Life in the Arctic: Super Cool Squirrels

Life in the arctic is defined by extremes! Winters in the arctic are long and dark. Strong, bitter winds whip through the landscape. Extreme cold temperatures drop as low as -50°C in the Arctic. (Antarctica is the world record holder for lowest air temperature. On July 21^st, 1983 at Vostok station the thermometer measured a numbing -89.2°C or -128.6°F.) Much of the soil is frozen in permafrost, which remains frozen year round.

Summer is merely a short reprieve for animals to build up their fat stores and prepare for the next long winter. Despite harsh conditions, artic animals have spectacular adaptations to survive extreme cold.

Arctic National Wildlife Refuge. Photo by Troutnut/Shutterstock

Why does cold adaption matter?

Maintaining a consistent internal body temperature is extremely important and energetically costly in cold environments. The temperatures inside our bodies maintain cellular integrity, impact proteins and enzymes, and allow our organs to function properly. When humans (or other animals) get too cold, we go into a state of hypothermia where our body temperatures get dangerously low. In humans, hypothermia sets in around 95°F (35°C) and, if we don’t warm back up, results in shivering, slurred speech, loss of consciousness, frostbite or even death! Researchers have been investigating how animals survive extreme cold for decades, but it wasn’t until 1989 that someone looked at an impressive, small hibernator in the Arctic… and they were in for a surprise!

Enter the Squirrel

Arctic ground squirrel. Photo by Daniel J Cox

Arctic ground squirrels, Urocitellus parryii, are the most northerly ground squirrel in North America. While soil temperatures drop to -18°C and air temperatures get even lower, these sleepy squirrels spend 7-9 months of each year hibernating underground in burrows. To figure out what temperature arctic squirrels maintain during hibernation, Barnes (1989) implanted temperature-sensitive radio transmitters (essentially a thermometer in their body cavity that sends temperature data to a transmitter at regular intervals) in Arctic squirrels. He then monitored their internal body temperature during hibernation in outdoor enclosures. Over the course of hibernation, Arctic squirrels keep their core body temperature below freezing for 2-3 weeks at a time and warm back up to normal body temperature for a brief 12-15 hours. The lowest body temperature recorded was -2.9°C!!

Barnes (1989) Figure 1A: Core body temperature in a female 
Arctic ground squirrel during hibernation. Body temperature is maintained
 BELOW FREEZING for 2-3 weeks at a time! 

By reducing their body temperature so dramatically, Arctic squirrels can hibernate in cold conditions down to -25.9°C before ‘alarm arousal’, where temperatures get too cold and squirrels must wake up to rewarm (Ritcher et al. 2015). These cycles of 2-3 weeks at sub-freezing body temperature and 12-15 hours at normal body temperature repeats throughout the entire hibernation period (Barnes and Ritter 1993), until spring when Arctic squirrels end hibernation and start packing on the pounds in preparation for their next deep slumber.

  

Barnes and Ritter (1993) Figure 3: Male (left) and female (right) Arctic ground squirrel body temperature during a hibernation cycle in outdoor enclosures. Notice that males wake up earlier in the season than females and that squirrels raise their body temperature every 2-3 weeks. 

After this initial finding, Barnes brought arctic squirrels into the lab, housed them in temperature controlled environmental chambers, and slowly lowered the temperature down to -4.3°C. During this process, Barnes monitored temperature in different parts of the Artic squirrels body. Arctic squirrels were able to maintain different temperatures in different regions of their bodies! Even though their core body and feet were kept at -1.3°C, the area around their brain and neck were kept at a ‘balmy’ 0°C.  By keeping their brains slightly warmer than the rest of their body and rewarming to a normal temperature every 2-3 weeks, Arctic squirrels keep their brains happy and healthy (i.e. not freezing solid) despite the impressive drop in body temperature.  

Barnes (1989) Figure 2: Regional body temperatures in Arctic ground squirrels 
during hibernation at -4.3°C. The area around the head is kept at a slightly 
higher temperature than the core body temperature. This keeps 
the brain 'cozy and warm'.... or at least not frozen!

