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Scientific name: Rana sylvatica
Common name: 
Wood Frog

(Information in this Species Page was compiled by Laura Pacuch in Biology 220W, Spring 2003, at Penn State New Kensington)

The wood frog (Rana sylvatica) is a relatively small (1 to 3 inches long) black masked frog that occurs in a great range of locally variables colors including red, yellow, gray and brown. Coloration is thought to be greatly influenced by benefits of camouflage over a wide variety of its potential habitats. The wood frog has a flat body, pointed head, webbed feet with un-webbed distal toe joints, and smooth, moist skin. In the more northern portions of its considerable geographic range, the wood frog increasingly displays a light, mid-dorsal stripe. Northern dwelling wood frogs also have shorter legs and what is described as a more “toad-like appearance and pattern of movement” than individuals found in southern regions. Range
The wood frog is found from northern Georgia all the way up to the Arctic Circle. Its range includes 31 states and 13 Canadian provinces. It is the only “cold blooded” vertebrate known to occur north of the Arctic Circle. Habitats favored by wood frogs include tundra, wet grasslands, and moist woodlands. Typically, they utilize temporary pools formed by spring rains and snow melt as breeding pools and spend most of the rest of their active season away from standing water.

Diet and Predation
Throughout their relatively short lives (less than three years) wood frogs rely primarily on a wide range of insects and other invertebrates for food.  They use their long, sticky tongues to capture prey and are said to eat “anything that they can fit into their mouths.”  They are also subject to predation by a great variety of snakes and birds. Cryptic coloration is their principle defense mechanism against predators. Their mucous covered skin also enables the frogs to slip out of the grasps of potential predators.

Physiological Adaptations to Cold
The ability of R. sylvatica to survive in high latitude ecosystems is dependant upon a number of specialized physiological adaptations to sub-zero temperatures and extensive body freezing. The wood frog can survive sustained body temperatures as low as -6 degrees C during its hibernation cycles. These adaptations include the presence of specific proteins in the blood that regulate ice crystal formation, circulatory controls that shunt blood preferentially into critical organs (heart, liver, brain), and a liver response that releases huge amounts of glucose into the blood stream and organs. These high levels of glucose act as a cryoprotectant which reduces the amount of ice formed in the protected tissues and cells. The eggs and embryos of R. sylvatica also exhibit physiological adaptations to cold (see 'Mating and Reproduction', below).

During hibernation, 60 to 70% of a frog’s body fluids may be frozen. Cardiac function stops and blood ceases to circulate. As their bodies thaw, their hearts resume contractions and their livers rapidly clear the high levels of glucose from the blood even before all of the body’s ice is melted. Thawing frogs display some short term movement and behavioral difficulties but are soon fully functional. Freezing for periods longer than one month, though, leads to rising levels of tissue and organ damage in the frogs. The longer a frog is frozen, then, the greater the magnitude of over-all tissue damage and thus the greater the probability of hibernational fatality.

Mating and Reproduction
As soon as an adult wood frogs thaws, it moves to its breeding pools. The isolated melt and rainwater pools it favors are typically free of fish and other potential egg and tadpole predators, but they are also inherently transient and dependent upon unpredictable weather conditions. The use of these temporary pools, then, represents a very delicate, ecological “cost-benefit” balance for R. sylvatica.

In the mating pools, males call to females with their “duck-like” songs. An attracted female enters the pool and is quickly grasped on the back by the smaller male (this is called “amplexus”). The male may remain in place on the female’s back for 24 to 72 hours. The male releases sperm into the pool water as the female ovulates and thus externally fertilizes the forming egg mass. A typical egg mass contains 1000 to 2000 eggs. The female moves the floating egg mass into the shallow areas of the pool in a large, communal raft. Counting these rafts in an area’s pools is an accepted, and highly efficient, way to determine the population density of R. sylvatica in a particular region.

Egg and Embryo Physiological Adaptations to Cold
Since mating and egg laying occur very soon after ice melt, the chance of seasonal, sub-zero temperatures re-occurring is quite high. The eggs and embryos of R. sylvatica have an interesting adaptation that enable them to survive both transient and sustained periods of freezing and sub-freezing temperatures. The melting point (i.e. temperature at which material changes from a solid (frozen) to liquid state) of the mucopolysaccharide and mucoprotein “jelly” that surrounds the eggs and the developing embryos is higher than that of the fluids inside the egg. As temperatures fall, then, the jelly freezes before the egg or embryo. This freezing osmotically draws water out of the egg into the jelly mass. The dehydrated egg and embryo, then, are more resistant to freeze damage and are thus able to better survive the early spring temperature fluctuations. Larger embryos in particular, are more tolerant of longer periods of freezing, so severe weather patterns may generate a selection pressure for faster growing, and, thus, larger and more resistant embryos. It takes one and a half to two months for the embryos to fully develop into free swimming tadpoles.

Tadpoles are under intense predator and parasite pressures. The tadpoles feed on aquatic plants and grow quickly eventually developing limbs and lungs as they emerge out of their pools as tiny frogs.

Ecological Pressures
All amphibians including the wood frog are under intense ecological pressures due to human activities. Loss of habitat and breeding pools due to de-forestation, road building, and agriculture is a significant cause of declining wood frog numbers. Acid rain and other forms of air pollution contaminate breeding pools and seriously damage adult frogs and also their eggs, embryos and tadpoles. Rising levels of ultraviolet radiation due to the degradation of the stratospheric ozone layer is also quite damaging to these delicate organisms.

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