The butterfly effect

12 April 2013 by Malcolm Campbell, posted in Biology

“What joy awaits you, when the breeze
Hath found you out among the trees,
And calls you forth again!”

From To A Butterfly (1801) by William Wordsworth

Sometimes small things create big consequences. In Chaos Theory, the name for this phenomenon is “The Butterfly Effect”. The term Butterfly Effect was coined by Edward Lorenz to describe those instances when a small change in one place elicits a much larger impact sometime in the future. Lorenz used a simple example to illustrate this effect. In Lorenz’s simple illustration, the flapping of a butterfly’s wings determines the emergence of a hurricane.

Befitting its name, if ever there was a creature that created a butterfly effect, it was the monarch butterfly.  The flapping of its regal wings has fueled a maelstrom of scientific activity, by professional scientists and the general public alike.  In fact, a strong case can be made that the beating wings of monarch butterflies generated the creative gust behind “citizen science”.

The monarch butterfly has captured people’s imaginations for good reason. Every year, millions of these beautiful, delicate aviators make their way from the Sierra Madre mountains in Mexico, upward through the United States, and onward into central-eastern Canada, before turning around to make the return journey to Mexico. The monarch migration is generally accomplished by three generations – two that travel north, and one “super generation” that completes the entire route from Canada back to the Mexican over-wintering sanctuaries.  This 8000 km return trip is truly one of nature’s wonders.

The monarch butterfly’s monumental migration was, in the middle of the last century, one of nature’s great mysteries. This mystery served as the inspiration for a remarkable couple, Professor Fred Urquhart and his partner, Norah.

Born in 1912, Fred Urquhart classified himself as a “not-very-good student” who had a passion for biology, music and teaching when he started studying biology at University of Toronto in 1931. By 1935 he had graduated top of his class, with a scholarship to pursue graduate studies. Attaining his PhD in 1940, the newly-minted Dr. Urquhart taught meteorology to enlisted personnel in the Royal Canadian Air Force until the end of the second world war, at which point he returned to his passion, biology.

From the time he was a child, Fred Urquhart had been fascinated by monarch butterflies. Growing up on the shore of Lake Ontario, at the northern edge of the monarch butterfly range, he would wonder where the monarchs went as they headed south across the great body of water. In 1945, when he and fellow butterfly enthusiast Norah Patterson were married, Fred began a partnership where they would address the question that had intrigued him as a child for the next 30 years of their lives together.

The Urquharts recognised immediately that the key to determining the route of the monarch butterfly was to develop a good system to track them. Tagging had worked very well for birds, but a good tagging system did not exist for butterflies. The Urquharts set out to invent one.

Through trial and error, the Urquharts found a way to tag monarch butterflies. Fred had begun experimenting with tagging butterflies in 1937 with printed labels and liquid glue, but these had failed miserably. Norah and Fred also tried labels that were like postage stamps, with gummy backing. These were susceptible to being washed away with water. It wasn’t until a friend recommended labels with special adhesive that was used to place price tags on glass items that the Urquharts finally had what they needed. Their new tags adhered to butterflies’ wings where scales had been removed, without interfering with flight, and, importantly, without being washed away by water. The Urquharts dubbed their invention the “alar” tagging method, from the Latin word for wing, ala.

And so began the three decade quest to discover the migratory path of the monarch butterfly.

The Urquharts were not alone in their quest. Each butterfly had been tagged with a small, 6mm x 12mm, label that indicated “Send to Zoology University of Toronto Canada”. The Urquharts operated with the hope that people finding their tagged butterflies would return them.

Their hopes were not in vain.

People returned the butterfly tags (often with butterfly attached) noting where they had been found, which the Urquharts then mapped. A pattern was starting to emerge. It was clear that the pattern needed more data points – more data points than the Urquharts alone could hope to generate.

In 1952, Fred Urquhart published an appeal for volunteers to help with the tagging program. That year, twelve people responded. By the mid 1970s, thousands were involved. Anywhere monarch butterflies could be found, volunteers tagged them, and set them on their course, with the final tag locations returning to the Urquharts to chart. Almost single-handedly, the Urquharts had created a new way of doing research that capitalised on the enthusiasm of amateur scientists to bring critical mass to a scientific question. Citizen science was born.

A number of trends emerged from these efforts. Critically, it was clear that, from late summer onward, monarchs’ took a southwest trajectory from Canada and seemed to be heading to Mexico. In the spring, the reverse route was taken. Importantly, the condition of monarch corpses indicated that multiple generations were involved in making the migration.

