During summer in the Pembina Valley, amphibians and reptiles are hard to miss.
From choruses of toads and slithering garter snakes to painted turtles lined up on logs in the sunlight, the animals are a fixture of the environment.
In winter, however, these sights vanish.
While it might seem as though the animals are absent, their lives under the snow and ice rely on overwintering strategies that brim with life.
Frogs: Freezing solid
Dr. Randy Mooi, curator of zoology at the Manitoba Museum, said that the animal kingdom in winter employs two overarching strategies to combat plunging temperatures: freeze tolerance and freeze avoidance.
For some frogs, freeze tolerance is key.
As amphibians who can’t create their own heat or regulate their temperatures like mammals can, they combat the winter by becoming a part of it — they freeze solid.
“[During warmer months], they can change their behaviours, like sitting in the sun … to raise their temperature or going into the shade to lower it, but during the winter, they take on the temperature of their environment,” explained Dr. Mooi.
We think things are quiet and dead and that not much is happening, but in the animal world, there are quite a lot of things. Even though they’re slowing down, there’s a lot of things happening in their bodies — and in their behaviours — that are pretty important for them to survive.
-Dr. Randy Mooi on winter in Manitoba.
Wood frogs, tree frogs, chorus frogs, and peepers are some of the amphibians that use this method during the cold season.
“They produce what is sometimes called an antifreeze. It’s sort of a high sugar content in their bloodstreams …. They keep ice crystals from forming in their cells because that would be deadly, and they’re actually able to almost create this sort of bubble of ice in their bodies around their important organs.”
Dr. Mooi added that the frogs usually tuck themselves into spots on the forest floor where they won’t be disturbed by predators over the winter.
“Then in the spring, when things warm up, they thaw out and go on their merry way,” he said.
“It’s a pretty amazing ability.”
Frogs: Taking cover beneath the frostline
For frogs that cannot freeze, the solution is to overwinter beneath the frost line.
One strategy is to dive to the bottom of ponds and spend the winter there, slowing their metabolisms while relying on the remarkable ability to absorb oxygen through their skin.
Dr. Mooi noted that while this approach works well, frogs overwintering in ponds are at risk of winter kill, a phenomenon that occurs when oxygen levels in the water fall too low.
“These frogs — and fishes, for that matter — will expire because they don’t have access to oxygen,” he said.
“When the ice melts in the spring, sometimes you’ll see these animals floating along the shoreline or on the top of the water because they’ve run out of oxygen during the winter as they’re trying to avoid the freezing temperatures.”
Sometimes you might have ponds where you’ll see a winter kill with fish and maybe some frogs floating around after the winter, but there’ll be a painted turtle swimming happily around because they’re able to avoid that issue.
-Dr. Randy Mooi on how anerobic respiration helps turtles survive the harsh conditions of winter.
The causes of winter kill can vary, but plants sometimes contribute to it.
“In the winter, the plants, for the most part, are not photosynthesizing, and they may be just rotting and using up oxygen,” he explained.
“And, of course, there’s ice on the top of the pond, so there’s no access to the oxygen that’s in the atmosphere.”
Toads, salamanders, and snakes: Winter house guests
For toads, salamanders, and snakes, which must also get beneath the frost line, winter preparation sometimes means waiting out the cold temperatures as uninvited houseguests.
“They have to make sure that they don’t freeze at all, so … toads usually burrow into the sand, or they might go into rodent burrows,” said Dr. Mooi.
He added that some animals, such as snakes, also use loose soil from ant nests to avoid freezing.
Young snakes, in particular, will seek shelter from cold weather in the underground tunnels and chambers left by ants.
Turtles: ‘A very unusual ability’
Manitoba’s two turtle species, the western painted turtle and the snapping turtle, also need to avoid freezing, which drives them to the bottom of ponds, like their amphibian neighbours.
Their strategy for oxygen, however, is different — they find a way to avoid needing it.
“They can actually use what’s called anaerobic respiration, which is being able to create energy to keep their bodies running through the winter without oxygen, a very unusual ability,” said Dr. Mooi.
The curator noted that anaerobic respiration also occurs in humans during large energy expenditures, such as a long run or an intensive workout, resulting in lactic acid buildup that causes soreness.
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Although the process is similar in turtles, there are several significant differences.
“When we feel the lactic acid burn, we’re using a lot of energy all at once, but during the winter, turtles have really shut down their metabolism quite substantially, so they’re hardly using any energy at all,” he said, adding that in winter, a turtle’s heart rate can be as slow as beating once every five or even ten minutes.
“So even if they’re using anaerobic respiration and creating lactic acid, they’re creating very, very small amounts of it,” he said.
“They are able to use their shells and their bones as a buffer, so they can neutralize that acid using the calcium that’s in some of their bones.”
He added that this process also helps turtles survive in ponds with low oxygen levels.
“Sometimes you might have ponds where you’ll see a winter kill with fish and maybe some frogs floating around after the winter, but there’ll be a painted turtle swimming happily around because they’re able to avoid that issue.”
Fish: The sweet spot of 4 °C
According to Dr. Mooi, fish also dive deeper during the cold season.
For aquatic animals, water temperature is an important factor.
“The densest water is usually around four degrees Celsius, and so that sinks to the bottom and forces the other water upwards, so colder water is actually near the surface,” he explained.
“That’s why some of these fish go down further, because they’ll have a stable environment in that [temperature].”
Frozen water at dawn.
Dr. Mooi added that because oxygen levels can be lower near the bottom, fish often spend time around the middle of the water column.
They may also find places to hide, such as rocky areas or logs, because they also slow down for the winter and expend less energy.
“That’s why, generally, when you’re ice fishing, you’re not dragging the lure around,” he explained.
“[Fishers] are just sitting there with flags, … and the fish will be attracted to those because they still eat, but they need to go after something a little slower.”
A new life in winter
While some animals adapt to the winter, for others, plummeting temperatures may even be something to embrace rather than endure.
Dr. Mooi said that research on species such as lake trout (which tend to seek colder water even in summer) indicates that having more cold water to traverse in winter alters the species.
“They end up moving from some of the deeper areas of the lakes that maybe don’t have as much complicated environment into some of the shallower areas where there may be rocks and logs and things like that,” he said.
“There have been some studies that suggest their brains actually increase in size slightly in the winter. The idea being that their brains are adapting to the kind of environment they’re in.”
He added that certain organisms even require a cold spell to complete their life cycle, like turtles, for whom the sex of offspring is affected by temperature.
“When eggs are laid, there are certain temperatures that will produce males, and at other temperatures, the eggs will produce females or a mix of males and females,” he said.
The hidden wonder of winter
For Dr. Mooi, looking at how amphibians, reptiles, and fish survive the winter highlights that beneath the seemingly still surface, the season is full of hidden activity.
“We think things are quiet and dead and that not much is happening, but in the animal world, there are quite a lot of things. Even though they’re slowing down, there’s a lot of things happening in their bodies — and in their behaviours — that are pretty important for them to survive,” he said.