| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
|
|
|
wild lives Wood Frogs On Ice
Alaska. The name conjures up a land of wild extremes and epic proportions, impenetrable wilderness and harsh seas, savage snowstorms and sunless winters. The animals that thrive in this forbidding landscape are no less impressive: grizzly bears, wolverines, musk oxen, and wood frogs. That’s right. Frogs. In Alaska. Though not as famous as its bigger, fiercer, tourist-attraction neighbors, the wood frog (Rana sylvatica) is in many ways more remarkable because it weathers Alaska’s bitter winters as a frogcicle. For several months of the year, this frog doesn’t breathe. Its heart doesn’t beat. It exhibits no measurable brain activity. And, oh yes, more than half the water in its body freezes into solid ice. By most yardsticks, it’s quite dead. Yet this frog is able to hop back to life after enduring body temperatures as low as ten degrees Fahrenheit. The frog capable of these superamphibian feats resembles wimpy Clark Kent more than a Frog of Steel. A full-grown adult could lounge comfortably on a credit card. Adults sport a dark “bandit” patch across the eyes and mottled brown skin that disappears among the umber hues of rotting leaf litter. But thanks to their subzero survival skills, wood frogs have been able to establish themselves over a wide diagonal swath of North America, extending from the northeast and Great Lakes states, through much of Canada and Alaska, and including isolated populations in Missouri and parts of Colorado and Wyoming. For most animals, major frostbite is tantamount to a death sentence. Ice kills in two ways: through cell destruction and dehydration. Because water expands as it freezes, the sharp edges of growing ice crystals can damage delicate cell membranes beyond repair. And ice forming in extracellular spaces (such as inside blood vessels and inside body cavities) pulls water molecules across the cell membrane, dehydrating the cell to the point of irreversible collapse. But the wood frog manages to avoid these problems thanks to a clever trick of physics and chemistry. As winter approaches, it snuggles beneath the leaf litter that blankets its forest home. Falling temperatures eventually coat the frog’s skin with an icy rime, stimulating its liver to manufacture huge amounts of glucose. The frog’s heart rate doubles, saturating all its vital organs with sugar-rich blood. As winter hardens its grip on the land, the frog’s large body cavities freeze first. Bacteria or dust particles in the fluid-filled abdominal cavity act as nucleators that help speed the growth of ice crystals. Ice spreading through these extracellular spaces sucks water out of the frog’s cells through osmosis. The process partially dehydrates the cells but also leaves behind a concentrated glucose slush that, like antifreeze, prevents the cells from collapsing or freezing solid. This nifty strategy can reduce the water content of organs such as the liver by as much as 25 percent, desiccating them enough to preserve them in cryogenic suspension safe from the ravages of ice. Scientists are studying this skill in hopes of extending the transplant viability of harvested human organs from mere hours to weeks or even years. Freezing one’s innards is so hard on the body that wood frogs seem to have developed a few more physiological adaptations to help withstand the experience. Ken Storey, a biologist at Carleton University in Ottawa, has shown that the blood of hibernating wood frogs contains extraordinarily high levels of the blood-clotting enzyme fibrinogen, which could help stanch any accidental bleeding caused by ice. And the reason wood frogs don’t slip into a deadly diabetic coma from all the glucose antifreeze in their blood may be because the frogs secrete a different form of insulin than other vertebrates. The mutations, Storey speculates, may reduce the enzyme’s ability to lower blood glucose levels during the low temperatures of winter, but could function normally under balmier conditions when the frog is active. How did a frog end up living in a biological niche that demands regular metamorphoses into a block of ice? Storey believes that the frogs developed freeze resistance at the end of the last ice age approximately 15,000 years ago. When the glaciers retreated from what is now the latitude of Tennessee, they left behind expansive lakes and lush valleys—an amphibian version of Paradise. “It looks as if frogs followed the glaciers back because as the ice melted a new niche was created. It was wide-open territory, with lots of water, lots of insects,” Storey says. He believes the frogs’ freeze tolerance amounts to a new application of the species’ pre-existing ability to survive dehydration. “The frogs just used a variation of the dehydration trick—water lost to the air—as a freezing trick—water lost to ice,” Storey says. Ten known vertebrate species can tolerate some level of freezing; all are reptiles or amphibians, and all are found only in North America. Of these, the wood frog ranges the farthest north and can survive with the greatest proportion of its body fluids locked in ice. When spring rolls around, the frog’s internal organs thaw out first. “I’ve had these frogs connected to monitors, and the heart just spontaneously resumes beating,” says Jon Costanzo, a physiologist at Miami University in Ohio. “There are still a lot of questions about what allows this frog to tolerate going without blood, without oxygen...and then he’s thawed out and back to normal.” Freezing over winter even gives wood frogs a jump-start on survival come spring thaw. Because wood frogs winter beneath leaves and snow rather than the frozen fastness of lakes or ponds, they can emerge while their predators are still winging their way north or slumbering in hibernation. But the frog’s adaptation to the cold may work against it in the coming years. “In general, I think wood frogs are going to be one species that will be in fairly serious decline if global warming scenarios come through,” says Mike Lanoo, a professor at the University of Indiana School of Medicine who is working to determine the cause of planetwide amphibian declines. “With global warming, you lose the gradual transitions between spring and summer, fall and winter. Wood frogs are among the first species to breed in the spring and their eggs are vulnerable to being frozen during cold snaps. If global warming predictions are true, you can expect that wood frogs will get whacked fairly regularly.” At present, the species seems to be holding its own in Canada and the continental United States, although the malformations plaguing many frogs around the world have recently been spotted in Alaskan wood frogs as well. On the other hand, the wood frog’s talent for tolerating freezing may ultimately provide a last-ditch method to preserve amphibian species looking over the extinction abyss. In a frogs-helping-frogs scenario, Costanzo has recently been using wood frogs to study how best to preserve amphibian sperm and eggs. But Lanoo hopes that in the end, freezing amphibian gametes won’t be necessary. “At this point, we don’t have to go to the ‘lifeboat’ yet. There is still a chance we can plug the hole in the boat if we have the societal and political will.” Either way, the wood frog’s extraordinary feats have plenty to teach. Pamela Turner is a freelance writer who lives in Orinda, California. |
Spring 2001
|
