Glass act: scientists reveal secrets of frog transparency

Glass act: scientists reveal secrets of frog transparency

Glass act: scientists reveal secrets of frog transparency

Some frogs found in South and Central America have the rare ability to turn their nearly transparent appearance on and off, researchers reported Thursday in the journal Science. (Jesse Delia, AMNH via Associated Press)

Estimated reading time: 2-3 minutes

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 night frogs sleep by hanging under tree leaves. Their delicate, greenish-transparent 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 hop around looking for insects and mates, they turn an opaque reddish-brown color.

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

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

Since they have transparent skin and other tissues, it is the blood circulating through their bodies that would otherwise betray them. The frogs also shrink and pack most of their internal organs together, Yao said.

The study “beautifully explains” how “glass frogs hide blood in the liver to preserve 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 very little blood circulating with oxygen for several hours would be fatal. And concentrating blood so tightly would result in fatal clotting. But somehow the frogs survive.

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

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 terrestrial 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 the frogs.

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

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

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