Some frogs in South and Central America have the rare ability to turn their nearly transparent appearances on and off, researchers reported Thursday in Science. (AMNH via Jesse Delia, Associated Press)
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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 appearances on and off, researchers reported in the journal Thursday. Science.
During the day, these frogs sleep under the leaves of trees at night. Their delicate, greenish-transparent forms cast no shadows, making them virtually invisible to birds and other predators passing overhead or below.
But when northern glass frogs wake up and jump around in search of insects and mates, they take on an opaque reddish-brown color.
“When they’re transparent, it’s for their safety,” said Junjie Yao, a Duke University biomedical engineer and co-author of the study. When awake, they can actively avoid 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 sleeping, frogs condense or “hide” about 90% of their red blood cells in their livers.
Because they have transparent skin and other tissues, it is the blood coursing through their bodies that would betray them otherwise. Yao said the frogs also shrunk most of their internal organs and gathered them together.
Frog biologist Juan Manuel Guayasamin of the University of San Francisco in Quito, Ecuador, said the research “describes nicely” how glass frogs hide blood in the liver to maintain transparency.
How exactly they did this and why it didn’t kill them remains a mystery. For most animals, few hours of too little oxygen circulating in the blood can be fatal. And concentrating the blood so intensely causes fatal clotting. But somehow the frogs survive.
Duke University biologist and study co-author Carlos Taboada said further research on the species could provide useful clues for the development of anticoagulant drugs.
Oxford University biologist Richard White, who was not involved in the study, said only a few animals, most of which live in the ocean, are naturally transparent. “Transparency is very rare in nature and is unheard of in land animals, essentially outside of the glass frog,” said White.
The transparent ones include some fish, shrimp, jellyfish, worms and insects, none of which carry large amounts of red blood in their bodies. The trick of hiding blood while sleeping seems to be unique to frogs.
“This is a really cool, dynamic form of camouflage,” White said.
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