The only amphibian to live north of the Arctic Circle, the wood frog (Lithobates sylvaticus) has an amazing ability to undergo freezing and thawing to survive cold winters.
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As you might guess from its common name, it lives mainly in woodland, only taking to shallow pools in springtime to breed, the males uttering a distinctive quacking call to attract mates.
What does a wood frog look like?
The most recognisable characteristic of this small frog is a band of dark brown or black running from the tip of its snout, out over its eyes like a mask. Otherwise, the wood frog iswell camouflaged for its woodland environment, its colouring varying from tan to dark brown, rust red to greenish, with splodges or bands of darker pigmentation.
Thanks to pigment-containing cells in their skin called chromatophores, wood frogs are able to change colour in response to environmental cues such as temperature, light and surrounding vegetation. Unlike with famous colour-changers like chameleons or octopus, however, this transition takes place over hours and days rather than seconds. Female wood frogs tend to be more brightly coloured.
How big is a wood frog?
Adult wood frogs typically grow to between 3.7 and 8 cm in length, weighing up to 8g. Females are larger than males.
Where do wood frogs live?
Wood frogs live on the ground in woodland, forested swamps, tundra and grassland across Alaska, Canada, and the northeastern states of the United States.
There are small populations living as far south as the state of Alabama. They are the only frog – and indeed the only amphibian – to live north of the Arctic Circle, capable of doing so because of their ability to survive being frozen solid during winter.
Unlike most amphibians, who spend much of their lives in water – ‘amphibian’ is derived from a Greek word meaning ‘living a double life’ – the wood frog lives mostly on the forest floor, only seeking out pools to breed in springtime.
How do wood frogs survive freezing?
With the coming of winter in northern climates, most frogs will hibernate underwater, where temperatures get very cold but crucially stay above freezing. Not wood frogs, who instead take shelter in leaf litter, ready to undergo an extraordinary transition. As the temperature drops below freezing, the frog freezes solid, with up to 70 per cent of the water in its body turning to ice.
Crucially, though, it is the liquid outside its cells and organs that freezes, rather than inside, where freezing would cause irreparable damage. The frog prevents its cells freezing by producing substances called cryoprotectants – essentially antifreeze made from glucose and urea.
More cryoprotectants are produced in populations in colder locations. This adaptation means that the wood frog can survive at temperatures as low as -16°C. During this time, the frog stops breathing, its heart stops beating and its kidneys stop functioning.
How long can a wood frog stay frozen?
Wood frogs have been recorded staying frozen for as long as eight months.
What happens when the wood frog thaws?
As temperatures rise in the spring, the frog thaws, starting with its heart and brain – within 10 hours, it is back to normal function. Scientists are studying this freeze-thaw process in the hope of developing technology for the storage and transportation of human organs for transplants – human organs cannot survive the freezing process currently.
How do wood frogs breed?
Breeding takes place in so-called vernal pools, areas of wetland that only fill up in the wet season. Because they are dry the rest of the year, these pools tend to be free from predatory fish that would feed on the frog’s eggs. Male wood frogs make duck-like calls to attract female from early springtime.
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Female frogs will follow the sound of the calls to the centre of the pool, where a mating frenzy takes place, with males grabbing on to other frogs indiscriminately until they find one large enough likely to be female.
Females are grabbed by multiple males, forming a ‘mating ball’ of up to more than a dozen individuals. The male who holds on the longest – using thumbs that have swollen up during breeding season for the purpose – wins making rights. The male then wraps his forelimbs around the female’s torso – a move known as amplexus – and squeezes her until she lays her a mass of up to 3,000 eggs into the water, ready for him to fertilise.
How long do wood frogs live?
Wood frog eggs hatch into tadpoles after between nine and 30 days. It takes a further two months for the tadpoles to turn into froglets, during which time they tend to associate with siblings from their own egg mass – scientists don’t know why. Wood frogs reach sexual maturity at around two years old, and have a lifespan of around three years in the wild.
What do wood frogs eat?
Adult wood frogs enjoy a varied diet of small invertebrates, including insects, spiders, snails, slugs and worms. Tadpoles, meanwhile, tend to be herbivorous, mainly feeding on algae and plant detritus. They will however sometimes feed on the eggs or tadpoles of other amphibians.
What threats do they face?
As adults, wood frogs are prey for a wide range of predators, including snakes, raccoons, snapping turtles, coyotes, foxes and birds. Tadpoles are preyed upon by salamanders, beetles, wood turtles and adult wood frogs. Wood frogs are a species of Least Concern on the International Union for Conservation of Nature (IUCN) Red List, with a stable population.
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