MANISTEE COUNTY – The most miraculous transformation in the life of the wood frog does not take place in the summer, when they change from tadpoles to frogs, but in the winter.
As northern Michigan begins to freeze, so do the wood frogs. These frogs stop moving and breathing; their hearts stop beating and they lose all brain activity in winter, according to biochemical research at Carleton University in Ottawa.
Despite all the features characteristic of death, these amphibians are still alive. When the ice begins to thaw, so does the frog.
Some scientists suspect that its remarkable biology could one day provide medical breakthroughs and even hold the key to cryonic discoveries.
How it works
While many frogs seem to “freeze” in winter, the wood frog takes it to another level.
Most frogs survive northern winters by hibernating, partially submerged in mud and water. Frogs are cold-blooded animals, so their body temperature is usually about the same as the surrounding air. Although they can be cold and dormant, unlike the wood frog, the body temperature of other amphibians never drops below freezing. This is reported by the National Parks Service.
Wood frogs hibernate on the forest floor instead. While this may provide some insulation from extreme cold, the frogs are not as protected from sub-zero temperatures as those that remain in the water, can be found on the website of the park service.
When the temperature drops below freezing, the frog’s metabolism grinds to a halt so its cells can survive despite minimal energy and oxygen.
Wooden frog
Lithobates sylvaticus
Conservation status: Least Concern
Mass: 0.28 oz
Service life: three years
Food: Insects, worms, arachnids and snails
Habitat: Usually found around spring pools in wooded areas of eastern North America and as far north as Alaska.
The frog’s blood contains special proteins that ensure that the water first freezes on the outside of the cells. Ice fills the wood frog’s abdominal cavity and encases its internal organs, according to Boris Rubinsky, a bioengineering researcher at the University of California at Berkeley.
In most species, freezing in this way would lead to desiccation of the cells and organ failure, as crystallizing ice draws water from the animal’s cells, Rubinsky mentioned in an article on PBS.org.
However, the wood frog’s liver responds to this process by producing large amounts of glucose, increasing its concentrations in the bloodstream to more than 50 times that of a human with diabetes, Rubinsky said.
This glucose acts as a kind of antifreeze, similar to ethylene glycol, a sugar alcohol used for commercial antifreeze.
This natural sugar is pushed through the frog’s body to prevent extra water from solidifying in the cells, preventing dehydration and the chance of freezing to death, Rubinsky said.
When the warmer weather arrives in spring, the wood frog thaws from the inside out. The heart restarts first, followed by brain activity, and eventually the rest of its motor functions return intact, the National Parks Service website states.
This specialized adaptation means that wood frogs are the only frogs that can live north of the Arctic Circle, according to the National Wildlife Federation. In Alaska, where winters are longer, these frogs have been known to revive after seven months, is in an article on nps.gov.
Wood frogs and medicines
Scientists hope to copy this frog’s survival technique for various medical applications, including preserving organs during transplantation.
An experiment from 1999 conducted by Rubinsky at the University of California used cryoprotective chemicals similar to those found in wood frogs to revive rat livers for transplant. The results showed that the livers were still partially functional at transplantation.
Similar experiments by Israeli researchers in 2003 showed that frozen rat hearts can remain viable for more than an hour after thawing.
Researchers like Rubinsky suspect cryopreserved organs could revolutionize the transplant process and one day save lives.
“Inspired by the way wood frogs survive freezing, my research team developed a cryopreservation protocol for mammalian liver,” Rubinsky explained in a column for NOVA. “We successfully preserved a liver in a frozen state and then transplanted it into an animal that survived. This was the first time this had been done with an organ, and we are now successfully cryopreserving organs for hours to days.”