Despite the inroads the Urquharts had made in fleshing out the monarchs’ journey, there remained one major frustration. They could not determine where the monarchs over-wintered. The trail went cold on the gulf coast of Texas.

In 1972, Norah extended their network to Mexico, asking newspapers to highlight their research and put out a call for volunteers. This connected the Urquharts with another couple, Ken and Cathy Brugger, who picked up the trail of the butterflies in Mexico. By  1975, the Bruggers hit the motherlode figuratively and literally. While exploring the Sierra Madre mountains, the Bruggers discovered a grove of oyamel trees, “sacred firs”, that were laden with millions of monarch butterflies.

In 1976, supported by the National Geographic Society, the Urquharts joined the Bruggers in the Sierra Madre. Climbing to oyamel grove at 3000 metres, Fred Urquhart was inspired to poetic prose:

“In the quietness of semidormancy, they festooned the tree branches, they enveloped the oyamel trunks, they carpeted the ground in their tremulous legions. Other multitudes – those that now on the verge of spring had begun to feel the immemorial urge to fly north – filled the air with their sun-shot wings, shimmering against the blue mountain sky and drifting across our vision in blizzard flakes of orange and black.”

The Urquharts had completed their quest. The route of the monarch butterfly, with its stunning connection to the over-wintering sanctuaries in Mexico, was resolved. For their efforts, Fred and Norah Urquhart were awarded the Order of Canada, the nation’s highest honour. Fred Urquhart continued to make a mark in both research, but especially in teaching. One of the founding faculty members at University of Toronto’s Scarborough campus, Fred Urquhart was also at the forefront of video lecture delivery, back when TED was just someone’s name, and a MOOC was a noise a sickly cow might make.

The Urquhart’s legacy continues to this day, not only because of their amazing discovery, but also more broadly through the impact of citizen science. Both are currently being leveraged to obtain a greater understanding of the mechanisms underpinning the monarch’s marvellous migration. At the time that the Urquharts worked out the path of the monarch butterfly, it remained to be determined how the butterflies followed the correct route from the north to the south and back again. Did they hold some sort of instinctive map in their heads? Did they follow a compass? How did they know when to return? In the decades since the Urquharts’ discovery, answers to these questions have emerged.

Extensive citizen science studies suggested that monarch butterflies travelling from the north were able to track a southwest route that was not dependent on map-based cues, but rather a compass. Indeed, detailed analyses found that monarch butterflies have an antenna-dependent time-compensated sun compass. That is, they navigate, using their antenna, in a manner that tracks both time and the sun to provide compass-like bearings. On the southbound journey, this compass is set dead straight to reinforce a southwest flight path. It’s a remarkable mechanism.

Recent evidence suggests that the antenna-dependent time-compensated sun compass is pretty darn robust. Butterflies were captured at the outset of their southbound journey, and moved 2500 km to the west. As had been predicted by previous citizen science data, this relocation didn’t change the orientation of the compass at all. It remained set so that individuals flew in a southwest direction. The neuronal basis for this compass is now being examined, and it is hoped that the monarch butterfly genome will provide additional insights into these navigational adaptations.

Recently, the trigger for the monarch’s northbound trip has been discovered. It’s the cold.

In 1976, Fred Urquhart speculated that the cold temperatures of the Sierra Madre were important for over-wintering of the monarch butterflies. As poikilotherms, where body temperature fluctuates with the ambient air, the cold temperatures enable butterflies to enter semi-dormancy.  Entering semi-dormancy ensures that the overwintering butterflies do not burn energy reserves that will be necessary for the start of their northward journey in the spring.  The role that these overwintering individuals play in the northern migration has recently been fleshed out.

It turns out that cold temperatures play a second important role in overwintering monarch butterflies. Cold temperatures help monarch butterflies turn north. Cold temperatures reorient the antenna-dependent time-compensated sun compass. On the journey from the north, this compass is oriented in a southerly direction. Following exposure to cold, this compass is reoriented to direct their flight northward. Crucially, absence of this cold signal will cause the butterflies to continue southward. This has profound implications for monarch butterflies under global climate change scenarios. There is a risk that, with elevated temperature over their normal range, the monarchs will not be exposed to the appropriate reorienting temperature in their overwintering groves. The monarch butterfly may also experience negative effects of climate change in the northern extremes of its range as well.

Climate is only one risk to the flight of the monarch butterfly. Its unique diet and habitat also create cause for concern.

