Awesome monarchs

Migration cycle of the fragile monarch butterfly extends beyond its average lifespan

 
By Ram Kumar
Last Updated: Saturday 04 July 2015

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The monarch butterfly (Danausplexippus) holds the awe of scientists as it migrates across the North American continent despite its diminutive size and fragile nature. The time taken to cover the staggering distance is well beyond the average lifespan of butterflies—while one generation migrates during winter, the returning butterflies are of the next generation. Yet, they find their migratory path and destination unerringly, a fact that has been observed diligently by several researchers for the past few decades.

It is widely accepted that the time-compensated sun compass helps butterflies orient themselves. In simple words, the sun’s relative position in the sky and its time variation in relation to latitude and longitude helps guide the insects. But this does not explain how the butterflies manage to fly despite cloudy skies. Even in the absence of directional daylight cues, the migrants have been observed flying in the expected southern migratory direction.

Steven Reppert, Patrick Guerra and Robert Gegear from the University of Massachusetts Medical School, and the Worcester Polytechnic Institute, Massachusetts, worked on a hypothesis that the monarchs could use a “weather-proof” mechanism to guide themselves during cross continental migration from the cold North East US and Canada to Mexico to spend their winters. “We hypothesised that one such back-up mechanism would be the use of a magnetic compass, as found in other long-distance migratory animals such as sea turtles and birds,” says Guerra.

The researchers studied butterflies which were made to fly through a wind tunnel surrounded by a magnetic coil system to see the impact of this extra magnetism on their internal magnetic compass. The researchers found that the antennae play an important role as they contain light-sensitive magnetosensors. The team also realised that a magnetic compass is in play in the navigation and also that this compass is light-dependent. The findings were published on June 24 in Nature Communications.

Uttarakhand-based lepidopterist Peter Smetacek, an authority on Indian butterflies and moths, says the findings are remarkable and fill the gap in our understanding of the world of butterflies. “Within the south Indian peninsula, we have butterfly migrations following a pattern. We have the Himalayan butterflies who descend to the plains during the winter,” says Smetacek. “Some species cross into Europe regularly from North Africa, some like the Dragonfly butterfly fly from the Himalayas to the Maldives, and the next generation goes to Africa, and then the subsequent generations return to the Himalayas via the Maldives,” he adds.

Krushnamegh Kunte, reader, National Centre for Biological Sciences, Tata Institute of Fundamental Research, says, “The migration could only be possible by a complex orientation technique which includes the migration path being ingrained in their DNA. This helps them withstand the onslaught of weather variations and human interferences.”

But a spate of recent studies has indicated that human interference does affect migration. A study on young steelhead trout (Oncorhynchusmykiss) shows that exposure to iron and steel, materials used to construct hatcheries, affects its navigation ability. The ability of the fish to navigate has direct effect on its survival, says Nathan F Putman from the department of fisheries and wildlife, Oregon State University, US, who carried out this study published in Biology Letter in June (see “We are causing magnetic noise”). The study indicates that small differences in the magnetic environment of hatcheries could help explain why some hatchery fish do better than others when they are released into the wild.

Smetacek, the author of the book Butterflies on the Roof of the World, says, “When we get to know the amazing complexities of nature, we learn to appreciate the need to conserve what is left.”

“We are causing magnetic noise”
 

Nathan F PutmanSeveral human-made fields, which are stronger than the Earth's magnetic field, are hampering navigation abilities of animals, says Nathan F Putman, a post-doctoral researcher at the department of fisheries and wildlife, Oregon State University, US. Edited excerpts from an interview

Why is the ability to navigate important for animals?

Different parts of an animal's range are best suited for different activities. Some locations might serve as good shelter, others as good feeding grounds, others as places where it is safest to reproduce or rear the young. Animals need to efficiently transit between these different locations, whether a few metres apart or across continents.

How do animals ensure they navigate correctly?

A butterfly cannot look from Maine and see that it is making progress toward Mexico, nor can a salmon see from the Aleutian Islands that it is getting closer toward Vancouver. What these animals need is an orientation system that informs them whether they are heading in the correct direction. The earth's magnetic field provides such information. They can determine where they are on the globe, based on what the magnetic field feels like. This is possible because there are predictable gradients in the field's intensity (stronger toward the poles and weaker towards the equator) and the angle that field lines intersect the surface of the earth (that is, field lines are perpendicular to the surface at the magnetic poles and gradually become parallel as one moves to the magnetic equator). These two gradients form a bicoordinate grid over many parts of the earth that allows animals to assess their location.

Given that the earth's magnetic field is quite weak (relative to some human-made fields), for animals to use such subtle magnetic fields for navigation requires an exquisite sensitivity. Unfortunately, this may put the animals in conflict with some of the things that humans do.

How is this happening?

Humans are doing all sorts of things that increase the magnetic "noise" in our environment. Much of this is being done while we still have a very rudimentary idea of how animals use magnetic cues. Whether offshore wind farms or wave energy devices will disrupt the migration of marine animals is difficult to know, but these projects are moving forward regardless.

What do we do to avoid the problem?

It would be wise for us to learn more about how animals use the earth's magnetic field to navigate as we go forward modifying what appears to be an important environmental feature for many species.

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