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When a tiny glass frog sleeps, its body becomes so transparent that it almost cannot be seen. The frog’s glass-clear skin makes no shadows. Even the red blood disappears. It’s an unusual trick—most see-through animals live in water all the time, which don’t produce red blood cells.

But when the frogs are active, blood begins to flow again, forming a pattern of bright red that can be seen. So, scientists set out to discover what happens to all that blood.

In a new study, researchers found out how, while sleeping, a glass frog sends most of the red blood cells to its liver(肝脏). In the process, the liver grows in size by about 40% to accommodate the extra cells. Like its heart and some other organs, the frog’s liver is covered in a mirrored part, which reflects lights, that helps the frog hide itself from enemies.

“If these frogs are awake, stressed or under anesthesia(麻醉), their blood systems are full of red blood cells, and they are not transparent,” explains an expert. “The only way to study transparency is when these animals are happily asleep, which is difficult to achieve in a research lab.”

Luckily, there is an imaging technology which can check red blood cells without breaking the skin of frogs. The team used this technique on frogs while they were sleeping. They found the animals moved a shocking 89% of their red blood cells to their livers while sleeping.

How exactly these frogs can pack their red blood cells together without getting blood clots (血栓) remains a mystery. Most other animals’ blood becomes very thick if the cells bump into each other, which can help cure a wound or—in a worse situation—stop blood from flowing to important areas. Understanding more about how the frogs stay healthy while jam-packing their livers with red blood cells could help advance blood clot research in humans.