PETE OXFORD/MINDEN PICTURES

See-Through Frog

There’s no doubting that this frog has guts. That’s because you can see them! A glass frog’s heart, intestines, and lungs are visible right through its belly. Glass frogs are so named because looking at them from underneath is like looking through glass.

More than 150 species of glass frogs live in the tropical rainforests of Central and South America. Most have green skin—except for on their abdomens. This area of their body lacks pigmentation, or color. Some scientists believe this strange adaptation might help the frogs camouflage themselves to stay hidden from predators, says Santiago Castroviejo-Fisher. He’s a biologist at the American Museum of Natural History in New York City.

THOMAS MARENT/MINDEN PICTURES

A northern glass frog in Colombia

Most green frogs blend in with the leaves they’re sitting on. But sunlight hitting the animal creates a dark shadow on the leaf that predators can see from below. “If the frog is partially translucent, light can more easily pass through it,” says Castroviejo-Fisher. That makes its shadow lighter and harder to notice.

There are drawbacks to translucent skin, however. Pigmentation normally blocks ultraviolet (UV) light from the sun. UV light can damage body tissues. Since light can pass through the skin of a glass frog’s abdomen, UV rays could potentially harm its organs.

This isn’t the only threat to glass frogs. They’re also susceptible to chytrid fungus—an infection that has wiped out many frog populations. Widespread deforestation is also destroying the frogs’ habitat, leading scientists to fear that some species are at risk of extinction or have already died out.