Glass frogs do not live a life of modesty. Semitransparent skin on their backs and clear belly, the tree-dwelling amphibians of the Central and South American tropics have no choice but to display their organs. If you look at specific species below, then you will be treated to an aquarium full of their innards. A beating heart, a matrix made of bones and the glowing silhouette of the gut are just a few of the many things that await.
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. This is a problem in the morning when the frogs sleep among the leaves. The frogs are at risk of being betrayed by the sunlight that filters through trees and casts shadows on everything it touches below. To patch their invisible cloaks, the glass frogs use a novel strategy. In the hour or so before they drift off to sleep, roughly 90 percent of their blood cells march into their pea-size liver. Rest of the animal’s body is forced into an oxygen-deprived state. This can cause damage to fragile 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. White said that the skin is easy. He suggested: Get rid of melanin and other pigments, as they absorb 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 z’s with most of its body in a shockingly bloodless state. “I remember thinking, This is crazy,” Delia told me. Taboada teamed up with Junjie Yao from Duke to discover how the frogs did it.
Scientists were able track individual blood cell movements as the animals went to sleep and awoke the next morning for their nighttime adventures by beaming lasers at them. Their body becomes transparent twice as fast as when they are awake, according to the team. 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 might think that their ploy is too clever. Others arboreal amphibians are able to hide themselves by imitating the green hues of their leaves. Glass frogs may have an advantage that is difficult to see. Becca Brunner is a glass frog biologist. “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. The frogs also appear to be able to quickly recover, redistributing blood cells in a matter of seconds after waking up. 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.
This maneuver is almost certain to have 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 Hobel, 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. Although they may not be masters at disguise, they are great escape artists. 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. She described them as “a kind of bump”. In her 10 years of studying glass frogs, Brunner has spotted just one snoozing. Brunner might not have noticed it if she hadn’t been paying attention. It could be a green lump or a trick of light.
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