Scientists reveal mysteries of frog transparency

Now you see them, now you don’t.

Some frogs found in South and Central America have the rare ability to toggle their nearly transparent appearance on and off, researchers report Thursday in the journal Science.

During the day, these nocturnal frogs sleep by hanging under tree leaves. Their delicate, greenish transparent shapes cast no shadows, making them almost invisible to birds and other predators passing overhead or below.

But when northern glass frogs wake up and hop around in search of insects and mates, they turn an opaque reddish-brown color.

“If they’re transparent, it’s for their safety,” said Junjie Yao, a biomedical engineer at Duke University and co-author of the study. When awake, they can actively evade predators, but when they are asleep and at their most vulnerable, “they have adapted to remain hidden”.

Using light and ultrasound imaging technology, the researchers uncovered the secret: While the frogs sleep, they concentrate or “hide” almost 90% of their red blood cells in their liver.

Since they have translucent skin and other tissues, it’s the blood circulating through their bodies that would otherwise give them away. The frogs also shrink and pack up most of their internal organs, Yao said.

The research “explains beautifully” how “glass frogs hide blood in the liver to maintain transparency,” said Juan Manuel Guayasamin, a frog biologist at the University of San Francisco in Quito, Ecuador, who was not involved with the study.

Exactly how they do this and why it doesn’t kill them remains a mystery. For most animals, having very little oxygen in their bloodstream for several hours would be fatal. And such a high concentration of blood would lead to fatal coagulation. But somehow the frogs survive.

Further research on the species could provide useful clues for developing anti-clotting drugs, said Carlos Taboada, a Duke University biologist and co-author of the study.

Few animals, mostly sea creatures, are naturally transparent, said Oxford University biologist Richard White, who was not involved in the study. “Transparency is very rare in nature, and in terrestrial animals it’s essentially unknown outside of the glass frog,” White said.

The transparent include some fish, shrimp, jellyfish, worms, and insects, none of which move large amounts of red blood through their bodies. The trick of hiding blood while you sleep seems unique to frogs.

“It’s just this really amazing, dynamic form of camouflage,” White said.

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