Monarch Butterflies Rely In Part On Magnetic Compass For Navigation
June 25, 2014

Monarch Butterflies Rely In Part On Magnetic Compass For Navigation

April Flowers for - Your Universe Online

Mass migrations have puzzled scientists for decades, and perhaps none more so than the North American monarch butterfly. Each fall, millions of the beautiful orange and black winged insects begin their journey from the eastern US and Canada. They travel over 2,000 miles to winter in specific groves of fir trees in the Michoacan Mountains in central Mexico, where they crowd together so densely that the air is filled with butterflies, monarch butterfly expert and University of Minnesota professor, Karen Oberhauser told USA Today.

The Monarch Butterfly website reports that the species goes through four generations every year. The first three generations begin as eggs, live as caterpillars (larvae), cocoon themselves into their pupa state, then live about six weeks as butterflies. The fourth generation, though, lives somewhere between four to six months, which is long enough to make their long migratory journey south in the fall and north in the spring. Despite the fact that no individual butterfly lives long enough to make both trips, the migratory group uses the exact same trees each year.

Scientists have long known that the butterflies use a type of solar compass in their brains, according to a Discovery report, but were unable to account for the fact that the butterflies were able to migrate under heavily overcast conditions as well. The fact that they can suggested a co-reliance on a magnetic compass.

A new study from the University of Massachusetts Medical School (UMass Medical) and Worcester Polytechnic Institute (WPI), published in Nature Communications, confirms the presence of the magnetic compass with the identification of a new component in the butterflies' sophisticated navigation system. The researchers found that to help them orient southward during their migration, the monarchs (Danaus plexippus) use a light-dependent, inclination magnetic compass.

"Taken as a whole, our study reveals another fascinating aspect of the monarch butterfly migratory behavior," said Steven Reppert, MD, the Higgins Family Professor of Neuroscience and distinguished professor of neurobiology at UMass Medical School. "Greater knowledge of the mechanisms underlying the fall migration may well aid in its preservation, currently threatened by climate change and by the continuing loss of milkweed and overwintering habitats. A new vulnerability to now consider is the potential disruption of the magnetic compass in the monarchs by human-induced electromagnetic noise, which can also affect geomagnetic orientation in migratory birds."

Robert Gegear, PhD, assistant professor of biology and biotechnology at WPI, explained, "Our study shows that monarchs use a sophisticated magnetic inclination compass system for navigation similar to that used by much larger-brained migratory vertebrates such as birds and sea turtles."

Located in the butterflies' antennae, a time-compensated sun compass helps to guide the insects on their journey. Under conditions that leave them without daylight cues, such as heavily overcast days, researchers have been surprised to note that the monarch continue their migration flying in the correct southerly direction. Some researchers have suggested that the monarchs use geomagnetic cues to help navigate when daylight cues are absent.

Attempts at isolating use of an internal inclination compass in the insects has previously been met with conflicting or unconvincing results. The current researchers believe these previous studies did not account for the possibility that the magnetic compass was influenced by ultraviolet (UV) light, which is able to penetrate cloud cover.

Reppert and his team knew that monarch cryptochromes (CRY), a class of proteins that are sensitive to ultraviolet A/blue light, have the ability to restore a light-dependent magnetic response in CRY-deficient Drosophila (flies). This suggested that the monarchs also possess a light-dependent magnetic compass.

Patrick Guerra, a postdoctoral fellow in Reppert's lab, used flight simulators equipped with artificial magnetic fields to examine the monarchs' flight behavior under diffuse white light conditions. Tethered monarchs in the simulators oriented themselves in a southerly direction, he found. He also discovered that the butterflies in the simulators employed the inclination angle of the Earth's magnetic field to guide their flight. By reversing the direction of the inclination in the simulators, Guerra was able to cause the monarchs to orient in a northerly direction.

Guerra tested the light-dependence of the magnetic compass by applying a series of wavelength blocking filters to the simulators' lights. Butterflies exposed to wavelengths above 420nm lost their direction and flew in circles. Those exposed to light in the range above 380nm exhibited clear signs of directional flight. This range of 380nm to 420nm light wavelengths falls in the UV A/blue light spectral range, proving that the monarch's magnetic compass, and thus their directional flight, was dependent on UV light exposure.

These results represent the first demonstration of a light-dependent, inclination compass used by the monarch butterflies in their long journey south to Mexico, as well as the first evidence of such a tool in a long-distance migratory insect.

"For migratory monarchs, the inclination compass may serve as an important backup system when daylight cues are unavailable," Guerra said. "It may also augment hand-in-hand with the time-compensated sun compass to provide orientation and directionality throughout the migration process."

Reppert also notes that the monarchs might have an internal map, much like sea turtles. This map might be based on the magnetic field of the Earth. Scientists are still unsure how animals are able to sense the magnetic field, but it has been suggested that there is a molecule that senses the field, processes it and sends the information to the brain.

"I think the monarch (butterfly) will be the fundamental tool to find what is happening on a molecular level," Reppert said.

The researchers intend to continue their investigation by evaluating the molecular and genetic mechanisms of magneto-reception in the monarch butterflies, and the role of CRYs.

This discovery is not all good news, however. Other studies have proven that birds that navigate using an internal magnetic compass have suffered disruptions of their guidance systems caused by interference from most types of electronic gadgets. Reppert warns that their results might also mean the discovery of yet another species vulnerable to human interference.

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