A sleeping glass frog can become more see-through without changing its skin.
Researchers studying Hyalinobatrachium fleischmanni found that resting frogs had far fewer red blood cells circulating through transparent tissues. Photoacoustic imaging showed that roughly 80 to 90 percent of those cells were gathered in the liver while the animals slept. When frogs became active, the cells moved back into circulation.
With fewer red cells passing through the skin and muscles, the resting frogs transmitted more light—an increase that may help them blend into a leaf while tucked underneath it. The change is temporary, and activity, stress, or handling can disrupt it. How the frog stores so many cells without harmful clotting remains a question scientists are investigating.