Glass Law: Scientists reveal secrets of frog transparency

Researchers reported Thursday in the journal Science that some frogs found in South and Central America have a rare ability to turn their translucent appearance on and off. (Jesse D’Elia, AMNH via The Associated Press)

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Washington – Now you see them, now you don’t.

Researchers reported Thursday in the journal Science that some frogs found in South and Central America have a rare ability to turn their translucent appearance on and off.

During the day, these nocturnal frogs sleep by lounging under tree leaves. Their thin, translucent, greenish shapes do not cast shadows, making them nearly invisible to birds and other predators that pass over or under them.

But when northern glass frogs wake up and roam in search of insects and friends, they take on an opaque reddish-brown colour.

“When 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 more vulnerable, “they have adapted to remain hidden”.

Using photoimaging and ultrasound technology, the researchers discovered the secret: While sleeping, frogs concentrate or “hide” nearly 90% of their red blood cells in their livers.

Since they had translucent skin and other tissues, only the blood circulating through their bodies would get rid of them. Frogs also contract and bundle most of their internal organs, Yao said.

The research “beautifully explains” how “glass frogs mask 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 in the study.

How exactly they do this, and why it doesn’t kill them, remains a mystery. For most animals, having too little oxygen circulating in the blood for several hours would be fatal. Concentrating blood too tightly can lead to fatal thrombosis. But somehow, the frogs survive.

Carlos Tapwada, a Duke University biologist and co-author of the study, said more research on this species could provide useful clues for the development of anticoagulant drugs.

Only a few animals, mostly ocean dwellers, are naturally transparent, said Oxford University biologist Richard White, who was not involved in the study. “Transparency is extremely rare in nature, and in wild animals, it’s almost unheard of outside of the glass frog,” White said.

The transparent ones include some fish, shrimp, jellyfish, worms, and insects—none of which carry significant amounts of red blood through their bodies. This trick of hiding blood while sleeping seems to be unique to frogs.

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

The Associated Press Health and Science section receives support from the Howard Hughes Medical Institute’s Science and Education Media group. AP is solely responsible for all content.

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