Fleischmann's glass frog hides its blood to look see-through

When a Fleischmann’s glass frog falls asleep on a leaf, it quietly parks about 89 percent of its red blood cells in its liver. That is the finding of a 2022 study in Science led by Carlos Taboada and Jesse Delia, and it solves a puzzle. The frog’s belly is see-through, but blood is not.

Red blood cells absorb green light. Duke University’s news release explains the problem: circulating blood would stand out against green leaves, however clear the muscles around it might be. So a glass frog that wants to vanish into a leaf needs clear tissue and a place to put its blood. The liver, wrapped in a mirror-like layer, does the job.

This article covers what the see-through belly shows, how the liver trick works, and how glass frogs are thought to hide while they rest. Some of the camouflage evidence comes from other glass frogs, and we flag it each time, because a result from one species does not automatically apply to another.

Meet Fleischmann’s glass frog

The Animal Diversity Web calls this animal the northern glassfrog. It lives from southern Mexico through most of Central America into northern South America, from near sea level to over 1,600 meters (about 5,250 feet), along clear, flowing, usually rocky streams. Its pale lime-green back has tiny dark spots and small yellow ones, its eyes are gold, and its belly skin is clear enough that the page says you can see the heart pulse.

The fathers deserve a line. Per the Animal Diversity Web, guarding males rotate over the eggs and even urinate on them to prevent drying, and a 2014 field study near San Gabriel Mixtepec, in Oaxaca, Mexico, found that brooding fathers are the eggs’ main source of moisture. For a different take on frog fatherhood, see the Darwin’s frog.

A see-through belly, and why blood is the problem

The Science paper reports that these tissues, the muscles and the belly skin, let through more than 90 to 95 percent of visible light while still working normally. That transparency is camouflage: the authors say it hides the frogs from predators while they sleep on vegetation during the day. The figure is for the tissues themselves. The paper adds that circulating red blood cells could still compromise the leaf camouflage, which sets up the problem below.

Clear tissue is only half the trick. Vertebrate blood is full of red blood cells carrying hemoglobin, which absorbs green light strongly. Transparent icefish and larval eels have apparently avoided the issue by not making hemoglobin or red blood cells, according to the paper. A glass frog is a normal, red-blooded vertebrate, so it needs another solution.

One caution: even the glass frog is not perfectly clear. A 2020 study in PNAS by James Barnett and colleagues, on two other glass frog species, describes the frogs as translucent rather than fully transparent, because their backs carry some pigment.

The liver trick

The Taboada team measured how much light passed through 11 frogs in different states: asleep, awake, calling to mates, after exercise and under anesthesia. Sleeping frogs let through on average 34 to 61 percent more light than frogs in any other state. A second set of tests on 13 individuals tied the change to hemoglobin: the signal was strong in exercised frogs and barely detectable in sleeping ones.

To find where the blood went, the team used photoacoustic imaging, a method in which a laser makes red blood cells give off ultrasound that can be mapped, with no dye needed. Resting glassfrogs removed about 89 percent of their red blood cells from circulation and packed them into the liver. Duke’s release says “nearly 90 percent,” which matches. In the paper, that shift raised transparency two- to threefold.

Picture a shop that clears the front window and stacks everything in a mirrored back room. Why does the liver not give the game away? The paper says the heart, liver and digestive organs sit inside mirrored sacs that contain reflective guanine crystals and block over 80 percent of incident visible light. The packed liver swelled by about 40 percent in ten frogs measured by ultrasound. And when the frog wakes and moves, the cells flow back into the blood.

Here is the cost. At rest, just 3.4 percent of the frog’s hemoglobin was carrying oxygen, which the authors say suggests low or non-oxygen-based metabolism. The authors also flag an open question: the frogs pack and unpack a huge share of their red blood cells each day without clotting, and the paper does not explain how.

How a resting glass frog hides

Blood management explains the belly. The edges are a separate story, and the evidence comes from other species. Barnett and colleagues studied Teratohyla midas and Espadarana prosoblepon, not Fleischmann’s glass frog. They found that the frogs’ legs shift in brightness toward the background, and they propose “edge diffusion”: translucent legs tucked against the body soften the outline so the frog fades into the leaf instead of ending sharply.

The field test used green gelatin models in Ecuador, left on vegetation for 72 hours. Translucent models were attacked less than opaque ones. The authors list their limits: the models were gelatin, not frogs, the predators were mostly inferred from beak marks and probably birds, and some tests used human viewers. So the picture is promising, not final.

What this looks like in practice

For example, a male calling for mates is less see-through than a sleeping one, because the study counts calling among the states in which less light gets through. The trick is for sleep.

Common mistakes and myths about glass frogs

Myth: a glass frog is invisible. The back is pale green and pigmented, and even the belly is clear only when conditions are right. Barnett’s group calls these frogs translucent.

Mix-up: clear skin means clear blood. The blood is the hard part, which is why the liver trick exists.

Mistake: the frog is see-through all day and night. The authors say activity and stress easily disrupt blood flow and make the frogs opaque. In the study, only sleeping frogs reached the high transparency.

Mix-up: one result fits every glass frog. The blood experiments focused on this species. The leg and edge results come from two others. The authors compared opaque tropical frogs for context, and those did not hide blood the same way.

Mix-up: every small tropical frog hides the same way. The golden poison frog wears loud colors that, according to the Animal Diversity Web, warn predators. The glass frog bets on disappearing instead.

Conservation status

Fleischmann’s glass frog is listed as Least Concern on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Least Concern means the species was evaluated and does not meet the criteria for a threatened or near-threatened category.

That does not mean it has no problems. The Animal Diversity Web says the frog has likely lost habitat to deforestation, though precise population statistics are unavailable. The same page names heavy predation on eggs and estimates that 80 percent of clutches are destroyed before hatching, a figure we could not confirm in a second source. We have no confirmed population trend, because none of our sources states one.

Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the assessment, so we do not state one.