Wood Frog

The most recognizable feature of the wood frog is the black “robber’s mask” eye stripe.

Common Name: Wood Frog – Frog is among the oldest of Indo-European words originating as the Sanskrit pravate, meaning “he jumps up.” It evolved to English through Old Norse as frauki. Wood frogs are found around wet areas in woodland habitats, but not on wood as the name suggests. The reference may be to its characteristic brownish hues which are similar in color to wood bark. However, brown frog would then be a better choice and no less uncreative.

Scientific Name: Rana sylvaticaRana is the Latin word for frog which differs from the Sanskrit origin as an onomatopoeia of their call …  like croak or ribbit in English. The Latin word for woodland is silvae. The scientific name is literally “frog wood,”  the opposite of the common wood frog name. Wood frog have been reclassified by modern DNA taxonomy to Lithobates sylvaticus from the Greek lith meaning “stone” and bates meaning “one who treads,” which would connote “stone walker.” This could be literal, as is the case for the wood frog depicted above climbing on a lichen-covered rock.

Potpourri:  Wood frogs appear in the spring after having endured even the coldest of winters as if immigrating from remote, warmer habitats, like anuran snowbirds. Surely an amphibian noted for its slimy wetness cannot have survived near frozen-through skateable ponds that dot the woods they inhabit. But they do. The extraordinary tenacity of life in the savagery of the wild is the result of the survival of mutants.  After the basics of what it took to be a frog were successfully worked out in the deep recesses of time, populations of jumping, amphibian carnivores lurking in or near water burgeoned. To escape the crowds competing for the same resources, the more adventurous individuals left for greener, but sometimes colder, pastures. The resulting diaspora to new environments is one driving force for speciation. Wood frogs, like humans among mammals, have managed by sheer luck to evolve in the right direction to become among the most successful of their amphibian cohorts. Not only do they survive arctic winters, but they are first to emerge in spring to fill any emergent pool of water with thousands of eggs. It is only a matter of time until a new mutation will offer better chances elsewhere.

The first question is how do thin-skinned animals survive iced-in ponds without the coat of a beaver or down like a duck? This conundrum perplexed naturalists whose warm blooded judgement was skewed toward bears denning in caves and caribou gathering in tightly packed  herds to share or conserve body heat. The cold blood of reptiles and amphibians lacks the metabolic wherewithal of thermoregulation. Consensus was that burrowing deep into the ground below the frost line was the only possible palladium; toads had been found buried up to four feet deep during excavations.  John Burroughs, an eminent nineteenth century American naturalist, chanced upon a frozen frog he found under some leaf litter and concluded that “… frogs know no more about the coming winter than we do, and that they do not pass deep into the ground to pass the winter as has been supposed.” [1] Finding an animal frozen and lifeless would lead most to conclude that it died of exposure, having failed to account for weather extremes. The foolish frog theory, which would make a passable subject for Aesop’s fables or a subplot in Disney’s Frozen, is false. Frogs freeze on purpose.

Science entered the picture in the 1980’s when a Minnesota-based researcher with some knowledge of frog adaptability took up the subject. The experiment consisted of collecting a number of frog species in the fall and subjecting them to freezing in the laboratory under controlled conditions. After six days at -6°C, the frozen frogs were moved to a refrigerator and thawed at +6°C. Wood frogs began to show vital signs and limb movement after three days but mink and leopard frogs subjected to the same conditions froze to death and stayed that way. The resulting paper concluded that “an accumulation of glycerol during winter was correlated with frost tolerance, indicating that this compound is associated with natural tolerance to freezing in a vertebrate.” [2] In other words, wood frogs seemed to be making antifreeze. In the four decades that have followed since this seminal experiment, further research has revealed the true nature of the wood frog’s magic.  

What better place to study frozen wood frogs than Alaska where arctic winter is the norm and spring thaw the exception? Researchers located frozen frogs in the wild and measured ambient temperatures with sensors placed directly on their skin. After two seasons with temperatures as low as -18°C and a seven-month long period of deep freeze suspended animation, every wood frog came back to life. [3] That the frogs survived the natural habitat test at much lower temperatures for a much longer time period than in the laboratory test led to some speculation as to the mechanics of freeze protection. Vertebrate metabolism is based on energy generated primarily from the oxidation of glucose derived from dietary carbohydrates. Excess glucose is stored in the liver and in muscle tissue as glycogen for future energy needs. The key to the deep freeze conundrum was that in the laboratory, temperature was lowered to below freezing just once and the frogs froze. In the wild, frogs are subjected to multiple freeze/thaw cycles according to weather fluctuations. It was discovered that each cycle ratcheted up the production of glycogen, ultimately increasing its concentration by a factor of five. To accommodate the stockpile, liver size increased by over fifty percent ― one researcher described the wood frog as a “walking liver.” When compared to wood frogs monitored in more moderate Midwest climates, the Alaskan frogs had three times as much glycogen. [4] While Darwin’s Galapagos finches provided a hint of adaptations for survival, Alaskan wood frogs are a compelling affirmation case study.

