Image of a hibernating thirteen-lined ground squirrel from University of Minnesota Duluth researcher Matthew Andrews.
Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) are really cute when they hibernate (above). During torpor bouts, their body temperature decreases to a few degrees Celsius and their metabolism drops by as much as 95% with heart rates ranging from only 3-10 beats per minute. These bouts of torpor are interrupted by periodic arousals every couple of weeks during which their metabolism increases as body temperature elevates to 37 degrees Celsius. What is so fascinating is that they are able to decrease their body temperature below what would cause other animals to undergo cardiac arrest. Dr. Matthew Andrews at the University of Minnesota, Duluth wanted to know how these squirrels protected their hearts from a similar fate during bouts of torpor. His team characterized changes in heart proteins from animals during warm months (controls), animals in torpor as well as those aroused from torpor. They discovered that dozens of proteins show differences in their expression patterns at each of these stages in addition to novel forms of proteins found in ground squirrels. Their findings are published in the Journal of Proteome Research.
Sources:
Vermillion KL, Jagtap P, Johnson JE, Griffin TJ, Andrews MT. Characterizing Cardiac Molecular Mechanisms of Mammalian Hibernation via Quantitative Proteogenomics. Journal of Proteome Research. 14:4792-804, 2015.
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Is anything known about what's happening in their brains during deep hibernation? I would expect that their EEG in hibernation would be substantially different to what occurs during regular sleep.
While we're at it, a behavioral experiment might be of interest, to ascertain whether the squirrels' brains are capable of integrating sensory information while sleeping and hibernating:
During deep hibernation as measured by heart rate, present them with a sequence of two audio tones. Follow the tones with the smell of food.
Do likewise with waking squirrels and sleeping squirrels.
The duration of the tones played, should be extended proportionally to decrease in heart rate in the sleeping and hibernating states as compared to the waking state. This on the basis that acquisition of sensory data is slowed somewhat as brain activity also slows down.
After some number of repetitions that should be sufficient for a learned association to occur, present the same tones to all of the squirrels when they are awake, and observe if the squirrels exhibit food-oriented behavior after hearing the tones, as compared to control groups in each category who did not hear the tones followed by the smell of food.