Winter walks can yield odd – and quite amazing – surprises. For example, how about seeing large tadpoles wriggling just under the ice at Bear Creek Nature Park’s playground pond? My husband and I couldn’t believe what we were seeing at first and neither could members of the birding group last Wednesday – but there they were.
About a half dozen of them cork-screwed up to the surface, snatched tiny bits of green Water Meal (Wolffia columbiana) floating within a hole in the ice and quickly wriggled back into the depths. Tadpoles in the dead of winter? A first for us and for many of you readers too, I imagine!
After doing a little research, I discovered that this is not as strange as it first appeared. Evidently, Green Frogs (Rana clamitans) lay their eggs from April until autumn. So some tadpoles hatch from their eggs late in the year and overwinter under the ice. It’s not easy. The water under the ice is low in oxygen since no air reaches the surface, and currents can’t mix oxygen into the still water.
When ice forms on a pond, the adult Green Frogs, which is the most common frog at in the Playground Pond, spend the winter resting on the mud below the pond. During hibernation, they can absorb sufficient oxygen through their skin. Their tadpoles, however, can swim and feed during the winter, provided it is not too severe. I immediately wondered, “Why don’t they freeze when the temperature drops?” Well, the North Woodlands website of the North Woodlands Association in New Hampshire explains that it all depends on the harshness of the winter. Tadpoles can move and feed because they have more skin surface related to their body size, or a “higher surface area to volume ratio.” As a result, they absorb enough available oxygen through their skin to power their winter activity.
Also, though the temperature may be 32° near the ice, as the tadpoles scurry back down into the pond, the temperature rises to 38 or 39° and may be 40° on the bottom. Perhaps that’s why I never got a perfect shot of one wriggling at the surface; it’s just too cold to stay there for long. Or at least that’s my excuse and I’m sticking to it!
By the way, it’s possible that the tadpoles are Bullfrogs (Lithobates catesbeianus) too. They can often take 2 years to metamorphose into adult frogs. But Green Frogs, like the one below, are the ones I most commonly see at the Playground Pond.
If those tough, wriggly tadpoles survive winter under the ice, they may be getting a jump on the tadpoles that hatch in the spring by being bigger at an earlier date. When the weather warms, the winter tadpoles are ready to metamorphose sooner and grow into bigger frogs. And bigger frogs are better at defending their food territories and finding mates. Maybe we should take that as inspiration for all of us to keep moving in cold weather!