Critter Of The Week #20: Wood Frog

At first glance, the wood frog seems like your everyday, little backyard frog. They don’t have any striking colors, aren’t too small or large, and don’t have a distinct call that makes them famous like a bullfrog. But what they lack in appearance, they make up for in their body’s incredible tolerance of cold weather.

 

 

Most frogs and amphibians need a warm, moist environment to thrive in. Their skin is soft and fragile, much like the lining of our mouth and lungs. This is why you see most amphibians in areas like rainforests, swamps, and the temperate forests like those found in the Appalachian Mountains. As you travel to drier areas like deserts, grasslands, and mountains, you find less amphibians.

 

 

The other limiting factor that keeps frogs and amphibians out of certain areas is the cold weather. They are not warm-blooded and have no feathers or fur to protect them from these extreme temperatures; so most amphibians with their soft, fragile bodies do not live in areas with long and harsh winters like Canada and Alaska. There is one exception: the wood frog. They are one of the only amphibians that live near the Arctic Circle.

 

 

 

Most frogs and amphibians will cope with winter by either burying themselves in the ground, or stay at the bottoms of lakes and ponds. Their metabolism drops so low, they do not need to breathe air or eat and get all of the oxygen they need by breathing through their highly absorbent skin. But most frogs don’t deal with the kind of long winters that wood frogs must endure. This is why wood frogs have an even more extreme and successful way of dealing with the icy winter, they freeze almost completely solid.

 

They can survive in this state off and on for 7 months in temperatures as low as 3°F! Their heart stops beating, blood stops flowing, and they go into suspended animation. Their individual cells are still functioning but they have no way of communicating with each other. When the spring rolls around, they thaw from the inside out. The heart and brain will thaw first because of the high concentrations of antifreeze and then eventually the limbs come last.

 

This makes sense considering the most important organs should be the first to function after the long freeze. Yet, it goes against conventional logic because one would assume they would thaw out like a popsicle, from the outside in. But this wouldn’t work for the frog because with its heart and brain being the last to thaw out, they would have no control over their limbs until they are completely unfrozen, which would put them at risk of being easily gobbled up by predators. By thawing from the inside first, they can gain full consciousness and be ready to hop away as soon as their limbs catch up to their brain and body.

 

 

How are they able to do this without dying? Their secret lies in the production of their own antifreeze in the form of glucose and urea. Their liver converts glycogen into glucose (sugar) that builds up in their tissues. Much like antifreeze in your car, this prevents ice crystals from forming in their body. They may appear frozen solid when you pick them up, but in fact their body is only about 65% frozen because of their natural antifreeze. If you bent one of their legs though, it would break off.

 

Scientists discovered another strategy the frog uses to stay alive. During the day in the winter, the temperature is slightly warmer, and sometimes the frogs thaw a little, they use this opportunity to build up and produce even more antifreeze, so as the winter goes on, they build up more and more to make sure they stay alive. Once the spring is in full swing, they hop into a pond and do some spring cleaning by flushing out all of the built-up glucose and urea.

 

Scientific Classification

Kingdom : Animalia

Phylum : Chordata

Class : Amphibia

Order : Anura

Family : Ranidae

Genus : Lithobates

Species : Lithobates sylvaticus

 

Conservation Status 

The International Union for the Conservation of Nature classifies the wood frog as least concern due to their wide distribution. They still face threats of development and wetland drainage from farming and other human activities.

 

Physical Description

 Wood frogs are a medium-sized frog measuring about 2- 3 inches with the females being larger as is typical with most species of frogs and toads. No other frog looks like the wood frog in North America. They range in color from a light tan to brown and rust colored. They usually have a dark eye mask like a raccoon and are slightly green on their belly.

 

The reason their colors vary so widely is the frogs of that region have adapted to the local leaf litter and vegetation that best suits their survival. Frogs from peat bogs without many trees in parts of Canada show darker colors, whereas frogs from areas with lots of pine trees show the lighter colors because of the tan pine needles that litter the ground.

 

 

 

Diet 

 Because of their wide distribution all over Canada and parts of the US, wood frogs have a highly varied diet. They are insectivores and will eat pretty much any small forest floor invertebrate they can overpower. This includes beetles, ants, worms, grubs, isopods or "rolie pollies", millipedes, and anything else they can catch.

 

 

Like most frogs, they hunt based on movement of their prey. To catch them, they wait patiently until something comes nearby and then lunge forward and use their tongue to grip them and bring them into the mouth. They will then swallow their prey whole.

 

Compared to other frogs like bullfrogs, green frogs, or leopard frogs whose tongue will flip forward to grasp their prey, the wood frog feeds more like a toad and uses the tip of their tongue to catch prey.

 

Have you ever noticed when a frog swallows, they close their eyes? Wood frogs do the same thing, and they do this because their eyes actually help push the food down! It’s a bizarre but genius way to make sure food stays down when you have no teeth and can’t chew!

 

 

 

As tadpoles, they are herbivores and will eat primarily algae but sometimes, if resources are scarce, feed on the eggs of other tadpoles. Once the tadpoles transform into little froglets and emerge from the water, they switch to their carnivorous diet.

 

This transformation from a complete herbivore to a carnivore is perhaps one of the most dramatic changes in anatomy and physiology in the animal kingdom. The tadpole’s digestive tract is very long and convoluted so they can process the algae’s thick cell wall, whereas the carnivorous adults have short and simple intestines with strong stomach acids that quickly digest insects and invertebrates.

 

 

This dramatic metamorphosis is like a cow transforming into a lion. All the instincts the tadpole had as a herbivore changes to become a predator as an adult.

