarchs typically crowd into just three. In 2025 the eastern population's winter footprint nearly doubled, to 1.8 hectares, though it remains far below its long-term average.
Source: Monarch butterfly migration
