WOOD FROG WINTER
This morning it was 8 degrees when I got up, and that was a big warming trend, 16 degrees toastier than when I turned in. It is a wonder to me that the tender, tiny animals of the forest survive when the temperatures plummet like this, and the snow piles up.
The animal that bring us our earliest spring song amazes me the most: The common wood frog. I marvel at this small animal, not as big as my palm, that looks so delicate and finely made. Yet it is incredibly hardy. In winter, wood frogs hibernate close to the ground’s surface, snugged into the soil and leaf litter. By now, in mid-winter, they are the living dead of the forest. They actually freeze, just about solid. Yet the wood frog survives. As winter comes on, they shut down their life functions, and hunker in their hibernaculum, to tough it out until spring. Even while frozen to a frogberg, glucose in the wood frog’s tissues works like anti-freeze, protecting its vital organs. When the ponds first start to thaw, so do the wood frogs. They are ready to go first thing, all set to mate and start a new cycle of life.
I start listening for this frog when the snow is still on the ground – but the first margins of open water show at the vernal pool at Hemlock Hollow, in the Harvard Forest. This small, shallow pool is hidden deep in a hemlock wood. By late May, bugs skitter on its surface, and the water is dusted with golden pollen. The water level is fullest in late winter and early spring. Perfectly crystal clear, it is like a lens, magnifying the waving green grass-like plants and thick matting of leaves on the bottom. In the rain, rings of falling drops bloom on its still surface, and moss deeply covers the rocks around the pool’s edges. If there are fairies in the Harvard Forest, I feel pretty sure they live here, with the wood frogs for their orchestra.
This pool is about mid-way on a 3-mile loop that John O’Keefe, the field phenologist at the Harvard Forest, has been walking for 25 years. On this walk John monitors the seasonal changes of the tree canopy, as a way of seeing how climate change is altering the ecology and function of the forest. He has been watching the leaf emergence and drop of the same 40 trees on his walk all that time, and clear trends have emerged. Spring on average is earlier, and fall is later – with the first hard frost now coming as much as six weeks later than it used to. The first calls of wood frogs are one of the things he also notes on his walks.
Wood frogs don’t make the classic, high trilling sound of spring peepers. My field notes for a walk with John on April 14, 2014 read: “Some larger frog or toad that sounds almost like a duck? Pool still 2/3 ice, but breeding call has started. First open water of the year.” Unlike tree frogs, or spring peepers, that carry on for weeks, wood frogs make quick work of their mating time in the gelid water. This seasonal pool will dry up entirely most summers, as the trees draw down the water. That’s a boon, actually to these frogs that by then have migrated back to the woods. Their young have hatched and metamorphosed from aquatic, vegetarian tadpoles to breathing, walking, carnivorous young of the year. Creatures of the forest as much as this pool, wood frogs return each year to this same pool of their birth. The water regime of the pool is critical to their survival. Too much water, and the pool would sustain fish that would gobble the tadpoles in their aquatic phase of life. Dry up too soon, and the tadpoles would not have adequate time to grow the legs and lungs they need to survive. Evolved perfectly to their place and time, their lives hang in a delicate balance of conditions across a varied – and changing – landscape, in a short window of seasonal time.
I walked that same trail a few weeks later, in early June, and encountered an adult wood frog. There it sat, by a deliciously fragrant, rotting log that was eared all over with turkey tail mushrooms, and clotted with moss. There was a rich carpet underfoot of brown curled oak leaves, left over from the previous year, and not yet rotted out. Crumbly and sweet scented of brown, rich earth, it was just the place for, well, a wood frog. It was the frog’s motion that caught my eye, a bit of a hop as I surprised it, walking by. I froze, and stooped down to have a good look, and found to my delight that the frog was holding still. I slowly lay down flat, to get on its level.
Up close, it was exquisite, with tiny nostrils, and gigantic, wise eyes, with their deep, amphibian stare. Its skin was tinted in many more shades than just brown, there was a hint of rosiness to its back, and there were dark speckles, too, mimicking the rot on the oak leaves around it. Its toes were deep in the leaf litter, and its back legs were folded, ready to hop, the delicate bones of its hips poking up the smooth plane of its back. The frog’s throat was white and smooth, marked with a bit of brown speckling that traveled in a bandit mask to its eye.
Wood frog seems like such as simple name for such a complex, ancient animal, extant since the time of the dinosaurs, and navigating its environment with such four-season finesse. Would the future be as kind to it as the present? Would its needs, so modest really, still be met in a changing world? Already precipitation in the forest had increased since the 1990s, and temperature too. What would the future mean for this frog? How much longer would its weird quacking call be the harbinger of spring in this forest, as the timing and nature of spring continue to change?
Climate change is a global phenomenon, but also entirely local, as real and tangible as the frogs I listen for in this pond, timed so sensitively to the year’s seasonal changes. I marvel that they can survive these winters. But will they be able to survive our rewind of the seasonal clock?