But why don’t the squirrels freeze solid?!

            Despite being below freezing for 2-3 weeks at a time, Arctic ground squirrels don’t freeze solid and don’t get frostbite! How do they manage it? Barnes (1989) sampled their blood and couldn’t find evidence for any of the known mechanisms other species use to withstand cold (i.e. antifreeze proteins; proteins that inhibit crystal growth in animals and lower their freezing temperature). The only remaining explanation is supercooling.

Supercooling is the technical term for when a liquid is below its freezing temperature without becoming solid. Ice formation requires low energy environments (cold) and structure from strong molecular bonds. What many people don’t know is that ice also requires a nucleation site where ice bonds first slide into correct formation. Nucleators are often minerals, dust, or other impurities. For example, snow crystals in the atmosphere form around bacteria! The danger of supercooling is that supercooled fluids are inherently volatile and can spontaneously re-freeze if water molecules align!

To make supercooled water at home, check out this awesome experiment: 

Through an unknown mechanism, Arctic squirrels are able to keep their body fluids at subfreezing temperatures without freezing! SUPER COOL SQUIRRELS! 

Arctic squirrels supercool their bodies during hibernation.
Sunglasses not required to be this cool!
Original photo by Øivind Tøien

Metabolism and Cost of Hibernation:        

            While arctic squirrels have a remarkable ability to supercool, there is a cost to maintaining body temperature. Even a body temperature that is below freezing. Buck and Barnes (2000) set out to understand the energy costs of hibernation in Arctic squirrels at different ambient temperatures. They measured Arctic squirrels metabolic rate during hibernation in a range of temperatures from -20°C to 20°C and found that metabolic rate increased substantially when ambient temperature is below -4°C.

Buck and Barnes (2000) Figure 1: Effects of ambient temperature on body 
temperature, metabolic rate, and respiratory quotient. When temperatures 
drop below -4°C, Arctic squirrel metabolic rate increases substantially!

Metabolic rates of Arctic squirrels during hibernation can be 36 times higher than resting metabolic rate (Richter et al., 2009), but warming up to normal body temperature for 12-15 hours every 2 weeks is even more energetically expensive with metabolic rates 300+ times higher than resting metabolic rate (Karpovich et al., 2009). Even though this energy expenditure seems alarmingly high, keeping their body temperature below freezing for the majority of the winter gives these squirrels an impressive energy savings.

For more information and adorable videos of hibernating Arctic squirrels check out these videos!

How an Arctic squirrel survives winter - Wild Alaska - BBC

Marvels of an Arctic Winter - America's National Parks

Hibernation Phenomenon of Re-warming Arctic ground squirrel with infrared camera

Citations:

Barnes, B. M. 1989. Freeze avoidance in a mammal: body temperatures below 0°C in an arctic hibernator. Science 244:1539-1595.

Barnes, B. M. and D. Ritter. 1993. Patterns of body temperature change in hibernating arctic squirrels. In “Life in the cold ecological, physiological, and molecular mechanisms”, Carey, C. et al. (eds). Westview Press Inc. pp. 119-130.

Buck, C.L. and B. M. Barnes. 2000. Effects of ambient temperature on metabolic rate, respiratory quotient, and torpor in an arctic hibernator. Am. J. Phsysiol Regulatory Integrative Comp Physiol. 279:R252-R262.

Karpovich, S.A., Toien, O., Buck, C. L., and B. M. Barnes. 2009. Energetics of arousal episodes in hibernating arctic ground squirrels. J. Comp. Physiol B. 179:691-700.

Richter, M. M., Williams, C. T., Lee, T. N., Toien, O., Florant, G. L., Barnes, B. M. and C. L. Buck. 2015. Thermogenic capacity at subzero temperatures: how low can a hibernator go? Physiol. Biochem. Zoology. 88(1):81-89.