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Class: Amphibia (full scientific classification)
Conservation Status: Least Concern
G5 – Secure
There are only six species of amphibians native to the state of Alaska. The wood frog (Lithobates sylvaticus – also known under the unaccepted name, Rana sylvatica LeConte) is the only amphibian residing in the interior of Alaska with a range extending north of the Brooks Range.
One reason the wood frog can survive the long and frigid winters in these areas of Alaska is that it creates antifreeze proteins (AFPs) and antifreeze glycolipids that give it the ability to almost completely freeze solid over winter and thaw in the spring with minimal cellular damage. Research has shown that the wood frogs in Alaska and northern Canada most likely have higher freeze tolerance than the wood frogs in other regions.
Adult wood frogs grow to between 3.8-8.2 cm (1.5-3.23 inches) in length. The color of their bodies varies greatly between specimens, most taking a gray or brown shade, and occasionally a shade of rust or green. The underside of the frog is paler than the top. Individual frogs also have the ability to change color in response to the darkness of their background [King]. The females are typically larger and more brightly colored. The easiest way to identify a wood frog is by the dark eye mask behind the eye and often dark stripe in front of the eye to the nose. The eyes themselves are usually goldish-brown with a round, black pupil.
Lithobates sylvaticus has two prominent ridges along each side of its back, known as dorsolateral folds. Some have dark markings or stripes running along these ridges, but not all. Most also have a lighter colored middorsal fold running down the center of the frog’s back. Adult male wood frogs may have an enlarged thumb, called nuptial pads, especially present during mating season.
Hibernation and Freeze Tolerance
The wood frog has the furthest north distribution of any amphibian on the planet. It even lives in northern and interior Alaska where the winter temperature routinely reaches below -40 °C (-40 °F). This cold-blooded amphibian is able to hibernate in these conditions because it has the ability to freeze solid (typically 35-45%, but up to 65% of the water in their body may freeze) overwinter and then rethaw in spring. During hibernation, wood frogs may remain frozen for over 190 days in interior Alaska.
Most hibernating frogs overwinter deep underwater, where they can avoid the below-freezing temperatures completely. Wood frogs have adopted a different strategy by burying themselves in decaying leaves approximately 4-10 cm (1.5-4 in) deep, typically close to their breeding ponds. One reason they may have adapted this way is that the land typically thaws much earlier than the lakes and ponds do, giving them a headstart and earlier breeding season in spring.
How do they freeze?
Lithobates sylvaticus possesses cryoprotectants that give them this incredible ability. The wood frog’s liver produces high levels of glycogen, which is converted to glucose. The glucose is then stored within the muscle and heart cells. This helps prevent intracellular ice formation that would otherwise destroy the cells. The sugary glucose also bonds to the water molecules preventing it from escaping the cells and avoiding desiccation.
Another cryoprotectant, a waste product known as urea, also builds up within the wood frogs cells. Urea protects the frog from cellular damage and acts as a metabolic depressant, slowing the metabolic rate as they enter hibernation.
The wood frogs in Alaska may have larger freeze tolerances than the same species in warmer, more southern climates. This is due to the successive freezing/thawing temperatures in early October amplify the glucose concentrations [Larsen].
As soon as the frogs thaw in the spring, they wake up and begin to move around. This is necessary, as the short summers in Interior Alaska don’t allow much time for breeding.
Reproduction and Lifespan
As soon as thawing and waking from hibernation, wood frogs migrate to their breeding ponds or pools (typically seasonal pools that dry later in the season). They breed from March to May, the males attracting females by making a duck-like quaking croak. They mate by amplexus, the male hooking its thumbs around the female so that she deposits her eggs (1000-3000 at a time), often in the deepest area of the water before depositing his sperm in the water.
The eggs hatch in 9-30 days, and the emergent tadpoles develop into froglets after approximately two months. The development time of the tadpoles is dependent on the temperature of the water and the availability of food. The males become sexually mature adults after 1-2 years, the females taking 2-3 years. They will reproduce every year they are sexually mature until death, with a lifespan of 3-5 years.
Wood frog larvae and tadpoles feed on algae or decaying plant or animal matter. The tadpoles may also feed on the eggs of other wood frogs. Adult wood frogs primarily feed on insects and spiders.
Range and Habitat
The wood frog is widely distributed across North America. Its range extends from western and northern Alaska, across much of Canada and the eastern midwest states, and the eastern US from New England to northern Georgia. It is also found in smaller regions in Idaho and Wyoming.
The wood frog gets its name from its primary habitat, woodlands, and boreal forests. While they breed in ponds, lakes, or other wetlands, they are frequently found away from the water, on the forest floor, or under rocks or logs.
References and Further Reading
Costanzo, Jon P., Lee, R. E., (2005). Cryoprotection by urea in a terrestrailly hibernating frog. The Journal of Experimental Biology. 208, 4079-7089.
Larson, D. J., Middle, L., Vu, H., Zhang, W., Serianni, A. S., Duman, J., Barnes, B. M., (2014). Wood frog adaptations to overwintering in Alaska: new limits to freezing tolerance. The Company of Biologists Ltd | The Journal of Experimental Biology. 217, 2193-2200.
King, R. B., King, B., (2011). Sexual differences in color and color change in wood frogs. Canadian Journal of Zoology. 1991, 69(7): 1963-1968
Van Bergen, Yfke, (2005). Urea Protects Frogs From Frost. Journal of Experimental Biology.
Wood Frog, The National Wildlife Federation (accessed August 2020)
Lithobates sylvaticus Wood Frog, Katie Kiehl, Animal Diversity Web (Accessed August 2020)
Biological Miracle, National Parks Service – Gates of the Arctic
All websites accessed July-August 2020 unless otherwise noted