托福听力

🎧 听力原文

Listen to part of a lecture in a biology class. The professor is discussing animal migration patterns. Professor: So, we've talked about how some birds migrate thousands of miles each year. But what about insects? Monarch butterflies are a classic example. They travel up to 3,000 miles from Canada and the northern United States to central Mexico. What's fascinating is that no single butterfly makes the entire round trip. Instead, it takes four generations. The first three generations live only a few weeks, but the fourth generation, born in late summer, enters a state called reproductive diapause. This allows them to live up to eight months, conserving energy for the long journey south. They use environmental cues like temperature and daylight to trigger their departure. Interestingly, they navigate using the sun's position and Earth's magnetic field. Once in Mexico, they cluster in oyamel fir forests, where the microclimate protects them from freezing. The return trip in spring is led by this same generation, but they only fly partway north, lay eggs, and die, and the cycle continues.