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The freeze-and-thaw cycle of the Yukon’s wood frogs

The amphibians have the special ability to freeze solid without sustaining any damage to their cells
Heidi Miller/Yukon News A wood frog in a bucket after being captured in Wolf Creek in Whitehorse in 2015. Wood frogs hibernate frozen-solid under mud during the winter months.

In the popular cartoon series Futurama, the main character, Philip J. Fry – a hapless pizza delivery boy with a kind heart and less-than-average intelligence played by voice-actor Billy West – finds himself living in the future when he accidentally flash-freezes himself on New Year’s Eve, 1999. Recklessly blowing a party kazoo while perched precariously on the two back legs of a chair – something your grade-school teachers doubtlessly warned you never to do – Fry tumbles backwards into a cryogenic pod and is immediately frozen like a popsicle, only to to thaw out, unharmed, exactly 1,000 years later.

While this is the stuff of great science-fiction (and hilarious animated hijinks) real, everyday living creatures die (most unpleasantly) when frozen solid – unless, of course, you’re a wood frog, says Carrie McClelland of Environment Yukon’s biodiversity program.

In many animals – including humans – unprotected exposure to cold temperatures can cause hypothermia, wherein the body temperature falls dangerously – even fatally – low. Unprotected skin exposure to sub-zero temperature causes frostbite, which occurs when blood cells constrict and ice crystals form in the blood, damaging (and sometimes killing) tissues. In the average living creature (or plant) death would occur long before being frozen solid (and, if you’ve ever had frostbite, would probably be pretty uncomfortable).

The innocuous-looking wood frog (Lithobates sylvaticus), however, has an unusual adaptation to the cold — it hibernates frozen-solid under the mud during the bitter Yukon winter.

Under normal circumstances, fully-frozen cells “burst like a frozen Coke bottle,” says McClelland, but wood frogs have a special chemical in their blood which allows them to endure punishing sub-zero temperatures during hibernation without damage.

“There’s pretty complex chemistry behind it,” says McClelland, “but basically (wood frog) cells produce a type of antifreeze that prevents the water from expanding and bursting their cells when it freezes solid. The reason the rest of us can’t freeze solid is because our bodies have so much water in them and water expands as it freezes, so all the water in our cells would expand and then burst.”

Wood frogs start their year much earlier than you would think, says McClelland, beginning their mating season as soon as the ice starts to get off the lakes. They begin breeding almost immediately, finding suitable holes in the ice and getting down to the all-important frog business of making more frogs. Many Yukoners have probably heard their mating call and not even known it, says McClelland; their amorous chorus is often mistaken for the calling of ducks, to which it sounds similar to an untrained human ear.

Female wood frogs lay their eggs underwater, amid aquatic vegetation. They hang out there for about a month in May, when they hatch into tadpoles — those wriggly, legless creatures so familiar to curious children — in June. By August, the tadpoles have undergone their metamorphosis and developed into land-dwelling but sexually-immature froglets. By September, they are all grown up and frantically feeding in preparation for the long hibernation period, which usually begins in October. The entire reproductive process takes between seven and 12 weeks — the fastest of any frog in North America, a necessary adaptation to the Yukon’s short summers.

These special adaptations not only allow the wood frog to survive in the North, but thrive, and the species is by far the most wide-spread and successful amphibian in the Yukon, says McClelland. Wood frogs — which grow up to six centimetres long and are identifiable by their white jaw stripe, cream-coloured underbelly, black eye-mask and a pale line running down their darker back — are found as far north in the territory as Old Crow, a much farther northern reach than their more southerly cousins. By contrast, the Yukon’s other two species of frog — the Columbia spotted frog (Rana luteiventris) and the boreal chorus frog (Pseudacris maculata) — occupy small areas in the southernmost regions of the territory.

No population studies have been done on wood frogs in Yukon, although they are a common sight and are assumed to be “an abundant species,” says McClelland, although she notes that amphibian populations around the globe are currently in decline. Amphibians — including newts, salamanders and toads, not just frogs — are extremely sensitive to environmental pollution and changes to their environments. Additionally, many species around the world have suffered from infection by Batrachochytrium dendrobatidis (called Bd for short), a type of chytrid fungus. Bd grows on the porous skin of amphibians, excreting chemicals which unwind the proteins in the animals’ flesh — essentially turning the skin into an amino acid slurry and devouring it – causing dermal shedding, slowed growth and death in a matter of weeks.

McClelland says that we “don’t seem to have a huge problem” with Bd in the Yukon, but that “it’s a good thing to remember, and remember not to be touching frogs, because you can transfer the fungus to them this way.”

As climate change ramps up — keeping in mind that the North, particularly the Yukon, is warming at three times the global rate — things might change for the wood frog, McClelland notes, as it might allow other frog species to expand their ranges.

“We don’t always know what kind of effects climate change will have on populations, but I would predict that the effect on wood frogs would be the introduction of other frog (species) and predators moving farther north,” says McClelland. “It’s not so much that conditions (would change to be) harder for wood frogs, but that it would be easier for other competitors to be here, which could impact wood frogs.”

McClelland says that Yukoners can help understand these changes in populations by reporting unusual frog species in their areas.

The boreal chorus frog is slightly smaller — growing to only two to four centimetres — than the wood frog, is green or brown, with a slightly rounder body, three irregular stripes on its back and a white or olive-coloured body and is typically only found in the La Biche River area, east of Watson Lake. The Columbia spotted frog — larger than the wood frog, growing to eight centimetres in length — is olive or tan-coloured, with red undersides on the back legs, a cream-coloured belly, and a pattern of irregular black spots with lighter centres. It is usually found at Bennett Lake in Carcross, or the Hyland River and Irons Creek Lake areas east of Watson Lake.

The Yukon also has one species of toad, the western toad (Anaxyrus boreas), usually found only in the Liard River basin area. Western toads are the largest of Yukon’s amphibians, growing up to 12.5 centimetres and are often found far from water. They are grey, brown or black with lots of warts and bumps, a stocky body with a stripe down the back and short legs.

Spotting these species — or others — outside of their normal habitats is not cause for alarm, but reporting them to Environment Yukon can help researchers better understand the changing environment, says McClelland, leading to a better understanding of amphibian populations.

Remember though: don’t touch the frogs!

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