Freshly Baked Science
Cold-Blooded Animals In The Cold
4th March 2019
Have you ever wondered what happens to cold-blood animals (ectotherms) in the cold? It’s not like you ever see a turtle wearing a puffer jacket or a frog with mittens on its feet. Do they migrate south like geese or hibernate like bears? Do they simply give up and freeze solid? Do they do something else entirely?
The answer to all of these questions is yes! Ectotherms use some strange and amazing strategies make it through the winter season.
Monarch butterflies move south
Credit: USFWSmidwest [Public domain]
Sometimes I wonder if birds are smarter than us. Huge flocks of them prudently migrate to the tropics – Florida, the Caribbean, Spain – when it gets nippy outside. But birds aren’t the only ones able to fly greater distances.
Many insects smaller than your hand, like the monarch butterfly, travel thousands of miles to escape the winter. Somehow these delicate insects fly over 4,800 km from Canada to the Sierra Madre Mountains of Mexico and come back. Less than 10 cm across, these butterflies keep a brisk pace of 80 to 160 km per day for 2 months. This is the equivalent of 2-4 marathons, every day!
Monarchs form massive groups once they reach their southern overwintering grounds, covering the branches of every tree, bush, and flower within reach. Researchers visit the mass congregation every year and measure how much land the butterflies occupy to estimate the total global population of the species.
Garter snakes snuggle up
Credit: Oregon State University [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)]
There’s nothing like spending a cold morning under a blanket with some hot tea and a good book. As the mercury dips, garter snakes find themselves a log, burrow or basement to curl up in and wait out the cold. Because the snakes stay awake during the winter, they look for underground spaces below the frost line or spots that are insulated enough to stay warm.
In northern Canada and the United States, where the air temperature dips to -40 degrees Celsuis, such cosy places are rare. Snakes will travel long distances to a proven hibernaculum like the Narcisse Snake Dens in Gimli, MB, Canada. They’ll also tolerate roommates… a lot of roommates. Groups of dozens, hundreds, and sometimes even thousands of snakes congregate in dens.
All that snuggling gets the snakes feeling frisky. Once the outside world thaws, the snakes emerge en masse and form huge mating balls, where dozens of males compete for time with a single female.
Turtles breathe through their bum
It’s one thing to spend the winter with your buddies, either on a tropical vacation or in a cosy cabin. Freshwater turtles instead stick out winter alone in the mud.
Before the water freezes, the turtles sink to the bottom of their lake or pond and bury themselves in mud. It sounds like a great idea, except that turtles have lungs and not gills, so can’t breathe underwater like fish.
Luckily, turtles can pull oxygen into their blood using a body part with lots of blood vessels: their butts. The polite term for this is cloacal respiration.
Now, even turtle butts with all their blood vessels are no lungs, and can only bring in so much oxygen. Though the turtle’s metabolism is very low in the winter, their butt breathing doesn’t bring in enough oxygen to fuel all their tissues. Turtles use another set of chemical reactions, called glycolysis, to make up for the shortfall.
A side-effect of glycolysis is that it produces a lot of lactic acid, the chemical the causes your muscles to burn during intense exercise. Turtles get around this problem with their shells. The magnesium and calcium carbonates in the shell react with the acid, just like an antacid for heartburn, and get rid of it. This helps keep acid levels manageable during the long months of low oxygen.
Carp and goldfish get drunk
Carp living in northern Europe and Siberia face the same problem as turtles: icy lakes have very little, if any, oxygen in them. Like turtles, they rely on glycolysis to fuel their bodies. But carp don’t have heavy, mineral-rich shells to buffer all that nasty acid. Bizarrely, these fish opt to drown their winter blues in alcohol.
To accomplish this feat, the fish rely on a strange protein similar to the one in (beer) Brewer’s yeast. This protein converts lactic acid into ethanol, which the carp can then dump out of its body through its gills. The strategy is so effective that carp blood alcohol levels can reach 50 mg per 100 milliliters: above the legal driving limit in Scotland, and in the Scandinavian countries they call home.
Wood frogs freeze solid
Wood frogs don’t flee south in autumn, or huddle up, or use some biochemical trickery to get rid of acid. If anything, these amphibians seem to hunker down, accept their fate, and turn into froggy popsicles. Literally.
As much as 70% of the wood frog’s body freezes solid. For any other animal, this leads to intense, lethal, frostbite. Somehow, the frogsicles melt and get on with their lives in the spring. Winter is simply a pause button in their lives.
The trick is that the frog freezes very, very carefully. It crams its cells full of an antifreeze-like sugary syrup. This lowers the freezing point of the fluid inside cells, and prevents the cells from shrivelling up and dying. The rest of the water, located outside the cells, in places like the skin, abdominal cavity, and in the lens of the eyes, freezes.
If you could have any of these warmth tactics in the winter, which would you choose? Send us your answer to or our social media (@wonkscience).
Cloacal respiration: Oxygen uptake through the cloaca of an animal. The cloaca is where the digestive and urinary systems excrete waste.
Ectotherm: an animal that relies on its environment to regulate its body temperature. Most animals, except birds and mammals, are ectotherms.
Glycolysis: A metabolic pathway that produces the high energy molecule ATP, but does not require oxygen.
Hibernaculum: a place or den where an animal undergoes hibernation.