 

Reproduction & Life Span

 If a wood frog manages to escape predators and disease, they can live to the ripe old age of 3 years old.

 

Like most frogs, they start that life as a humble tadpole. For wood frogs especially in Alaska they need to hurry to get things done in less than 5 months. This means the adults need to mate and lay eggs and those eggs need to develop from tadpoles to adult frogs in that short time window. Compare that to a bullfrog which takes 2-3 years to grow into an adult.

 

Its only after the second year will the wood frogs actually be ready to reproduce on their own.

 

When they are ready to reproduce, they will thaw out from their winter hibernation and find the nearest body of water.

 

Wood frogs prefer to breed in ephemeral wetlands or vernal pools. These are places that temporarily hold water after heavy rain or snow melt. The reason the frogs choose these areas is actually a smart survival tactic as they are usually free of fish and other predators, which the perfect place for tadpoles to grow up.

 

 

The risk with laying eggs in these types of pools is there is always a chance they will dry up. This is why the wood frog lays eggs so early because the chance of evaporation is reduced earlier in the season since there is more rain and lower outside temperatures in the early spring.

 

Wood frogs are also some of the very first frogs to emerge in the spring, and the earlier they get started, the better chance their offspring have at surviving. This is the reason scientists think they freeze instead of hibernate at the bottom of a pond or lake.

 

Frogs that hibernate in lakes take longer to emerge and get a later start in the spring, whereas the wood frog is one of the first to emerge and lay their eggs as soon as it’s warm enough. This strategy has worked great for them and has helped them expand into areas no other frog could go.

 

Once they find a good pool of water, the frogs will begin calling to each other. Once a male and female locate each other, the male will grasp around her waist and lock his thumbs together in a position called amplexus, it looks much like he is hugging her. As she lays her eggs, he will fertilize them in this position. Oftentimes frogs will lay eggs near other frogs and create large masses of eggs to ensure even better odds of survival for the tadpoles.

 

 

 

Once the eggs are laid, their rate of development depends on temperature, population density of other tadpoles, and the abundance of food. Depending on temperature, the eggs hatch in a few days to weeks and tiny tadpoles swim out.

 

 

If the pools they are in are crowded and warm, they will develop faster than normal. If they are cool, large and sparsely populated with plenty of food, they can take their time and grow larger and stronger.

 

An interesting observation scientists have made in the field is that tadpoles from the same family will often congregate together. If temporarily separated they will seek out their siblings and stay together in small groups.

 

In about 2 -4 months, the tadpoles will develop into tiny froglets.

 

Most of the adult frogs will come back to this same pool to breed. Only about 20% will migrate to other areas. It is the 20% though that have spread the population all over Canada and Alaska, which means they are key to the species expansion and distribution.

 

Once the froglets have hopped out of the water, they will continue growing, and in about 2 years for the males and 3 for the females, they will be ready to reproduce. Most frogs will breed only once in their lifetime.

 

See their complete development in this amateur video.

 

  

Distribution & Habitat

Wood frogs live in moist woodlands, bogs, freshwater wetlands, and swamps. They are more terrestrial than other frogs and spend a lot of time on land. This makes sense considering they breed and reproduce in temporary pools and bodies of water. When the water dries up, they need to survive until it returns. When they are on land, they will stay in areas with more moisture like ravines and heavily forested shady areas.

 

 

Wood frogs have a wide distribution and are found throughout the northeastern US, Canada, and Alaska.

 

Population

Exact population estimates of the wood frog are unknown, but due to their wide distribution, it is believed their population is healthy in the northern parts of their range. In the United States, they face many challenges because of their unique habitat preference. Most pools and standing water is drained during development so their populations are becoming fragmented.

 

Relationship with People

Wood frogs are not used in any major human industry nor do they have any culinary value. This has protected them from over-harvest that other species have had to endure. The biggest challenge they face from humans is pollution and development. The temporary bodies of water wood frogs prefer are frowned upon in the developed world as they are seen as breeding grounds for mosquitoes.

 

The truth is these temporary pools host a unique variety of life that isn’t found in normal bodies of water that have fish. These mini ecosystems are becoming an increasingly rare sight. Fish-free ponds are havens for amphibians and other animals.

 

Vernal pools and wetlands also improve water quality by slowing down draining of water so the land has a longer time to absorb and filter pollutants.

 

Despite these challenges in the northern parts of their range, they are doing much better and are relatively safe from heavy development and have garnered increased interest from scientists in recent years.

 

 Wood frogs are being studied more and more these days because of their extreme freeze tolerance. There are obvious applications in organ transport, space travel, and even helping stroke victims recover from brain injury.

 

The key to preserving the organs and tissues lie in the natural antifreeze in the frogs’ tissues. If humans can replicate this, there is a chance at successfully transporting organs for longer periods of time.

 

In 1999, researchers successfully preserved rat livers using what they learned from the wood frog and transplanted it to other rats. Research on humans has yet to happen but could be in the near future.

 

Hopefully next time you see a wood frog, you will have a newfound respect for what they can do in their short and amazing lives!

  

CritterFacts CritterKnowledge Ranking:

7

We rank each new Critter of the Week based on how much information and resources we find online about each animal. We rank on a scale of 1-10. 1 being very little information available or only very basic information available and there is a need for more research. An example would be a newly discovered deep sea creature like the blobfish. A ranking of 10 means there is abundant and easily accessible information available on all aspects of a species. An example of this would be chimpanzees. We will use this gauge to help us guide future research efforts to learn more about new animals we include in our Critters of the Week!