Blue Ridge Country - January/February 2020 - 7

C R E AT U R E F E AT U R E

FROZEN FROGS

Hardy Wood Frogs Can Even Lose
Heartbeat for a Time
One Blue Ridge species adapts to cold weather with special "antifreeze."
By NANCY HENDERSON

osh Ennen's early fascination with wood
frogs grew even stronger during his senior
thesis project at Maryville College in east Tennessee, when he spent several months in the early
2000s recording the amphibians at the edge of Cades Cove and listening to cassette tape soundbites
to see how temperature and rain affected the timing of their mating calls.
"In these ephemeral ponds, there'd be thousands of
frogs calling in late January into February, maybe early
March," says Ennen, an aquatic conservation biologist at
the Tennessee Aquarium Conservation Institute in Chattanooga. "I would go out there and the ponds would be
just full of ice, solid ice, and the frogs would be waiting
until it'd be warm enough for it to thaw out so they could
get back in and call and breed. It was a really cool system."
One of the widest-ranging frogs in the world, the
wood species is found from north Alabama to Alaska and
the Canadian tundra.
"They basically hopped and followed the glaciers
as they retreated north at the end of the last ice age,"
Ennen says. In the southern Appalachians, the blackmasked frogs sometimes show up in water-filled tire
ruts and drainage ditches but prefer pristine ephemeral
ponds-fishless, seasonal pools that fill with rain-in
forested areas.
One of the most intriguing things about wood frogs is
that they can temporarily freeze solid if the weather gets
cold enough, with their bodies producing a sort of anti-

freeze from sugar and waste that helps them withstand
frigid temperatures.
"This is similar to throwing salt on your
driveway," Ennen says. "It lowers the freezing point of water, so the temperature outside
would need to be well below freezing to form ice
on your driveway. The antifreeze is pumped inside the cells, which prevents ice formation
and the rupturing of the cell membrane."
Wood frogs seek out leaf litter, usually under logs, before freezing. "If they're in a pond,
they're thawed out, looking for a girlfriend," says Ennen.
In the Blue Ridge area, where in many spots the temperature seldom dips below 32 degrees, most frogs don't
freeze solid, using their antifreeze instead to lower their
temperatures as they hunker down for the season. But at
higher elevations, such as those in West Virginia, and especially if it's extremely cold for a prolonged period, the
species may freeze solid, which temporarily shuts down
their breathing and heartbeat. This is different from hibernation, in which bears and other mammals are sustained by their own body heat.
Another thing that sets wood frogs apart is that, unlike most amphibians which prefer to mate in warmer
months, they are actually quite cryptic in summer but
"explosive breeders" in winter. "It's their strategy," Ennen
says. "If you're the first in the pond, your offspring will
have no heavy competitors because the only other things
that will be there are salamander species. There aren't that
many predators out.
"Take a hike in February and March and you might be
able to hear hundreds, if not thousands, of frogs calling,"
he adds. "It sounds like a bunch of clucking chickens. No
other frog is going to make that sound in the dead of winter. It's pretty phenomenal how hardy these species are."
If you happen to run across a frozen wood frog, Ennen
advises, "Cover it back up. These are winter-adapted animals and they know what they're doing."

JANUARY/FEBRUARY 2020

Blue Ridge Country - January/February 2020

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