Mating Begins
The first reports of mated pairs were noted this week. Mating will increase in earnest leading up to departure in March. Once in reproductive condition, a monarch's biological clock is ticking. With energy directed toward reproduction, the butterflies won't live much longer. They'll race northward to produce the next generation.

Lipids Are Low
The monarchs' energy reserves should be about 70% depleted by now, as the lipid graph shows. By March, the typical monarch may have only 60 mg of the 140 mg it had when it arrived in November.

Habitat Connection
Running out of fuel is a common cause of mortality in Mexico. In order to survive the winter, monarchs must obtain energy from milkweed and flowers before they reach Mexico. The cool temperatures of their Mexican forest allows them 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 winter survival in Mexico.

• Slideshow: No Food for Five Months
• Journal: Describing Lipid Levels