It takes four generations of Monarch butterflies to complete the migratory path from the Rockies to Mexico each year, so how do they know where to go? The answer may surprise you; it certainly surprised the scientists who figured it out!...

Four video-segments highlight the study of seasonal change through the "Journey North" website: "Introduction," "Seasonal Migration," "Plants and Seasons," and "Sunlight and Seasons."

Paul Andersen explains how organisms respond to the external environment. He begins with a discussion of behavioral responses like hibernation and migration. He ends with a discussion of physiological responses like shivering and...