These turtles freeze, but that's okay

Science News, Dec 10, 1988

These turtles freeze, but that's okay

Residents of northern climes will note that few reptiles share their environment. Those that do generally take elaborate measures to deal with frigid temperatures -- such as hibernating underwater or in deep underground dens. But the young painted turtle has evolved a simpler strategy: It freezes.

Researchers removed 13 hatchlings from their winter nests and transported them in a bed of sphagnum moss to nearby Carleton University in Ottawa, Ontario. There, they froze a box of four hatchlings to 24.8[deg.]F, and two more hatchlings to 12[deg.]F. After 24 hours, all hatchlings were thawed back to 32.6[deg.]F. The entire 24[deg.]F batch survived, even though on average 52 percent of the body water in each froze solid. Just one turtle frozen to the lower temperature survived. Survival seems to depend on how much body water freezes: Anything much above 54 percent appears lethal, the researchers say.

Freezing doubled blood levels of glucose, tripled liver glucose levels, tripled blood glycerol levels and increased amino acid levels in blood 2.25-fold over those of nonfrozen hatchlings taken fromt he same nests. The researchers suspect that glucose and glycerol, which can limit the amount of freezing, helped the frozen animals survive. Taurine, an amino acid that appears to limit freezing in bivalves (SN: 7/4/87, p.9), also may have played a role, they say. However, high levels of these chemicals apparently don't entirely explain the animals' survival, the researchers add, since painted turtles observed after spring freezes have survived "despite low levels of cryoprotectants."

While four frog species share a similar tolerance to freezing, the painted turtle is the only reptile and "the highest vertebrate life form known to tolerate the natural freezing of extracellular body fluids during winter hibernation," write Kenneth B. Storey and his co-workers in the November 1988 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol. 85, No. 22). An understanding of how these animals survive could suggest techniques for cryopreserving human organs (SN: 8/29/87, p.138).