The Tadpoles of Winter

Illustration by Adelaide Tyrol

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.

 
Discussion
  1. Ted Watt → in Amherst, MA
    Nov 07, 2013

    Rachel
    Thanks so much for your article about winter tadpoles.  I knew some of what you wrote about, but I loved how you presented the 4 factors that influence how quickly tadpoles develop.  I never thought about their situation quite like that and it makes really good sense.  I wondered if over-crowding might actually speed up development because, as the pool dries, the habitat may be disappearing?  But this would be difficult in terms of feeding and getting oxygen?  I have found the dessicated carnage in vernal pools that have dried before the tadpoles are ready—seems so sad, but I know it is part of the cycles…

    Thanks again for your thoughtful treatment of this topic.

    Ted

  2. David McGraw → in Berlin, VT
    Nov 20, 2013

    I have heard reports of hibernating frogs that actually freeze solid during the winter months.  Obviously this capability would be highly adaptive during long cold winters!  I wonder however, is this capability commonly used by amphibians in the northeast?  Or, is this a capability something that is limited to just a few unusual arctic species. 

    Nice article about our frogs friends!

  3. Kay → in United States
    Sep 23, 2014

    All of the above is very interesting. My kids and I have raised frogs all summer long this year. our first batch took about 6 weeks and were mainly Cope’s gray tree frogs. We handled that as best we could with limited knowledge, but lost too many of them. The information that we found -tadpoles only need boiled lettuce- is nonsense and it’s unfortunate that this is such a widespread concept.we lost many frogs due to a lack of protein and calcium in their diet.

    The second batch we raised have all been extremely healthy. We included spirulina wafers, added liquid calcium to their water, and did not boil the all the nutrients out of lettuce. We blanched dark green lettuce, romaine to be exact, and froze it so it would retain all the healthy nutrients that our little hoppers needed.

    The result from our second batch was a whole lot of healthy strong little jumpers that were ready to go out and conquer the world. We did have a few problems with air bubble disease, but that was about it. I wish every article encouraging those who want to raise a few ttadpoles to feed them only boiled lettuce would be stricken from the internet. It broke our hearts to raise tadpoles who ended up with useless back legs.we know better now. we have the system down pat. we know what to feed them to make sure they’re healthy and we now have at least 3000 froglets hopping around in our backyard. It’s been one heck of a summer! :-)

  4. Victoria → in Northern MN
    Oct 12, 2014

    HELP!  We still have tadpoles in our TINY plastic ponds!  Unfortunately, our ponds become big blocks of solid ice, during the winter.  It’s not unusual for us to have temps of 50 degrees BELOW zero…or even lower, so I don’t think pond heaters are an option.  I suppose the tads are doomed, but I can’t understand why they’re in there so late in the season.

  5. Shannon → in United States
    Nov 14, 2014

    Wondering what to do with the 25 tadpoles I brought in from our water feature in the backyard when it started getting very cold here.

    Knowing that the limited water in the fountain would freeze solid eventually, I hated to think of the little tadpoles in there doing the same thing. I scooped up enough water from the fountain in which they had hatched and poured it—and the tadpoles -  into a large plastic container that is now sitting in my kitchen. I live in Atlanta, and although we are currently getting freezing nights, our little pond in the nearby park never freezes over.

    Should I just transport the tads there and let nature take its course? They’ve been inside for about two weeks & although they’re getting larger, they do not seem to be developing further. And, I think they are likely bullfrogs, which I have read can take two to three years to metamorphosis! Any advice?

  6. Peter → in Driftless Area IN WI
    Aug 22, 2015

    I also have frogs and tadpoles in a small shallow pond. Did Victoria in Northern MN ever get a solution? Would a deicer help? I read sweeping snow off and letting sun reach the plants and algae helps. I will start with that. My pond will freeze solid top to bottom, can the tads and frogs take this and survive?

  7. Ethan → in NV, USA
    Feb 18, 2016

    Thanks for the article.  My family’s pet pollywog’s stagnated metamorphosis mystery solved.

  8. Julie Wilkins → in North Carolina
    May 24, 2016

    Your article on frog metamorphisus is interesting.  I am just now learning about their development as they laid eggs in my little dogs plastic pool.  Looks like I will be tending tadpoles all summer.  However I have noticed that new tiny tadpoles keep hatching, so it looks like the frogs have been returning at night to lay even more eggs.  I currently have about several hundred in a 4’ wide pool.  I hope this will be large enough for them to complete their transition to frogs.

  9. Sarah → in Ky
    Oct 16, 2016

    I also have tadpoles in the water trough in Mid Oct….MANY of them. I was wondering if they are a different type of frog. Not sure what to do with them.

  10. Carlie → in Atlanta GA
    Nov 07, 2016

    Best advice is to keep the water fairly cool to prevent them from sprouting legs! Keeping the water in a cool dark place will trick them into hibernation sagely worn no danger of freezing. .. you can even place the water in an area cooler than the house that is safe from freezing.  We keep our house at 65 in the fall and winter and that is cool enough but you can keep them in an area 45 F to 65 F and that will keep them from morphing. The cooler the better.

  11. Kim Hornsby → in Washington
    Dec 07, 2016

    I brought my tadpoles in as their habitat started to freeze solid. They thawed out of dormancy and are now swimming around in their tub. Couldn’t stand the possibility of them dying after watching them for 6 months.

  12. Cassandra Podmore
    Jan 10, 2017

    I found two tiny tadpoles with no legs in a puddle with almost an inch of ice. They are not frogs, so I don’t know what to feed them. Does anyone know?

  13. Patrick → in Boston, MA
    Jan 23, 2017

    I learned a lot but am concerned. There’s a pond near me that has thousands of polliwogs in different stages but mostly in the early bullfrog polliwog stage. I believe there are so many they are over-crowded but at the same time I am catching great sized shiners too both very active its Jan.23rd I set 2 minnow traps both filled equally with polliwogs and shiners. Is this normal?

  14. KP → in PA
    Sep 09, 2017

    It’s mid September here in western PA and we’ve had a long stretch of nights in the 40’s. I have four pods each 24"x6” with fairly large numbers of tadpoles (mostly Hyla versicolor, possibly some Lithobates sylvaticus) & a large food supply they’ve been living well on since hatching. They’re taking longer to morph this year than others. My two concerns are that I don’t know if these 2 species can cycle through winter as others can & if so what water temp range is optimum for the process? Any help would be greatly appreciated.

  15. Karen → in Milwaukee, WI
    Oct 15, 2017

    Just wondering, I have tadpoles in a small water fountain base, bought seaweed for them, My fountain will freeze solid, Will they live thru winter in the solid ice?

  16. Jill → in Franklin TN
    Oct 19, 2017

    What should I do with the tadpoles that are in my fountain? I have to empty for the winter as the fountain cannot have frozen water in it or it will crack.  Can I take the tadpoles out and put them in a near by pond?  I live in Middle Tennessee and we can get cold winters.

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