They survive colder core body temperatures than any other known vertebrate, sustaining a temperature below freezing yet not becoming frozen. They emerge from hibernation with clock-like accuracy despite having spent 8 months in underground burrows below Arctic tundra and layers of snow, out of sight of the Sun. The Arctic ground squirrel is an adorable furball of mystery.
Loren Buck, University of Alaska Anchorage Department of Biological Science professor, has been enticed by the truths he might discover through researching Arctic ground squirrels. These are creatures which have adapted to thrive despite their immensely challenging environment.
“When the animals end hibernation they enter reproduction right away. And their reproductive season is fast and furious, it’s roughly a week: 10 days in length. So if you stayed in hibernation a little bit too long you missed it. Or if you come above ground a little bit too early… Think about these animals, they are coming above ground in early April. Early April up at Toolik on the North Slope is characterized by 100% snow cover, ambient temperatures minus 20, minus 30, even minus 40 [degrees]. And you are a one kilogram animal sitting out there on the snow waiting to get a date. If you get up a little bit too early, there’s a good chance that you will not survive until green-up in the middle of June. So they have to remain rhythmic.” ~Buck
Arctic ground squirrels maintain a strictly regimented schedule, a rhythmic cycle that I probably couldn’t adhere to even with an alarm clock. And they’re rodents who’ve never heard of a light switch living in a land where the summer Sun never sets.
Plenty of questions
Science is about seeking the answers to questions. These often lead to more questions, more answers. We have no shortage of questions about Arctic ground squirrels.
“Looking at their natural history and their timing, just in a very cursory fashion in the early years, led to further investigation. Led us into this idea: we’ll let’s start looking at clock function. How does it do what it does? How do these animals maintain rhythmicity when they should be arhythmic? How do they maintain their phenology of entry and exit of hibernation when what we know of the clock tells us it shouldn’t be functioning at loss tissue temperature? […] We span from the ecological to the neurobiological to the molecular, trying to understand the persistence of clock function.” ~Buck
It seems the question most relevant to humans is how the animals’ internal clock stays so steady, even in the extreme environment they call home.
“So many of the disease states – pathologies – that are exhibited by humans today are characterized by a breakdown in clock function. And it runs the gambit, the types of diseases that you think of. The one that comes right to mind is near and dear to us in Alaska: seasonal defective disorder. That costs the state and health in a huge way. Well, that has to do with clock function. Some that you may not have thought of are cardiovascular disease, diabetes, alzheimers, old age senility. The list goes on and on. Virtually any pathology that you can think of is tied to clock function.” ~Buck
Summer vacation
Arctic ground squirrels are active for as little as 3 to 4 months of the year, during which time they are driven to breed, as well as to prepare for the upcoming winter, all while avoiding predators.
“These animals have a really extreme physiology and natural history. They are sequestered in their hibernacular for 270 days per year, during which time they are not eating, and they are not drinking. Then they have only three months to do all the things they need to do: to not only sustain their own lives and to reproduce but to grow and fatten sufficiently so they don’t have to eat again for another 270 days. Wouldn’t it make a lot of sense to just poke your head out of the burrow any time of the day and say ‘It’s nice out, there’s no predators, it’s not raining, I’m going to go eat!’? That’s not what they do. They poke their head out of their burrow in mid summer around 7 o’clock in the morning and they call it a day at about 7 o’clock at night.” ~Buck
What environmental or internal cues allow the ground squirrel’s strict clock to remain on-schedule? Buck asks: “What is it that they are entraining to?” Something sets the species’ clock, some signal which entrains or guides it to synchronize to a specific schedule. The scientists look to learn the answer. Further, they want to discover what the Arctic ground squirrel gains by following a schedule reminiscent of a human with a first-shift job.
Why do these creatures maintain a persistent rhythm? Other animals which reside in the Arctic, like reindeer and ptarmigan, let their biological clocks free-run arhythmically in the land of the midnight Sun: eating when they feel hungry and sleeping when tired. But not the ground squirrel. Perhaps the Arctic ground squirrels which returned to burrows every night to save energy or to avoid predation were more successful; they stayed alive, and bred, and passed down genetic information which encourages that behavior.
We don’t just want to know why the schedule is kept, but how. One hypothesis for how the animals maintain such a regimented schedule is that they can detect the time of day through light color or quality.
Brian Barnes is a University of Alaska Fairbanks Biology and Wildlife Department professor. He’s also the director of the Institute of Arctic Biology. The Institute of Arctic Biology operates a National Science Foundation -funded research site in the northern foothills of Alaska’s Brooks Range called Toolik Field Station. “The Sun doesn’t set at Toolik for much of the summer, it just goes ‘’round a lazy circle above the horizon,” Barnes explains. “It is light all of the time.”
The Arctic summer Sun at noon is higher in the sky and the sunlight that reaches the ground is subtly bluer in color than that of the Arctic summer Sun at midnight, which sits near the horizon and creates light appearing redder in color. Relatedly, small differences in temperature may give clues to the Arctic ground squirrel about what time it is even though the Sun never sets.
“We are dissecting these cues and trying to understand which are more important for them. But it is always with an eye on how it is they are getting ready for hibernation. When is it on an annual basis that they decide to ‘check out’ in the fall to enter the hibernation state? And then in the spring, there they are, a meter deep in the frozen tundra; they can’t see the changing day length, they can’t feel the warming temperatures. How do they know when to come out of hibernation?’ ~Barnes
A strict calendar
While the circadian clock governs daily schedules like eating, sleeping and waking, a circannual clock governs activities across the year. That circannual clock seems to govern Arctic ground squirrel timelines for entering and exiting hibernation. The scientists are investigating how the ground squirrels’ circannual clock synchronizes with the environmental year.
