Frozen Frogs
and other tales of the connection between human health and biodiversity
In a new book from Oxford University Press, two Harvard physicians look at one of the least-talked about casualties of environmental destruction: medical research. In addition to describing imperiled ecosystems that may hold solutions for human health problems, they delve into fascinating examples—ranging from “denning” bears that recycle, instead of excrete, waste and could serve as models in kidney failure research to a sea slug yielding a compound now in tests for cancer treatments. Here’s an excerpt about the extraordinary wood frog.
For centuries, people have dreamed of being frozen so that they might be thawed at some point in the future. Some have even paid high prices to be stored post mortem in this way, hoping that scientists will someday figure out how to bring them back to life. While frogs cannot return to being alive after dying, frozen or not, at least five frog species—the wood frog (Rana sylvatica), gray tree frog (Hyla versicolor), spring peeper (Pseudacris crucifer), chorus frog (Pseudacris triseriata), and Cope’s tree frog (Hyla chrysoscelis)—can survive after being frozen solid. In The case of a wood frog, upon its first exposure to ice in the fall, it undergoes a remarkable transition, worthy of a science fiction novel, to a dormant state in which its heart ceases to beat for up to several weeks and the water that surrounds its cells turns to ice. The contact with ice first sets off a modified fight-or-flight response (the body’s way of preparing for acute stress, which includes an increased heart rate, dilated pupils, and mobilized energy stores) that yields an enormous outpouring of sugar into its bloodstream (as much as 4,500 milligrams per deciliter has been recorded—more than ten times the level needed to diagnose diabetes mellitus in humans, and more than enough to kill us) in addition to other substances that together act as antifreeze. These substances are taken up by the wood frog’s cells, while at the same time proteins are released into its blood that promote the formation of ice. Thus, the cells are protected—if ice crystals were to form inside the cells rather than in the extracellular spaces, they would be torn apart. Come spring, wood frogs reverse the process, though they do so from the inside out. In a somewhat inconceivable turn of events, despite warmer temperatures outside, they manage to thaw their brains and hearts first. The frog’s ability to survive freezing has drawn the attention of many, including those involved in organ transplantation, who are trying to apply some of what the frogs do to prolong the viability of organs for eventual transplantation. ❧
From: Sustaining Life: How Human Health Depends on Biodiversity
Edited by Eric Chivian and Aaron Bernstein, Oxford University Press, 2008
