What happens to wood frogs in winter?

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February 1, 2021

In the warmer months, frogs are a common sight in backyards, around ponds, and in parks. But when winter hits, it’s as if they’ve just vanished into thin air!

We talked to our reptile and amphibian expert, Dr. Allison, to help us learn more about what frogs, particularly wood frogs, do in the winter.

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Dr. Allison holding a frog egg mass

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A wood frog metamorph

Q: What do frogs do in winter around Chicago?

A: Frogs like other reptiles and amphibians brumate during the winter. Brumation is kind of like hibernation. Their metabolic rate slows down, but they’ll still move around a little bit on warm days.

Q: Where do frogs spend the winter?

A: Green frogs will overwinter in flowing, well-oxygenated water, sometimes even under ice! Toads will overwinter on land and bury themselves in mud or muck. Wood frogs will excavate a little hollow in the ground called a form and stay pretty close to the surface under leaf litter or just under the substrate.

Q: Why do wood frogs stay close to the surface while other frogs and toads bury themselves?

A: Wood frogs have a really cool adaptation that lets have up to 65% of the water in their body freeze and still survive! Most other frog species have to stay underwater or below the frost line, but wood frogs produce an antifreeze in their bodies and superload or concentrate the antifreeze in certain organs of their bodies. The antifreeze, glucose, prevents ice crystals from forming that would damage the wood frog. This adaptation allows wood frogs to be found as far north as Alaska!

Q: Where can you find wood frogs in Chicago?

A: Generally, wood frogs are the first frogs to wake up in spring, but in our area they are second to the chorus frogs. They are one of the earliest frog species to breed, around late March to early April. Wood frogs are explosive breeders, which means they all gather to breed around the same time. Male wood frogs will gather to call and then mate with the females for 5-8 days. This is when it’s easiest to find wood frogs, when you can hear them calling! They lay their eggs in vernal ponds, which are ponds that are seasonal and aren’t home to any fish, and then move away from the pond after a week or two into the surrounding woodland where they can be hard to see. Metamorphs, tadpoles that have metamorphosed and grown legs, will leave the ponds in mid- to late- June. Wood frogs will stay active until the temperatures start to drop around October, and then they’ll start brumating over winter.

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