Glass act: Scientists reveal secrets of frog transparency

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WASHINGTON — Now you see them, now you don’t.

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

During the day, these nocturnal frogs sleep hanging under the leaves of trees. Their delicate transparent greenish shapes cast no shadows, making them nearly invisible to birds and other predators passing above or below.

But when northern glass frogs wake up and jump in search of insects and mates, they turn a dull reddish-brown color.

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

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

Because they have transparent skin and other tissues, it is the blood that circulates through their bodies that would otherwise supply them. Frogs also shrink and pack away most of their internal organs, Yao said.

The research “explains very well” how “glass frogs hide blood in their livers 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 it, and why it doesn’t kill them, remains a mystery. For most animals, having very little oxygen circulating in the blood for several hours would be fatal. And concentrating the blood so strongly would result in fatal clotting. But somehow, the frogs survive.

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

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

Those that are 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 sleeping seems to be unique to frogs.

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

The Associated Press Department of Health and Science is supported by the Howard Hughes Medical Institute’s Educational and Scientific Media Group. The AP is solely responsible for all content.

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