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Glass frogs on a leaf
Photo: Jesse Delia
For scientists, sleeping glass frogs are a wonderful thing: they can comfortably see what's going on inside from the outside.
How things work in the digestive system of animals.
Or how the heart is working – all without an X-ray machine or MRI.
The only a few centimeters large animals from South and Central America are partially transparent.
Muscles and abdominal skin appear transparent and let up to 95 percent of the light through.
This complex type of camouflage gives amphibians advantages because they are better able to hide from predators.
Experts led by Carlos Taboada from Duke University in the US state of North Carolina have now examined the special camouflage trick more closely using the species Hyalinobatrachium fleischmanni.
The green animals are nocturnal and sleep during the day.
When they go to rest, their transparency increases two to three times - they can then hardly be distinguished from a leaf.
But how is that possible?
Camouflage through transparency is well known in the animal kingdom.
Above all, aquatic animals such as jellyfish or zooplankton have mastered the art of merging with their environment.
Vertebrates that live on land are less likely to do this.
Just like in humans, the glass frog's cardiovascular system is usually made of red blood cells that are full of hemoglobin.
This blood pigment absorbs blue and green light.
Even transparent tissue becomes colored and visible due to the oxygen transport of the blood.
Complex camouflage mechanism
Clear eel larvae seem to have overcome this blood problem by not producing hemoglobin and red blood cells.
Glass frogs, on the other hand, developed a complex camouflage mechanism in the course of their evolution, the current analysis shows.
During periods of rest, they remove about 89 percent of their red blood cells from the circulatory system and shove them into a kind of interim storage facility: the liver, which is responsible for blood purification and is thus something of a body's garbage dump.
Nevertheless, the small frog's body functions normally during this phase, there are no circulatory or metabolic disorders.
In the resting phase, clots would have been expected due to the few red blood cells in the blood.
Oxygen starvation could also have occurred since red blood cells are responsible for transporting this vital element.
Apparently, the animals get by with very little during the sleep phase.
After the frogs wake up and start moving, the liver releases the red blood cells again.
At the same time, the color returns to the animals' bodies.
The experts tracked all of this with imaging methods and ultrasound examinations – in particular the distribution of blood and hemoglobin in the vessels.
"This strategy allows the frogs to achieve great transparency when they are most vulnerable," the team writes.
It is still completely unclear how the animals manage to temporarily store their red blood cells in the liver.
It is also unclear whether they can control this mechanism independently - for example, if a predator approaches, write Nelly Cruz and Richard White in a comment on the study.
An answer to these questions could provide clues to a general understanding of blood flow - possibly with results that are also relevant for humans.
If the intriguing mechanism that prevents blood clots during the process is understood, new cardiovascular drugs or therapies could potentially be developed.
The little frog should at least allow researchers to look more deeply in the future.
joe