Below freezing but still moving: How salamanders stay active in winter

Source: The Conversation – Canada – By Danilo Giacometti, São Paulo Research Foundation Postdoctoral Fellow, NUPENS (USP)

It’s a cold night in a Canadian forest. Temperatures are below freezing, snow is on the ground and most animals remain hidden to avoid the harsh conditions. But one creature is braving the cold weather.

Blue-spotted salamanders (Ambystoma laterale) are on the move. These small amphibians have left their underground winter refuges and started migrating toward breeding pools, often risking freezing.

For animals whose body temperature closely follows the environment, like salamanders, freezing can be deadly, as the formation of ice inside the body can damage cells and disrupt vital functions. However, blue-spotted salamanders remain active and apparently unharmed under the freezing conditions.

As a group of biologists who study how temperature shapes animal behaviour and physiology, our team has been long fascinated by animals that keep active at low temperatures.

In a recent study, colleagues and I documented salamanders migrating with body temperatures below 0 C in the wild. Our findings highlight field-based evidence that these amphibians can function at sub-zero temperatures without freezing. In doing so, salamanders are possibly extending their breeding season.

Survival strategies

Amphibians have a thin skin that is supported by many blood vessels. This allows them to breathe through their skin; however, it also means they lose a lot of water through evaporation, especially under warm conditions. That means both exposure to extremely cold (risk of freezing) and warm temperatures (risk of desiccation) can be harmful. Consequently, amphibians are often described as extremely vulnerable to environmental change.

At the same time, amphibians are not passive victims of their environment. Many species rely on different strategies to cope with winter, a season that can last several months in parts of Canada. As winters become more unpredictable, with frequent freeze–thaw cycles and reduced snow cover, understanding these strategies is key.

Amphibians generally survive winter through freeze tolerance or freeze avoidance. Freeze-tolerant species can survive even if parts of their bodies are frozen solid. The wood frog (Lithobates sylvatica) is a well-known example: during winter, the frogs allow ice to form in their tissues while their cells are shielded by large amounts of natural antifreeze, like glucose. As temperatures rise in spring, the frogs thaw and resume their normal activity.

Instead of tolerating ice formation, freeze-avoidant animals avoid freezing altogether by keeping their body fluids in a liquid state. This can be achieved by moving to underground refuges, or, as in the case of blue-spotted salamanders, through a physiological process called super-cooling.

Supercooling: staying liquid below zero

To understand how blue-spotted salamanders cope with early spring cold, we studied migrating individuals in Ontario’s Algonquin Provincial Park. We used infrared thermal cameras to measure salamanders’ skin temperatures (an approximation of body temperature) as they moved naturally across the forest floor to make their way to Bat Lake.

This approach allowed us to record body and forest floor temperatures without the need to handle the animals, which is relevant because direct contact can quickly alter their body temperature.

As freeze-avoiders, blue-spotted salamanders would likely die if their body fluids were to freeze. This makes their early spring movement particularly risky, but this is where super-cooling comes in. Super-cooling occurs when liquids inside the animal’s body remain unfrozen below their normal freezing point.

This strategy occurs through a combination of processes. The animal removes surfaces in its body where water molecules can collect and start crystallizing through gut evacuation and partial dehydration. Producing antifreeze (typically sugars) and accumulating them in certain body parts (like the liver) also help in the process, although this occurs in smaller proportions than in freeze-tolerant species.

Previous lab-based research had shown that blue-spotted salamanders could super-cool down to about -1.5 C. Our field measurements not only validated the importance of super-cooling under natural conditions, but also demonstrated that salamanders can remain active at temperatures far below their known minimum super-cooling point, with values as low as –3.6 C.

Since we recorded individuals actively moving over ice, our observations confirm that the animals were not frozen. These results show that salamanders can push their limits further in nature than expected from lab tests alone.

Why do salamanders take the risk?

Super-cooling is unstable. Contact with ice or sudden temperature changes can trigger rapid freezing. That means salamanders moving over ice are operating close to their physiological limit. Why take the risk, then?

The answer lies in timing. Blue-spotted salamanders breed in temporary ponds formed by snowmelt and spring rain, and their breeding season lasts only a few weeks. Arriving at the ponds early increases access to suitable egg-laying sites and reduces competition. Although waiting for consistently warm nights would be safer, it could also mean missing the breeding window altogether.

Early migration appears to be a trade-off: higher short-term risk of freezing in exchange for long-term reproductive success. This behaviour also shows that salamanders respond to small changes in environmental conditions, not just average temperatures.

Winter brings with it various challenges, and opportunities, for animals. Whether they hibernate, develop antifreeze or stay super-cooled, various species have developed fascinating ways of surviving the freezing temperatures.

Danilo Giacometti receives funding from the São Paulo Research Foundation (Brazil).

– ref. Below freezing but still moving: How salamanders stay active in winter – https://theconversation.com/below-freezing-but-still-moving-how-salamanders-stay-active-in-winter-274100