Mexico’s monarch butterfly more than doubles this winter — Why?

This Monarch Butterfly Annual Migration represents an intriguing example of gene-environment interactions. Each winter, Monarch butterflies — from Canada and the northeastern and midwestern United States — flock to the pine and fir forests of central Mexico to hibernate, covering trees and turning many acres of forest from “green” to orange and black.

The annual cycle of these animals is fascinating: after wintering-over in the Mexican mountains, the offspring fly to southeastern United States. Then a second generation flies further north, a third generation flies even further north, and the fourth generation (now in southern Canada) makes a wild-and-crazy quick trip back to Mexico in the early autumn. How do these insects know what to do, each year? This is an example of transgenerational epigenetics [the brains of each generation ‘are programmed’ (‘imprinted’) with the ‘migrating signals’ necessary to navigate northward throughout spring and summer, then southward again at the end of summer].

This year, the butterfly population wintering-over in Mexico is more than double (according to World Wildlife Fund Mexico, which oversees the annual count). The butterflies covered 14.8 acres (6.05 hectares) of Mexican forest, up from 6.13 acres for last year. This year (2018-19) is the largest wintering 1-year expanse of Monarch butterflies since the winter of 2006–07. In recent years, the total number of migrating monarchs has been struggling — with an all-time low, arriving back in the Mexican mountains in 2013–14 — when they covered a mere 1.66 acres (0.67 hectares). The (political) Science magazine postulated that “this year’s uptick reflects a boost from mild climatic conditions in the U.S. last year, which allowed more Monarch larvae to survive and mature there and eventually make the 4,000-kilometer journey to Mexico.” However, please recall the reports of “an extremely hot summer” in the U.S. in 2018 (i.e. the U.S. did NOT have ‘mild climatic conditions’ last summer, according to Science magazine).

Perhaps part of the explanation (for this increased number in Monarchs) is that plants (including milkweed on which Monarchs feast) are growing better, due to rising amospheric CO2 levels, which have been increasing ever since the end of the Little Ice Age (1300-1850 AD) — which, in turn, is caused mostly by rising planetary temperatures. It is known that, as oceans warm, the sequestered CO2 in cold oceans (during the Ice Age) is again released into the atmosphere. Based on data from the Modis and AVHRR sensors that have been carried on American satellites over the past 33 years, the instrumentation shows significant greening (15-20%) of Earth’s land, as the result of rising CO2 levels that benefit plant growth. Another contribution: Earth is currently at a solar minimum (which occurs every ~11 years). The bottom line: Scientists do NOT know know why the Monarchs went through a lowest point (nadir) in 2013-14, and why they reached a much higher point in 2018-19. Will the number go higher next winter? Stay tuned. 🙂


Science 6 Feb 2o19; 363: 564­­

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