How Glass Frogs Weave the World’s Best Invisibility Cloak
The amphibians pull off a death-defying magic trick to stay hidden while they sleep.
Glass frogs do not live a life of modesty. With their semitransparent skin—green on the back, clear on the belly—the tree-dwelling, gummy-bear-size amphibians, which are native to the tropics of Central and South America, have little choice but to put their organs on display. Gaze up at certain species from below, and you’ll be treated to an aquarium of innards: a beating heart, a matrix of bones, the shimmering silhouette of the gut.
The frog’s see-through stomach is an ingenious ruse. It turns the animal’s underside into a living, light-transmitting window, camouflaging the creature from skyward-gazing birds and snakes. There’s just one problem with the frog’s otherwise convincingly ghostly garb: the latticework of bright-red blood vessels laced throughout its tissues. It’s an especially big issue in the daytime, when the frogs are asleep amid the leaves. As sunlight filters through the trees, casting shadows off whatever it hits below, the frogs’ own blood threatens to betray them.
To patch the holes in their invisibility cloak, glass frogs deploy a radical option. In the hour or so before they drift off to sleep, roughly 90 percent of their blood cells march into their pea-size liver. The rest of the animal’s body plunges into an oxygen-starved state, risking damage to delicate organs. This grants the frog the temporary gift of imperceptibility—all because “they’re basically able to hide their blood” every day for about 12 hours at a time, says Carlos Taboada, a frog biologist at Duke University who co-led the discovery.
The transparency tactic solves glass frogs’ greatest dilemma: vanishing from view on land. Several animals have managed the feat in water, where it’s relatively easy for fluid-filled bodies to blend in. But when air is the backdrop, animals have to maintain clear outsides and insides—a triumph that, to scientists’ knowledge, only glass frogs have managed, among terrestrial beasts, says Richard White, a cancer biologist at the University of Oxford who wrote a commentary on the new find. The skin part is “pretty easy,” White told me: Just get rid of pigments such as melanin, which absorb and reflect light. Blood, though, presents a conundrum. Its opacity comes from hemoglobin, a protein necessary for ferrying oxygen throughout the body; the frogs can no more rid themselves of it than they can jettison their need to breathe.
So instead, they move the light-absorbing hemoglobin around. Jesse Delia, a frog biologist at the American Museum of Natural History, first clued in on the phenomenon a few years ago, when he spotted a glass frog in Panama catching daytime zzz’s with most of its body in a shockingly bloodless state. “I remember thinking, This is crazy,” Delia told me. He partnered with Taboada and a team of other scientists, including Duke’s Junjie Yao, to suss out how the frogs were pulling it all off.
By beaming lasers at the frogs, the scientists were able to track the movements of individual blood cells as the animals fell asleep and then woke for their nocturnal jaunts. The team found that as the frogs hop around dreamland, their vasculature fills almost entirely with plasma—colorless, save for a gauzy bluish tint—interspersed with just a few red cells, turning their body two or three times as transparent as it is while the animals are awake. Even the blood-cell-filled liver plays its own deception game: The organ’s outside is coated with a film of tiny, reflective crystals, which essentially conceal the redness behind a veil of white.
The frogs’ feint might seem like overkill. Other arboreal amphibians can conceal themselves just by mimicking the emerald hue of leaves. But glass frogs could have a hard-to-see leg up. “It’s very clear when frogs are sleeping on a leaf,” says Becca Brunner, a biologist who studies glass frogs. “Their silhouettes are picture-perfect”—an ideal cue for a predator. Slumbering glass frogs, though, generate no such outline. “You just see two little blobs: the heart and the liver,” Brunner told me, “which could be anything.”
How the frogs’ tissues endure their bizarrely bloodless state for hours at a time is still a mystery. “If I took 89 percent of your blood and put it in your liver at night, you’d probably be dead by morning,” White told me. Nor do scientists understand how the liver handles the daily influx. Jamming so many red blood cells into such a small space should trigger catastrophic clotting, but the frogs get by just fine. They also seem to recover remarkably quickly, redispersing the blood cells within seconds of waking. Taboada and his colleagues don’t yet know how the frogs execute their death-defying magic tricks, but they could find hints elsewhere in the animal kingdom: Aline Ingelson-Filpula, a biologist at Carleton University, told me that many other creatures have to deal with similar challenges when they enter and exit torpor, in order to survive extreme cold or starvation.
The maneuver almost certainly has costs. Although glass frogs seem able to rapidly refill their vessels come nighttime, their muscles may take a while to recalibrate—leaving them potentially discombobulated or stiff in the joints. “What price would they pay if a predator finds them?” says Gerlinde Höbel, a frog biologist at the University of Wisconsin at Milwaukee. Any animal that sees through the gimmick might find itself rewarded with a sluggish, snaggable snack.
Still, Brunner likes the frogs’ chances. They may not be great escape artists, but they’re masters of disguise. Brunner has seen the frogs, when they’re heading to bed, flatten their body against their leafy mattress, tuck in their legs, and pull their eyes into their head. “They’re like a sort of bump,” she told me. In her 10 years of studying glass frogs, Brunner has spotted just one snoozing. Had she not been looking closely, it might have eluded her notice entirely—just another green lump in the forest, or perhaps a trick of the light.