Has anyone ever seen a wood frog? I’m not sure if I have or not, as I’m no frog expert. I’ve certainly seen frogs in the wild, and who knows, maybe some of them were wood frogs. The reason I ask is that apparently wood frogs live all across Canada, and in Alaska and the northeastern United States. They are also quite common, so if you’ve seen a frog in that area, it may have been a wood frog. Wood frogs are quite amazing little animals, so if you do see one, take a second to appreciate its awesomeness.
Wood frogs are range in size from 3.5 to 7.6 cm, with females being much bigger than males. They are usually brown or rust coloured, but can also be different shades of green or grey. Wood frogs can be distinguished from other frogs in their range by dark black patches on their eyes, which looks a bit like a robber’s mask. Their undersides are light coloured, either yellow or greenish white.
Wood frogs have a very early breeding season, and are the first frogs to start calling in the spring. Their calls sound almost like a quacking duck, so if you hear that noise around March or April, you’re hearing wood frogs. Male wood frogs actively search for mates, and really aren’t very good at it. In fact, they can’t tell the difference between a male and female frog unless they grab the frog and give it a big hug. If the frog is nice and fat, the male knows it is a female with a lovely bunch of eggs waiting to be fertilized. If not, he knows the frog is either a male or a female who has already laid her eggs, and he will release the frog and try again with another frog.
Once fertilized, the female lays her eggs in water, usually in temporary pools of water. She lays anywhere from 1000 to 3000 eggs at once, in a mass that usually is 10 to 13 cm in diameter. The egg mass is conspicuous on the surface of the water, but tiny algae start to grow on the eggs, disguising the mass as a lovely chunk of pond scum. It takes two weeks for tadpoles to turn into frogs, and they become sexually mature after two years.
One of the most amazing things about wood frogs is their ability to survive sub-zero temperatures. In fact, a wood frog can survive having up to 65% of its body frozen. The frog manages this by stocking its blood with urea before winter, and converting most of its liver glycogen into glucose. These two molecules limit the amount of ice that forms in the frog’s body, and prevent the cells from drying out. While frozen, the frog’s heartbeat, blood flow and respiration stop. A single frog can be frozen and thawed multiple times throughout the winter without experiencing any harm.
I can’t imagine what it’s like to be semi-frozen, but I bet it’s unpleasant. Still, it’s a key survival strategy for a frog that lives in Canada, where freezing temperatures are frequent. So next time you see a frog in Canada, check if it’s a wood frog, and if it is, give it a thumbs up for being awesome.
Cover image By DDauri Daniel D'Auria – Own work, CC BY-SA 3.0,