Unraveling the Mystery of Monarch Migration

monarch butterflies

Monarch butterflies fly between 50 and 100 miles each day during their migration. (Photo credit: Didier Dorval / Radius Images)

The monarch butterfly is the only butterfly species that makes an annual round-trip migration. Scientists have wondered for quite some time what triggers the monarch’s migration behavior. New research may finally provide an answer to that question.

Despite their diminutive size, the length of the monarch butterflies’ migration is quite impressive. Over the course of several months, monarch butterflies travel approximately 2000 miles from their northern habitat to their southern habitat. These butterflies typically travel between 50 to 100 miles per day. Rather than flapping their wings the entire way, monarchs rely on air currents and thermal winds to help power their flight from one location to the next, thus conserving energy and increasing their chance of successfully reaching their final destination.

Populations of monarch butterflies that live in eastern North America overwinter in the Sierra Madre Mountains of Mexico — where their overwintering habitats are limited to fewer than 14 locations. Monarch butterfly populations that live in the western North America overwinter in California along the Pacific Coast near Santa Cruz and San Diego.

During their migration, the butterflies only fly during the day and spend the night roosting in trees. The butterflies tend to utilize the same trees and roosting areas year after year.Upon their arrival at their overwintering habitats, the monarch butterflies cluster together tightly in colonies — tens of thousands of butterflies may be found in a single tree.

What makes their migration behavior particularly interesting is that while one generation of butterflies typically makes it from their summer habitats in the north to their winter habitats in the south, it takes multiple generations of butterflies to make the return trip. That is, a female adult butterfly that leaves Mexico for Canada will not make it much farther than Texas after it lays an egg; and its daughter will likely not make it farther than the Dakotas; and the granddaughter will give rise to a great-granddaughter that hatches from an egg laid in Canada.

As if an epic migration wasn’t enough of a challenge, monarch butterflies also face hazards on many different fronts. Their habitats are threatened throughout their range, and illegal logging continues to decrease the size of their overwintering habitat in Mexico. One of the primary food sources eaten by monarch butterflies is the milkweed plant, which provides monarchs with their characteristic bitter taste, which predators such as birds find unpalatable. Increased use of pesticides has greatly diminished milkweed populations. In addition, climate change is affecting the timing of wildflower blooming, which in turn affects food source availability for the monarchs on their journey back north.

Monarch butterflies are the subject of a number of scientific studies and long-term projects. There are many questions that remain about the butterflies’ migration patterns and how the butterflies know where to go year after year. Recent research from scientists at the University of Massachusetts Medical School has shed some light on the details behind the butterflies’ migration. Researchers led by Dr. Steven Reppert captured southbound monarch butterflies on their way to Mexico. The captured butterflies were separated into three groups. One group was placed in an incubator for 24 days in which the temperature was turned down to 4°C during dark periods simulating nighttime, and up to 11 Celsius during light periods simulating daytime. A second group of butterflies experienced the same conditions, but in addition, experienced a small increase in daylight each day, similar to the increase of daylight as winter progresses toward spring. A third group experienced served as the control, and experienced no changes in temperature or daylight.

When individual butterflies from each group were tethered inside a flight simulator, monarchs from both experimental groups flew north. Butterflies from the control group continued to fly in a southerly direction. The researchers found that exposure to a period of just 24 days of colder temperatures triggered the butterflies to switch their migration from south to north.

“The more we learn, the clearer it becomes that the monarch migration is a uniquely fragile biological process,” Steven Reppert said in a press release about the research. “Understanding how it works means we’ll be better able to protect this iconic system from external threats such as global warming.”

More to Explore
The University of Massachusetts Medical School Monarch Project
Chill Turns Monarchs North
Coldness Triggers Northward Flight in Monarch Butterflies: Migration Cycle May Be Vulnerable to Global Climate Change
National Geographic: Monarch Butterfly
World Wildlife Fund: Monarch Butterfly
Monarch Watch
Monarch Butterfly Migration

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