The monarch butterfly, Danaus plexippus, is known for its extraordinarily long migrations between central Mexico and the central-northern United States, which it makes twice during its two-year life span. The longest recorded flight for a tagged adult is 2900 km (1800 mi). Large numbers of monarchs spend their winters in the mountains west of Mexico City. Scientists speculate that the mountainous climate provides a favorable mix of moist air and cool, but not freezing, temperatures. These conditions keep the butterfly from drying out and keep its metabolism low enough to conserve fat stores but high enough to maintain life.