5 months in Mexico, monarchs remain largely inactive. While they do fly
out of their colonies on occasion, most of the time they are motionless,
hour after hour, day after day, night after night, week after week. The
butterflies must live off their stored lipid reserves all winter. They gained
this fat while they were caterpillars and during the fall migration. At
the end of the overwintering season, there must be energy left for mating
activities and the spring migration northward.
Surviving the Season on Stored Energy
The overwintering habitat is perfect for conserving energy: The high mountains
are cold enough so the lipids burn slowly--but not so cold that the butterflies
Alonso-Mejia, Eduardo Rendon-Salinas and Lincoln Brower studied the rate
at which butterflies burn their lipid reserves during the overwintering
season. Each month, the researchers took samples of butterflies that were
resting on the trees. They measured the amount of lipids stored in the butterflies?
bodies and watched how it changed, from November until March.
But do the butterflies also need nectar? It?s common to see overwintering
butterflies drinking from flowers when visiting the region. So a quick conclusion
might be YES, nectar is important. In fact, some people argued that, by
thinning the forest, more flowers would grow and their nectar could help
the butterflies survive the winter.
A closer look at the flower-visiting butterflies by Alonso and his team
lead to a fascinating conclusion: ?Monarchs that visited flowers at the
over-wintering sites had highly depleted lipid reserves. It appears that
flower-visiting monarchs do not have enough lipid reserves to migrate back
to the breeding areas of the southern United States.?
In other words, the monarchs that are visiting flowers for nectar are "running
out of gas." They're so low on lipids that they resort to nectar to
try to stay alive. In contrast, the monarchs in the clusters still have
enough stored lipids.
- Make a
graph so you can see how the lipids dropped each month. Compare the
lipid mass of butterflies found in clusters to that of butterflies found
at flowers. What do you notice?
- How many
milligrams of lipids does the average monarch burn during the overwintering
season, from November through March?
- A monarch
tag weighs 10 mg. How many times that weight does the average monarch
lose by the end of the overwintering season?
- any of a group of organic compounds including fats, oils, waxes,
steroids and triglycerides, that are insoluble in water, but soluble in
common organic solvents.