Hardy Wood Frogs Can Even Lose Heartbeat for a Time
One Blue Ridge species adapts to cold weather with special “antifreeze.”
CarolinaSmith Getty Images/iStockphoto
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Wood Frog, Rana sylvatica, looking directly at you (macro, isolated, 12MP camera).
Josh 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 black-masked 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 antifreeze 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.”
The story above appears in our January/February 2020 issue. For more like it subscribe today or log in to the digital edition with your active digital subscription. Thank you for your support!