Cory Williams, postdoctoral fellow at the University of Alaska Anchorage, works with Loren Buck on decoding Arctic ground squirrel mysteries.
“They have a really strong circannual clock. They have this endogenous clock in their brain that tells them what time of year it is. This is likely for triggering their end of torpor: that reduced metabolism period. But we also see a tremendous amount of plasticity when they actually emerge to the surface. And so we are interested in understanding how they determine what conditions are like on the surface when they are sequestered in their hibernacula with a meter of snow above them. ~Williams
Based on gender and age, Arctic ground squirrels select an extremely precise time to enter hibernation (immergence chronology) and to end hibernation (emergence chronology). Young ground squirrels emerge from their mother’s burrow on July 4th, after which you could spot all ages and genders searching the Arctic tundra for food, gaining enough fat to carry them through the winter. Adult females enter hibernation before the end of July; meanwhile, their male counterparts continue to squirrel away food in a burrow, entering hibernation no later than October 1st.
The males will awaken from hibernation earliest in the spring; to be reproductively competent they need to go through puberty every year, eating the extra food cached away in their burrow and undergoing testicular maturation. Then they’re out and about, Buck says, “Charging around looking for love on the tundra,” ready to procreate as soon as females awaken and exit their burrows.
“After 21 days of sitting in this cold dark burrow and eating they pop above ground and witness this winter wonderland and start looking for females. […] The males have been up since early May – have had a high body temperature since early May. Females come to high body temperature say on the 28th of April, something like that, and come right above ground and they are impregnated within 24 hours. They end hibernation. The next day they’re pregnant. Gestation occurs over the next 25 days or so and then they move into lactation. And as soon as they gain back their body mass in July they enter hibernation. It’s just this really fast and furious life cycle.” ~Buck
Supercool
Buck describes the furry animals he works with as roughly the size of a softball, weighing about one kilogram, “And possibly the most handsome animal you’ll ever meet.” I know I keep re-watching the footage of him uncurling then re-rolling a hibernating Arctic ground squirrel. Too cute.
During deep hibernation (torpor) the Arctic ground squirrel’s core body temperature measures -2.9°C [26.78°F]. “The Arctic ground squirrel is the only vertebrate that we know of that can tolerate sustained sub-zero temperatures,” Buck explains. “That is below the ice nucleation point of their tissues. So they are in a meta-stable state that we refer to as being supercooled,” and yet “They don’t have anti-freeze proteins, they’re in this state that at any point in time they could freeze.”
”Why they don’t freeze? We really don’t know the answer to that but supercooling is a very well-described phenomena for liquids and solutions where you can cool water down below zero and it won’t freeze unless you provide an ice nucleator or provide some energy into that system. And that’s essentially where these Arctic ground squirrels are: their tissue temperatures are below the freezing point. And that is unique across all hibernators and unique among vertebrates.” ~Buck
The benefits of the big slowdown
“While these animals are in the hibernation state they are naturally protected from low rates of breathing and slow rates of blood flow, conditions that occur in [humans] following a heart attack or a stroke. We go ischemic (which means the blood stops flowing) or hypoxic (not enough oxygen). In a human, or a rat for that matter, if you induce those same circumstances there will be terrible damage in just 5 to 10 minutes. You’ll begin to lose brain cells. Your heart will begin to die. That doesn’t happen in hibernating species; they evolved to naturally face these conditions. So they’ve protected their heart and their brain from these conditions of ischemia and hypoxia. We want to know how.” ~Barnes
During hibernation, everything slows down. The Arctic ground squirrel’s heart beats 3 or 4 beats per minute instead of 300. They might breathe only every few minutes. Their metabolic rate drops by 90%.
“They are using 2% or 3% of the energy that they would be using if they were at normal body temperature. So Arctic ground squirrels are quite unique in that they have to generate heat to prevent themselves from freezing; they still do use a fairly substantial amount of energy. But that’s nowhere near the amount of energy they would use at high body temperatures.” ~Williams
Arctic ground squirrels have adapted to live in the Arctic tundra and permafrost. Measured burrow temperatures average -10°C [14°F] during the hibernation season, and have been recorded at -27°C [-16.6°F]. That means to maintain a core temperature of -2.9°C [26.78°F] the animals must expend energy to warm themselves.
“Even a low core temperature of -2.9° requires that they be actively thermogenic during hibernation. That means they have to produce a large amount of heat through a lot of fat in order to keep their body temperature from falling lower than -2.9°.” ~Buck
Despite their enormous fluctuations in body fat, and rapid regaining of fatty tissues later in the year, the ground squirrels somehow escape the detriments we humans would face if we gained large measures of body fat as swiftly as they do.
During torpor (deep hibernation) Arctic ground squirrels generate heat using brown adipose tissue. “Although brown adipose tissue looks a lot like white adipose tissue, typical white fat that we all have, it’s actually quite different and more closely related to muscle tissue,” explains Cory Williams.
“Brown adipose tissue is packed with mitochondria. It has what we call an uncoupled transport chain which generates heat. And this heat is what they use to maintain their body temperature though non-shivering thermogenesis, so, a way of generating heat without shivering.” ~Williams
In torpor they don’t shiver. In fact they’re so still for so long, it’s a wonder they don’t waste away.
“There’s also interest in hibernating species by space agencies because of their capacity to reduce their metabolism and because they don’t exhibit loss in bone mineral density and muscle tissue when they go through prolonged periods of inactivity.” ~Williams
Once we answer more questions about how Arctic ground squirrels have adapted to achieve their remarkable lifestyles, we may be able to apply the information to help humans, to treat a wide range of diseases or pathologies, and perhaps even to aid in space travel.
Laura Nielsen
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
Arctic Ground Squirrel project
- Interviews with quoted scientists, 2013 & 2014
