It's mid-October and fall is in full swing: foggy mornings, cold rains, and falling leaves. Time to talk about ... tadpoles!? That's right, while we may be accustomed to discussing tadpoles in spring and summer, they're still around and they're gearing up for winter.
Imagine your local pond. Under a slate gray autumn sky, the pond is mostly quiet. Only an occasional peep (called the "fall echo") escapes from the reeds, where previously an amphibian chorus declared its presence. Yet despite the chill and silence, frog life continues. Most of the summer's broods hopped onto land at least a month ago. Others will hibernate in the coming months as polliwogs.
So how do tadpoles "decide" when to change into frogs? And why do some of them stay in tadpole form all winter?
Let's go back to the beginning: it is spring and egg laying is in full swing. Peepers fill the air with their songs, punctuated by the slower, heavier calls of wood frogs. Cold, clear water fills with lumpy egg masses, which will hatch into little black or gray tadpoles. Amidst this new activity, some polliwogs born last summer are stirring to life again, ready to resume eating and growing.
As the pond's diversity suggests, the life cycle of a frog is flexible. Green frogs, a common frog in our region, will start breeding and laying eggs in April and continue into the end of summer. Their eggs will hatch after just a few days, the tiny new tadpoles wriggling out of the egg jelly to spend the summer eating detritus at the bottom of the pond. Metamorphosis begins in June and continues into early autumn; tadpoles born in late broods may overwinter as tadpoles.
Tadpoles get their cue to metamorphose from their thyroid hormones. These hormones move through the frog's tissues, delivering orders to mobilize ranks of cells to action. Some tissues get the message to grow while others are told to cut back. Once begun, messages from the thyroid hormones keep the frog's metamorphosis on the right track - for example, by preventing a tadpole from reabsorbing its tail before it has grown legs.
Studies have shown that key conditions affecting metamorphosis are water temperature, crowding, diet, and physical stress such as drying out. Warmer water temperatures may speed up tadpole metamorphosis, while many tadpoles crowded together tend to delay it. Plenty of food may also delay metamorphosis, but stress may cause a frog to start metamorphosis earlier. How these conditions send the thyroid hormones into action is a puzzle researchers are still working on.
Timing of metamorphosis is partly determined by species; for example, frogs that commonly breed in vernal pools will grow faster, completing metamorphosis six to eight weeks after hatching, while frogs that breed in ponds can delay metamorphosis for two or even three years. The bullfrog is the perfect example of delayed metamorphosis, usually taking two years to become a frog.
There are advantages to overwintering as a tadpole instead of as an adult frog. The longer a frog spends in the tadpole stage eating and growing, the larger it will be when it finally undergoes metamorphosis. Larger frogs are better able to escape predators and to compete for territories and mates.
Another advantage is that tadpoles are better suited than frogs at surviving low-oxygen conditions. Picture your pond in winter: a layer of solid ice over near-freezing, still water. Ice blocks air from the water surface, and there are no currents to mix oxygen into the water. Overwintering frogs require less oxygen than they do in summer, but low oxygen conditions still take a toll. Tadpoles have a higher surface area to volume ratio and can respire across their skin more efficiently.
As we prepare ourselves for snow and cold, remember the tadpoles hidden below cooling pond water, settling down for a long dormancy, and awaiting new growth in spring.
Rachel Sargent is an educator with the Fairbanks Museum, as well as a freelance nature writer and illustrator. The illustration was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation.