Do all Monarchs survive the migration to Mexico or is the trip more hazardous for some?
The Monarch butterflies' migration can be quite long, sometimes more than 2500 miles, and hazardous. Unfortunately, many of the butterflies do not survive the journey. Some of the deaths are accidental but others may be due to the inability of certain individuals to survive when conditions are somewhat stressful.
Little is known about which Monarchs survive the journey and which do not. Is size and/or mass related to survival?Maybe individuals representing all size and mass combinations survive equally from their origin in the north to the roosts in Mexico or perhaps individuals of some size and mass groups are more apt to die along the way. If this occurs, samples of Monarchs obtained along the migration should reflect these changes in survivorship.
For example, if most of the smallest Monarchs died during the migration, we would see fewer and fewer small individuals as the migration moved southward and the average size of the individuals in each sample would increase.
But it may not be this simple, nature seldom is. Mass, in particular the fat body the butterfly accumulates from consuming nectar, may be as important or even more important than size. So, our question is: how is size and/or mass related to survival during the migration.
What determines size and mass?
After an adult butterfly emerges from the chrysalis, the wings and body stay the same size throughout the life of the butterfly. But not all butterflies are the same, some are small while others are much larger. What determines size?
The three most important factors determining size are
Mass is determined by these three factors as well but is also determined by the ability of the butterfly to obtain nectar from flowers.
The nectar is rich in carbohydrates which are converted into fat tissue (fatbody) stored in the abdomen. The fatbody is an important reserve, and during bad weather when the butterflies are unable to visit flowers, breakdown of the fats produces the energy and water needed to survive. Thus, it is possible for the butterfly to gain or lose mass depending on the availability of flowers and on the weather conditions.
Actually, the differences in mass can be quite large and two butterflies which are identical in all size measurements can differ by 40% in mass. Although these differences in size and mass seem small to us, they could have significant effects on the success of butterflies making the long trip.
How can we learn more about the relationship between migration and Monarch size and mass?
We need help from students throughout the country to find out more about which Monarchs are best equipped to make the journey.
The challenge is to collect butterflies in September and October, make size and mass measurements, record the data, and send it to us. Once you have the data there are lots of questions you can ask.
How do you capture a butterfly?
See the advice in the tagging section.Briefly:
How can you tell female Monarchs apart from male Monarchs?
Because male and female Monarchs may differ in both wing measurements and mass, it is important to record the sex of each individual you measure.
It's pretty easy to tell the difference between male and female Monarchs.
What parts of the butterfly do you measure?
Two measurements should be made on the butterfly:
How do you keep a butterfly still while you measure it?
The easiest and least damaging way to measure a Monarch is to place the butterfly in a small, clear plastic or acetate folder.
To make a folder:
How do you weigh a butterfly?
If you have a scale that is accurate to 0.01 grams, you can weigh or obtain the mass of the butterfly.
Not all of you will have such a scale available in your school, but if you do following the instructions below:
What are the difficulties you might encounter?
The greatest challenge here is making accurate measurements.
Even though it may seem accurate at the time, there may be slight errors in measures of the wings due to butterfly movement, etc.
One easy way to limit the effect of this kind of random error is to make each measurement twice, each time by a different student or at different times, and use the average of these measured values as your recorded value.
To obtain accurate measures of mass, you will need to use an electronic balance accurate to 0.01 grams. Most high schools and some junior high and middle schools will have these.
If, however, you are unable to measure the mass accurately enough, please send only the length measurements and stress the accuracy of these measurements in class.
What other observations can you make on Monarch butterflies?
*This is a modified version of an article entitled "The Great Dragonfly Challenge: Measuring Monarchs" which appeared in Dragonfly Magazine vol. 1, September/October 1996.