The actual mechanism employed not only by wood frogs, but also by spring peepers, gray tree frogs and chorus frogs to revive after freezing to death (heart stoppage and breathing cessation) is now understood to involve both glycerol and glucose in addition to some specialized proteins. Glycerol lowers the freezing point of water to protect membranes from freezing just as it does for automobile cooling systems. Glucose in high concentrations prevents the formation of ice crystals inside cells. Ice crystals are like small daggers, shredding cell membranes and wreaking havoc with organelles. This is why freezing is normally lethal to animals and why frozen vegetables that are not dehydrated turn to mush when defrosted. When a frog senses first frost, adrenaline is released to convert liver glycogen into blood glucose. This is the same mechanism that provides energy for fight or flight (and freeze in frogs). It originates in the amygdala, the brain region that provides for immediate action in emergencies known as the sympathetic nervous system.  The difference with wood frogs is magnitude. Human glucose ranges from 90 to 100 milligrams per deciliter with a diabetic threshold at 200 mg/dl. Frogs boost their glucose to as high as 4,500 mg/dl, well over lethality for humans, and probably for just about every other living thing. The specialized proteins act as ice nucleation sites outside the cells where about 65 percent of total body water ends up frozen. [5] Cryobiology may well be the next frontier in the quest for life everlasting if the lessons learned from wood frogs can be mastered. [6]

Male wood frog in amplexus grip of female amid fertilized eggs.

The spring thaw fills vernal pools with the cacophony of male wood frogs courting, a behavior known as explosive breeding. As the amphibian exemplar of the early bird gets the worm, the quest for sex begins in early March, even before wet areas are free of ice. Filling the air with their duck-like quacking, male wood frogs frenetically search for something to mate with, not infrequently grasping other males and even other species, including large salamanders. The tenacious grip is called amplexus, aided and abetted by swollen thumbs and expanded foot webbing that won’t let go. [7] It is necessary because females are generally larger than males and slimy frogs are slippery. Mating success of male wood frogs is dependent on physical size,  one of nature’s enduring correlations. It is also true that larger females are more likely to mate as size in this case correlates to the number of eggs produced. After an embrace that can last for over an hour for egg fertilization, the female deposits as many as 3,000 eggs in a gelatinous, globular mass about four inches in diameter. After a time, the ball flattens and collects algae for disguise as pond scum. One month after oviposition, the eggs hatch into aquatic tadpoles for the race against the clock to metamorphose into terrestrial wood frogs before the pool, which may be seasonal, dries up and they expire. Wood frogs can freeze but their young need water. The odds are stacked against survival, but only one tenth of one percent of the eggs in the brood must reach adulthood for survival of the species, at least for those that are fittest. [8]

Amphibians first appeared in the Devonian Era about 400 million years ago as something like a walking fish and have never broken free from their aquatic “roots” even as evolution has run its course. True frogs of the family Ranidae, which don’t appear in the fossil record until 57 million years ago, are long-legged, narrow wasted, and web-footed with horizontal pupils including wood, green and bull frogs. Since their origination occurred after the breakup of the supercontinent Pangaea, global dispersal required continent jumping. DNA assessment of 82 Ranidae species revealed that the North American clade of true frogs came from East Asia, hopping across Beringia and spreading across the New World by 34 million years ago. The first genetic split of the true frogs that spread out in North America was the mutation that became the wood frog, suggesting a significant adaptation. [9] Are wood frogs still evolving? The short answer is yes because everything does, including humans. It is just too slow to notice.

Amphibians are the proverbial “canary in the coal mine” when it comes to planet Earth. They need both clean water because they are aquatic for at least a portion of their life cycle and clean air because we all do. Wood frogs offer a case in point. With climate getting warmer and not colder, ice survival may not have quite the same importance in the future.  One study found that pond temperature had a marked effect on wood frog tadpole development time. Those in colder ponds grew faster. Conversely, warmer water not only slowed tadpole growth but also evaporated more quickly. Rising ambient temperatures will thus reduce the chances for slower growth tadpoles to metamorphose into lunged froglets before the water evaporates due to accelerated desiccation. [10] On the other side of the survival ledger, empirical data from the beginning and end of the last century revealed that temperatures had risen about 3°F and that male wood frogs were calling for mates about two weeks earlier. This would then move conception time up to account for more time needed to gestate and grow. [11] Given their historical evolutionary success over the last 34 million years, it is reasonable to conclude that Rana sylvatica is more likely to survive climate change than Homo sapiens, who have only been around for less than one million.

References:

1. Heinrich, B. Winter  World, Harper-Collins, New York, 2003, pp 169-175

2. Schmid, W. “Survival of Frogs in Low Temperature” Science, 5 February 1982,  Volume 215, Issue 4533  pp. 697-698.

3. Pennisi, E. “How to Freeze and Defrost a Frog”, Science, 8 January 2014.

4. Servick, K. “The Secret of the Frozen Frogs”  Science, 21 August 2013.

5. Heinrich, op cit.

6. Costanzo J et al  “Survival mechanisms of vertebrate ectotherms at subfreezing temperatures: applications in cryomedicine”. The FASEB Journal. 1 March 1995 Volume 9 No. 5 pp 351–358.

7. https://www.nasw.org/users/nbazilchuk/Articles/wdfrog.htm    

8. https://animaldiversity.org/site/accounts/information/Rana_sylvatica.html

9. Yuan, Z. et al. “Spatiotemporal diversification of the true frogs (genus Rana): A historical framework for a widely studied group of model organisms”. Systematic Biology.  Issue 65  No. 5, 10 June 2016  pp 824 – 842.   

10. Renner, R. “Frogs not croaking just yet” Science, 12 My 2004.

11. Wong, K. “Climate Warming Prompts Premature Frog Calls” Scientific American 25 July 2001’

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