With the milder weather this year, wood frogs (Lithobates sylvaticus) have been heard calling on warm, wet days here at Bernheim Forest. Wood frog eggs have also been found in many of the ponds in the forest.
Adult wood frogs live in woodlands and breed in vernal ponds (temporary wetlands that fill from snow or rain), and one of the first frogs to breed in late winter. Wood frogs are the only frog found north of the Arctic Circle, because they have the ability to completely freeze in winter. During this freezing, they stop breathing and their heartbeat stops. They produce an antifreeze substance that prevents their cells from freezing, which would normally be deadly to them. When the weather warms, they thaw out and will move to their breeding ponds.
Males will begin calling to the females to begin the breeding season. Because the wood frogs breed so early, the risk is high that the ponds will freeze again when the eggs are in the ponds. However, their eggs are adapted for this fluctuating temperatures. The mucoprotein jelly around the eggs have a melting period higher than the fluids found in the eggs. So when the ponds freeze, the jelly will freeze before the egg and will pull water out from the egg. These dehydrated eggs are more resistant to freeze and are more able to survive the fluctuating temperatures in late winter and early spring.
So when you’re enjoying a hike in the natural areas on a warm, wet day, listen closely and you might hear the wood frogs.