“This is how I got my eye amoeba,” Cortni Borgerson says, staring up into the massive branches of a 100-foot-tall tree in search of a rare red ruffed lemur, a cat-size primate that lives only in this area of northeastern Madagascar. She and Pascal Elison, a Masoala National Park tour guide, had raced to the area a few minutes earlier, as the animal’s roar-shriek echoed through the forest.