John Burroughs, the nineteenth century American writer, was a keen observer of nature. In his book Signs and Seasons (1886), he wrote about an encounter with a frog during the winter of 1884-1885. While walking through the woods on a warm December day he heard a frog calling from under the leaf litter. He found the frog, uncovered and observed it, then returned it to its hibernaculum. He returned on warm day in March after the snow had melted and found (assumedly) the same frog tucked down in amongst the still-frozen leaves. Several days later Burroughs again checked on the frog, this time finding it resting above the leaves soaking in the spring warmth.
I was reminded of Burroughs’ observations recently when I was asked “How do frogs hibernate without freezing to death? They cannot possibly dig down to below the frost line?”
Many people believe that frogs hibernate in the muck at the bottom of a lake or marsh. It might surprise most people, but much of the oxygen frogs get is not from breathing; it is absorbed through their skin. So if a frog dug into the oxygen deficient mud at the bottom of a body of water, they would suffocate. However, some frogs may simply lie on the lake bottom where they are still exposed to the oxygen in the water.
Frogs will generally spend their winter hibernating in one of three places: at the bottom of a body of water, amongst the vegetation in a wetland, or somewhere under the leaf litter in the forest. Some frogs that hibernate on land are freeze tolerant. These frogs include the wood frog, grey tree frog, chorus frog, and spring peeper, all North Dakota natives with the exception of the spring peeper. These frogs use glucose as an antifreeze to protect their vital organs.
The specific mechanisms, however, are not well known. The frog then relies on an imperceptible rate of anaerobic respiration to get through the cold winter.
Some of these frogs may appear to be frozen solid, and they basically are! But there is a catch to this. Although ice crystals form in the body cavity and between cells, they do not form inside cells, which kill the cells by ripping them apart. Sixty-five percent of their body water may be in the form of ice. These frogs are known to tolerate temperatures down to about 18 degrees Fahrenheit.
Some toads are good diggers, and can dig to below the frost line. Manitoba toads hibernating near Waubun, Minnesota have been found to burrow into gopher mounds to hibernate. The toads are able to stay just below the frost line by digging deeper into the ground as the soil temperature dropped. The toads kept just below the frost line, and ended up digging down four feet, staying in soil that was a degree or two above freezing.
Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.