Monarch butterflies are powered on their journey by milkweed. Like the butterfly that feeds on them, milkweed is a remarkable plant species. Named after the milky latex it produces, milkweed is a prodigious producer of chemical defences. To defend itself from herbivores, milkweed latex is loaded with powerful protective compounds, including a range of alkaloids and other novel molecules.  Milkweed flowers throughout the summer months, producing a large pod that releases seeds that float away on gossamer silk filaments. North America hosts around 100 milkweed species, half of which are found in Mexico.

Monarch butterflies have a special relationship with milkweed. Adult females lay their eggs on milkweed. Their caterpillar offspring are specialist feeders on these plants. This provides monarchs with an incredible advantage.

Monarch caterpillars are tolerant of the chemical defences that milkweeds produce to defence themselves from herbivores. This includes a novel class of molecules known as cardenolide glycosides. Cardenolide glycosides are toxic compounds that induce heart arrest. They are therefore very effective herbivore deterrents. Except for monarch butterflies. Adult monarch butterflies contain large amounts of cardenolide glycosides, which they accumulated as caterpillars by consuming milkweed. This makes monarch caterpillars and butterflies toxic to many predators, who will avoid eating them.  Monarch’s specialist diet protects them throughout their tortuous travels.

Once a ubiquitous plant genus in North America, milkweed is now confined to the margins of agricultural and urban landscapes. While citizen science has shown that monarch butterflies are adaptable in terms of their roost sites, their choice of diet is restricted. Consequently, it’s important to provide habitats containing milkweed to support monarch butterflies along their migratory route.

Ensuring that landscapes harbour suitable food for monarchs is an important component of preserving the route the Urquharts discovered. Another is preservation of habitat that the monarchs need. The Mexican government has taken measures to preserve monarch habitat, the oyamel groves of the Sierra Madre, and to stave off the logging that threatens it.

The preservation of the migratory path of the monarch butterfly is important. It goes without saying that it is critical from an ecological perspective. But it also has symbolic importance. The preservation of the monarch butterfly migration would honour the legacy of the Urquharts to be sure, but, well beyond that, it would serve as a powerful symbol of what large groups people can discover when they work cooperatively. This is truly the butterfly effect in action. The workings of individuals, connected together, creating a force so powerful that we all soar.


Davis AK  et al. (2012) Identifying large-and small-scale habitat characteristics of monarch butterfly migratory roost sites with citizen science observations. International Journal of Zoology (in press)

Etheredge JA et al. (1999) Monarch butterflies (Danaus plexippus L.) use a magnetic compass for navigation. PNAS 96:13845-13846

Guerra PA & Reppert SM (2013) Coldness triggers northward flight in remigrant monarch butterflies. Current Biology (in press)

Heinze S et al. (2013) Anatomical basis of sun compass navigation II: The neuronal composition of the central complex of the monarch butterfly. Journal of Comparative Neurology 521: 267-298

Luna T & Dumroese RK (2013) Monarchs (Danaus plexippus) and milkweeds (Asclepias species) The current situation and methods for propagating milkweeds. Native Plants Journal 14: 5-16

Miller NG et al. (2012) Migratory connectivity of the monarch butterfly (Danaus plexippus): Patterns of spring re-colonization in eastern North America. PLOSONE 7: e31891

Mouritsen H et al. (2013) An experimental displacement and over 50 years of tag-recoveries show that monarch butterflies are not true navigators PNAS doi:10.1073/pnas.1221701110

Zhan S & Reppert SM (2013) MonarchBase: the monarch butterfly genome database. Nucleic acids research 41: D758-D763

Zipkin EF et al. (2012)Tracking climate impacts on the migratory monarch butterfly. Global Change Biology 18: 3039-3049


5 Responses to “The butterfly effect”

  1. Khalil A. Cassimally Reply | Permalink

    Beautiful. And especially poignant considering that the organism which kickstarted an important science movement now needs science to protect it.

    • Malcolm Campbell Reply | Permalink

      Thanks, as always, for the kind feedback Khalil. It's nice to see interest in the important work that the Urquharts did - particularly the role that they played in promoting citizen science.

  2. Ina Warren Reply | Permalink

    Thank you for a delightful article; I especially love the closing statement.

    As an aside, I believe the mountain range in Mexico where the monarchs overwinter is called the Transvolcanic Belt.

    • Malcolm Campbell Reply | Permalink

      Thank you for the very kind feedback, Ina. Thank you also for providing information on the overwintering ground! Very helpful!

  3. Lucinda Reply | Permalink

    Thanks for some other fantastic post. The place else may just anybody get that type of info in such a
    perfect manner of writing? I have a presentation next week, and I'm at the
    look for such info.

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