Frogs burn ample fuel while freezing solid

Wood frogs freeze so solidly that you could "use one as a doorknocker or something like that," says researcher Brent Sinclair. (EmilyK/Wikimedia Commons)

Frogs that cope with winter weather by freezing into a solid lump may be hit hard by climate change, Canadian researchers have found.

Wood frogs, which hide in the leaf litter of forests through much of North America, have the amazing ability to freeze their bodies into hard, icy little pucks when the temperature drops below 0 C.

"You could use one as a doorknocker or something like that," said biologist Brent Sinclair in an interview with Quirks & Quarks that airs Saturday on CBC Radio One.

The frogs thaw out and hop away when it warms up again.

While freezing looks as simple as a matter of keeping still as the temperature plunges, Sinclair, a professor at Western University in London, Ont., and colleagues including at Carleton University in Ottawa discovered that it's not as easy as it looks. In fact, freezing and thawing burns a surprisingly large amount of energy, the researchers reported in a recent issue of the Journal of Experimental Biology.

The researchers measured the frogs' energy consumption in the lab during freezing and thawing.

While other cold-blooded animals slow down their metabolism consistently as temperatures drop, wood frogs suddenly increase their energy consumption when temperatures drop to about a degree above zero, the researchers found. The frogs' metabolism increased suddenly again when they completely thawed out after being frozen, just before they start to breathe again.

By putting temperature sensors in the ground in an Ottawa-area forest during winter, the researchers discovered that the frogs freeze and thaw 15 to 25 times per winter, and calculated that the process consumes a large portion of the energy that a frog burns over the winter.

The problem is that the energy molecule used to fuel the freezing and thawing — glycogen — is the same one used by the frog to manufacture the glucose antifreeze that protects its cells from frostbite while the spaces between them are filled with ice. If the glycogen is depleted, the frog won't have enough antifreeze to survive the next freeze-thaw cycle.

Sinclair said that means the frog may be vulnerable to changes in snow cover and snowfall that are expected as the climate changes.

In areas where there is less snow cover, the frogs may go through too many freeze and thaw cycles to survive the winter, he added.

"We may see some counterintuitive patterns where places that become warmer actually become less habitable for the frogs because of the number of times they freeze and thaw during the winter."

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