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Does Scent Attract Monarchs to the Same Roosts?
Contributed by Dr. Bill Calvert

"I am amazed that, of the hundreds of trees in our yard, the monarchs consistently manage to find the very same tree AND the same branch that the larger aggregations roosted on nights before. At first I thought it was just chance, but as the incoming butterflies select the same places night after night, I really wonder if the others leave a trail of some sort."

Dr. Calvert discusses these questions below. But before reading further, answer this question in your science journal, and/or discuss as a class:

Journaling Question:
"How could you test the hypothesis that scent attracts monarchs to roost on the same branch each night?"

Photos by Patricia Rose

Now here's Dr. Calvert:
A person with hundreds of trees is in an excellent position to answer some very basic questions about roost formation. To study this, a person should note the exact position of the roosts each year, along with daily observations of:

  • Wind direction
  • Humidity (dew point is the best measure of this),
  • and, if in a drainage channel, their position with respect to the sides.

Has Anyone Done Research Into This?
One year along the migration route in Mexico's Sierra Madre Oriental, we encountered an immense flower field with isolated huisache trees in it. Each evening monarchs that had been nectaring on the flowers would stop, and fly towards the huisache trees. They would find each other and form transient, gregarious roosts. One evening we carefully noted the branch they were on. The following day we cut that branch out of the tree and tied it in position in another quadrant of the tree. That night we waited with anticipation. Well the wind had changed. The butterflies did not go to our "special" limb, but rather went to the lee side of the tree, as is their custom.

So what can we conclude from this? Not much. The cut tree limb may have been a bit drier than the tree limbs attached in the regular manner and therefore, rejected by the butterflies. But it seems that the dominant effect was the wind and not some sort of pheromone deposited by butterflies on the branch the previous night.

Dr. Lincoln Brower and some of his students tried to find an attractant at a Mexican overwintering colony where there were millions of butterflies. They found lots of chemicals associated with the fir (oyamel) trees, but nothing that could be measured as a pheromone.

It seems likely that the butterflies are looking for these things:

  1. A tree where they can be off the ground and therefore safer.
  2. A place out of the wind where they will not be blown from their perches.
  3. Each other, so they can be safer in numbers.
  4. In dry habitats, they also seek trees in riparian areas where the humidity is high.

So How Do They Find Each Other?
On observing the formations of roosts in the evening, one is struck with several things:

  1. There is active patrolling amongst the canopies of trees. Many butterflies appear to be flying around looking for something.
  2. Some butterflies land on perches. When another butterfly approaches their position the perched butterflies open their wings, often rather abruptly.
  3. After this "disturbance" they may continue to open their wings for a few seconds more, usually much more slowly than initially.

Researchers have considered two possible meanings for this conspicuous behavior.

One is the obvious one: That the perched butterfly is signaling for other butterflies to come join it. But why only signal when another butterfly is approaching? Again the answer may be the apparent one: Why bother to signal when there is no one around?

Still there is another intriguing interpretation. The perched butterfly may be signaling to come join it, but with conditions. The conditions would be to go to the periphery, a position more dangerous than the center. In any sort of school or aggregation, it is always safer to be in the center. Your predator is likely to strike at the periphery.

So far no one has figured out a method of teasing out an answer to this question. Maybe you would like to try?

National Science Education Standards

Science as Inquiry
Science investigations involve asking and answering a question and comparing that to what scientists already know about the world. (K-4)

Scientists use different kinds of investigations depending on the questions they are trying to answer. Types of investigations include describing objects, events, and organisms; classifying them; and doing a fair test (experimenting). (K-4)

Different kinds of questions suggest different kinds of scientific investigations. (5-8)

Scientists develop explanations using observations (evidence) and what they already know about the world. Good explanations are based on evidence from investigations. (K-4)

Current scientific knowledge and understanding guide scientific investigations. (5-8)

Scientific explanations emphasize evidence, have logically consistent arguments, and use scientific principles, models, and theories. (5-8)

Behavior is one kind of response an organism can make to an internal or environmental stimulus. (5-8)

Life Science
The behavior of individual organisms is influenced by internal cues (such as hunger) and by external cues (such as a change in the environment). (K-4)

History and Nature of Science
Although men and women using scientific inquiry have learned much about the objects, events, and phenomena in nature, much more remains to be understood. Science will never be finished. (K-4)

Many people choose science as a career and devote their entire lives to studying it. Many people derive great pleasure from doing science. (K-4)

Scientists formulate and test their explanations of nature using observation, experiments, and theoretical and mathematical models. (5-8)


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