frogs.

Frogs are among the most ancient and ecologically sensitive animals on earth. As amphibians, they are permeable to their environment in a way that makes them one of the most accurate indicators of ecosystem health available — where frogs thrive, the water is clean, the air is clean, and the habitat is intact.

There are over 7,000 known species of frog, found on every continent except Antarctica, inhabiting environments from tropical rainforest to high-altitude alpine meadow to arid desert. Poison dart frogs — some of the most toxic animals alive — derive their toxicity entirely from their diet, and captive-raised individuals are completely non-toxic.

Glass frogs have transparent abdominal skin through which their organs, beating heart, and developing eggs are directly visible.