Species Spotlight: Wood Frog ( Lithobates sylvaticus)

By Ruth Forsgren

Wood Frog on the forest floor at Brunet Island State Park
(image by Ruth Forsgren)

For me personally, Wood Frogs and their friends Spring Peepers and Chorus Frogs are the true harbingers of spring. Robins, Bluebirds and some other early migrants will return at approximately the same time each year regardless of the weather. The frogs on the other hand must wait for temperatures that are warm enough. Starting in late March as the ice leaves the area waterways I look for a string of three days in a row with temperatures in the upper 50’s (or higher) and especially if the temperatures are accompanied by sunshine, then I will start listening for frogs in the evenings. When listening for frogs remember that only Kermit and his west coast, Hollywood friends will speak “Ribbit”. The Wood Frog’s song is described as sounding like ducks when they are feeding. I think if you listen carefully and use a little imagination you will hear them saying “lick it up”, but they say it very, very quickly. They do not project their voice as well as some frogs so you have to be pretty close to hear them.

When Wood Frogs sing you can see the inflated air sacs on
both sides of the head, behind the ear. (image by Ruth Forsgren) 

Of the early frogs, I especially like the Wood Frog. They have such an interesting way to get through the winter and I’ll get to that in a second. First off let me more properly introduce you to the Wood Frog. As frogs go they are a small to medium sized frog, reaching a snout to vent (where an animal goes to the bathroom) length between 1.5 to 3 inches. Their skin is usually a brown to red-brown color but can vary to gray or green. Wood Frogs look like they are wearing a dark mask that extends from their nose, over the eyes, ears and then curves down to the front leg. The back legs are usually striped and these stripes in combination with the mask reminds me of a raccoon.

Their preferred habitat is right in their name, Wood Frogs can be found in wet wooded areas, bogs, wet meadows and on the tundra. (They are one of the only frogs that can survive on the tundra). Even their scientific name gives away their habitat. Lithobates comes from the Greek words: litho, meaning stone and bates, which means that walks or haunts; sylvaticus comes from Latin and means pertaining to the woods or forest.

Back to my favorite thing about Wood Frogs and the reason that they are the one of the only frogs that live in the Arctic is their ability to freeze during hibernation.(click for video) During hibernation Wood Frogs enter a state of suspended animation where their heart stops beating and they stop breathing. They do this by making changes at the cellular level as weather gets colder, in a sense they produce their own anti-freeze. The liver makes a glucose product that helps to support the body’s cells by preventing cell collapse from water loss. After spending what can be several months frozen, the Wood Frogs body “wakes up” and is ready to go without any muscle loss or atrophy. It boggles my mind that if scientists can unlock this secret we may be able to freeze people that need a heart transplant until there is a suitable one available. Total game changer!

Once they come out of hibernation the Wood Frogs are ready to mate. They will move from the forest floor to woodland pools, flooded ditches, marshes or other water areas that are mainly fish free. (Wood Frogs are tasty little snacks for fish.) The females lay egg masses that can contain between 1,000 and 2,000 eggs. They attach the mass to a plant stem or stick if there is one available.  

Wood Frog egg mass
(image from Flickr)

After a few days the mass will flatten out, turn green from algae and float on the surface looking like pond scum. Depending upon temperature the eggs will hatch in as little as 10 days if it is warm or in a month if it is cooler. Tadpoles take about 2 months to metamorphose into a frog. After mating the adult frogs will leave the ponds and return to the woods in search of their prey, which includes beetles, spiders, worms and other invertebrates. During the warmer summer months, they are primarily nocturnal (active/hunting at night) but during the cooler nights of spring and fall they become diurnal (active during the day).

So during the first warmer days of spring be sure to keep an ear open for the “lick it up” call of the Wood Frog.