Glass Frogs Hide Their Blood to Become Nearly Invisible

The glass frog, a tiny amphibian native to Central and South America, looks like something assembled at a science fair by someone who ran out of skin. Its underside is completely transparent, giving any passing predator or curious biologist a live feed of its beating heart and digestive system. It is the only vertebrate known to pull off this level of see-through-ness, and it gets much stranger from there.

In 2022, scientists discovered that glass frogs don't just have transparent skin. When they settle down for a daytime snooze, they actively hide their red blood cells by pumping about 89% of them into their liver. The liver, conveniently covered in light-reflecting crystals, then disguises itself against the leaf beneath it. The frog essentially cloaks itself by hiding its own blood.

This is remarkable for a reason beyond the obvious weirdness. Red blood cells packed that tightly in a living creature should form dangerous clots. Somehow, glass frogs evolved a way to do it safely and completely reverse the process when they wake up, a biological trick that has medical researchers paying very close attention.

Scientists believe the ability evolved to help the frogs hide from predators while sleeping on leaves above rainforest streams. One imagines evolution's pep talk going something like: you already have see-through skin, but have you considered also making your circulatory system disappear? Nature, as usual, said absolutely yes.