Sloths appear defenseless, slow, and vulnerable in ecosystems filled with predators. Yet despite these disadvantages, they persist in environments where speed usually determines survival. Scientists ...

MIAMI — Nine-year-old Emmy Egan has spent more than two years battling leukemia. On a recent day at Zoo Miami, she got to forget about treatment and fulfill her biggest wish: meeting a sloth up close.

Sloths appear to have life figured out, and once the details are revealed, being envious makes sense. These tree-dwelling mammals have survived for millions of years by doing things their own way, and ...