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Monarch Migration

A History

For most, monarch butterflies aren’t much more than brightly orange coloured documentary characters. For others, they are regular guests in the backyard. Many still will barely recognise the name.

For the esteemed Toronto-born lepidopterist – a person who studies butterflies and/or moths – Dr Fred Urquhart, the monarch represents a life’s work.

Every year, hundreds of millions of North American monarch butterflies venture off for thousands of kilometres to and from their southern overwintering sites. And it is in these truly spectacular mass-migrations where have monarchs claim their fame.

Having just completed his MA in 1937, Urquhart began what was to become decades of answering what seemed like such a straightforward question: what happens to the millions of these large, conspicuously coloured butterflies between leaving North America for the fall and winter months and their reappearance come spring?

Beginning that year, Urquhart began tagging individual monarchs in an attempt to track their movements. The effort gradually multiplied/morphed/cascaded into thousands of volunteers affixing tiny “Send to Zoology University of Toronto Canada” labels to the forewing of countless individual butterflies. A sprightly 38 years later, Mexico City residents Kenneth and Cathy Brugger heeded Urquhart’s plea, reporting to him what turned out to be the first scientifically recognised overwintering site.

Since this time, the understanding of monarchs has built to accommodate for hundreds of more documented sites stretching from Mexico to California. Armed with a richer grasp of their cross-continental migration patterns, the attentions of monarch researchers has since shifted from questions of “where” to questions of “why” and “how”.

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Monarchs in 2013. Johnna Madjedi

Why

Around late August, monarchs in their summer ranges to the north begin to sense changes in the environment around them heralding the onset of winter.

A 1997 study conducted by Liz Goehring and her supervisor Dr Karen Oberhauser found three key variables monarchs use to tune into these environmental changes: day length, temperature, and the age of their host plants. All three, or even one alone, were able to trigger diapause – a state of arrested development and suspended reproduction in the butterflies and a sure sign of migration preparation. Once those three measures reach a tipping point, the monarchs leave en mass in search of greener pastures.

 

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Carrying on Urquhart’s work, monarch butterflies are tagged to better understand their migration patterns. The Wild Centre.

How

As they leave monarchs mix at least three innate biological processes to find their way, each well above the pay grade of a paper-thin insect.

The Time-Compensated Sun Compass

Monarchs typically migrate during the day, and take advantage of the sun’s rhythms using a ‘time-compensated sun compass’ to angle themselves towards their destination. This “compass” combines an internal clock situated within the antenna of the butterfly (the “time-compensated” bit) with the position of the sun in the sky.

Take 8 AM for example. At 8 AM, the butterfly’s antenna feed information to its brain so it can tell what time it is. From that it knows that the sun should be sitting just above the horizon to the east. Its eyes then feed its information to its brain to locate the sun.

Let’s say this particular butterfly is wanting to head south. Using those two feeds of information, it can then line up the sun to be off on the left-hand side of its vision and be on its way.

Magnetic Compass

Either as a compliment to or a backup for the solar compass, monarchs have also been shown to run a second compass, this one magnetic. By fixing monarchs into small boxes with an artificially produced magnetic field, it has become quite clear that they are tuning into magnetic forces to guide their journeys. The method they use to pick up on those magnetic fields is still up in the air, but some evidence is suggesting that the butterflies are picking up on magnetically-manipulated UV light.

Genes

But despite these lofty abilities, many monarchs do not migrate at all. Since their dispersal into the southern reaches of America and through the Atlantic and Pacific regions, many monarchs have limited or altogether ceased migrating. By analysing the genetic differences between those who do and those who don’t, several pieces of research have suggested that genetic factors may also be playing a part in driving those that mass migrate.

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The monarch butterfly has spread into South America and parts of the Atlantic and Pacific. Picture taken Qld. Australia.

 

Unfortunately, the monarch’s dependence on crossing huge expanses of ever-developing land is threatening the preservation of their migratory behaviours. For the academic descendants of Urquhart, understanding their migrations is not an intellectual pursuit, but a necessary one.