Glass frogs become much harder to see while they sleep, and they do it by getting their blood out of the way. While resting, the frogs pull roughly 89 per cent of their red blood cells out of circulation and pack them into the liver, which makes them two to three times more transparent. When they wake and need to move, the cells return to circulation and the frogs become less transparent again.
The mechanism was worked out by Carlos Taboada, Jesse Delia and colleagues at the American Museum of Natural History and Duke University, reported in Science in December 2022.
What they actually do
The frog’s tissues are the easy part. The muscles and skin of a glass frog are already translucent, which is why their undersides look glassy. The obstacle to true transparency is blood. Red blood cells carry haemoglobin, which absorbs light strongly and is what keeps most animals opaque, so an animal trying to disappear has to deal with its own circulation.
The glass frog’s answer is to hide the cells rather than lose them. During the day, when the nocturnal frogs rest motionless on the undersides of leaves, almost all of their red blood cells are crowded into the liver, an organ lined with reflective crystals that mask the red colour. As Science’s coverage of the study reported, the liver swells by about 40 per cent with the stored cells, and the heart is left pumping a nearly clear fluid. Other frogs can shift some red blood cells out of circulation, but glass frogs do it at an unusually extreme scale, and that scale is what makes the difference between translucent and close to invisible.
It is not total invisibility. The internal organs still show as a faint shadow, and the effect is a large increase in transparency rather than a vanishing. On a green leaf, against a predator’s eye, that is enough.
How they caught it happening
The finding was hard to make for a simple reason: the trick only works while the frog is genuinely asleep. An awake, stressed or anaesthetised glass frog fills its circulation with red blood cells and turns opaque, which means any ordinary attempt to examine the mechanism destroys it.
The team got around this with photoacoustic imaging, a technique borrowed from biomedical engineering. A safe laser pulse is absorbed by the red blood cells, which warm very slightly and give off ultrasound, and those sound waves are used to map where the cells are. Because it requires no dyes, no injections and no handling, it could track the blood in a frog sleeping undisturbed in a dish, which is the only state in which there is anything to see.
The part that is genuinely puzzling
The transparency is the headline, but the open question is what happens to the blood while it is parked. Packing close to 90 per cent of the body’s red blood cells into one organ, holding them there for hours, and then releasing them, is the kind of thing that in a human would tend to produce dangerous clots. The frogs do it twice a day without apparent harm.
How they avoid clotting is not known. That is precisely why the researchers have pointed to glass frogs as a possible model for studying clotting and vascular problems, since an animal that routinely does something our circulation cannot is a natural place to look for the mechanism. This is an opening for research rather than a result with any medical application attached, and it is worth keeping the two apart.
What is settled, and what is not
The settled part is the behaviour. Glass frogs control their own transparency by moving red blood cells in and out of a reflective liver, on a daily cycle tied to rest and activity, and the imaging shows it clearly.
The unsettled part is the physiology underneath it: how the blood is concentrated and stored without clotting, and how the frog manages its oxygen supply while most of its red blood cells are out of circulation. Those are the questions the work has opened, and they are the reason a small translucent frog has become a subject of interest well beyond the study of camouflage.
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