The main idea of the passage is that sloths are uniquely adapted to thrive in their rainforest environment.
One piece of evidence that supports this main idea is the description of the sloth's features, such as its "curved claws" that allow it to dangle from branches and its "long tongue" that helps it grasp leaves, demonstrating how these adaptations facilitate its lifestyle. Additionally, the mention of the algae in its fur providing camouflage from predators further emphasizes its suitability for life in the trees.