The Backyard Naturalist – Frozen Frogs
In fall, when the days were still warm but the nights were sometimes freezing, I was orchestrating daily migrations of our pepper plants and some tender herbs into the kitchen at dusk, and then back out to the yard in the morning, trying to wring a few days’ more growth out of these sensitive ones. I left the rest of the potted plants to fend for themselves — either die for good when the frost hit hard, or gather themselves down and wait for spring. I also set up a micropond: an enamelware lobster pot that I filled with water and embellished with some branches and water plants. If you build even such a tiny pond, frogs will come, and they did. One of them liked to sit on a piece of wood just at the waterline, and this frog I called Frog-In-A-Pot. There were other frogs in there, but I didn’t deign to name them, though they were at least lowercase frogs in a pot, too.
As the fall wore on, I stopped seeing the frogs. They didn’t sit out to sun themselves anymore. I assumed they’d wisely moved out of the pot. When the frosts came and stayed, I dumped out the pot, spilling a thick disk of ice studded with pondweed, some branches, and two frogs, now no longer in a pot. They were grayish, bloated, lolling, unresponsive. I thought about pitching their bodies into the compost, but I remembered what I learned in vet school about cold water drownings and hypothermia patients: “Not dead until warm and dead.” The day was reasonably warm, so I left the frogs on a bed of brown leaves and did some other yard chores. As the day progressed, they changed. The clouded cataracts over their eyes cleared; their gray skin showed hints of green; their bloating reversed and they de-puffed. When I poked them, they moved their legs slightly. By the time I was done with my work, they were sitting upright, though not yet mobile. I could pick them up. I brought them both to my big pond, dug to a depth of about 2 feet, and set them at the edge. In that pond they’d have a chance at surviving the winter.
These were bullfrogs, and they aren’t famous for their survival strategy like some other land-wintering frogs. Wood frogs accumulate sugars and other compounds in their cells, and these molecules disrupt ice crystal formation, so the fluid in the cells remains fluid and the cells don’t burst from expanding ice. In the spaces between the cells, water within the body freezes solid. Metabolism slows, the heart stops. In spring, they thaw out, and within a matter of hours can hop away. Bullfrogs don’t dazzle with such a Lazarus routine. They hunker. They sit at the bottom of ponds, not burrowed into the mud, but on top of it. They slow, but don’t stop their bodily machinery. Very cold water holds high oxygen levels as long as the water isn’t stagnant and the ice isn’t too thick. They dwell there, oxygen diffusing directly through their permeable skin.
My frogs in the pot may have suffered from a lack of oxygen when the water froze solid over their heads. Bullfrogs don’t hibernate so deeply that they would spill out on the ground utterly unresponsive like that. They winter just sitting, awake, at pond bottom — even swim around a bit from time to time. My frogs in a pot were likely on their way to dying when I found them. As I sat by the bigger pond with one of them still in my hands, I grinned and said to the frog, “Like death warmed over.”