No Food for Five Months?
Monarch butterflies arrive in Mexico in November and stay until March. Scientists say they can survive all winter with little or no food at all. How is this possible? Let's explore where monarchs get the energy they need to survive.
Fat is Fit
When butterflies arrive in Mexico, some are big, fat and fresh. Others are already thin, tattered, and torn. Look at the size of each butterfly's abdomen, where its fat is stored. Which of these two butterflies do you think is more likely to survive the winter? Running out of fat is a common cause of death. Monarchs must survive on the food they ate before they went to Mexico.
Voracious Eating as a Caterpillar
When a monarch caterpillar eats milkweed, some food energy is stored as fat. A monarch caterpillar that finds plenty of milkweed becomes a big, healthy butterfly with a reserve of fat. This fat reserve will help a monarch survive the winter.
Voracious Eating as an Adult
Adult monarch butterflies drink nectar from flowers. Extra food energy is stored as fat in the monarch's abdomen. Monarchs eat hungrily in the fall before—and during—their migration. "They're so intent on eating in the fall you can sneak up on them and catch them with your fingers!" observed Dr. Brower. "Monarchs won't let you do that any other time of year."
How Do Cool Temperatures Help?
The secret behind the monarch's survival without food is the cool habitat it chooses in Mexico. "Cool temperatures in the forest slow down the monarchs' metabolism so they can stay alive longer," says Dr. Karen Oberhauser.
Fat Reserves Month-by-Month
This graph shows how much fat (lipid mass) the typical monarch has in each month of the year. Look how fat reserves change during the five months the monarchs are in Mexico!
So Many Butterflies, So Few Flowers!
There can be 50 million monarchs sharing a single hectare. Imagine how many flowers millions of hungry monarchs would need. "There are some flowers near the colonies, but nowhere near enough for all of the butterflies," says Dr. Oberhauser. This is why stored fat is essential to survival.
What About Water?
Monarchs do need water to survive the winter, and often leave their clusters to find it. On warm days, butterflies fly out of the colonies by the millions to drink. At first, scientists didn't realize water was so important. By observing overwintering monarchs, scientists learned that protecting the watershed of the forest is one of the most important conservation goals.
The Habitat Connection
In order to survive the winter, monarchs must find milkweed and flowers to obtain energy before they reach Mexico. Then they need the cool temperatures of their Mexican forest to conserve energy. The next time you see milkweed and flowers in your backyard, think about how habitat on the northern breeding grounds is connected to monarch survival in Mexico.