Thirteen-lined ground squirrels survive extraordinarily low blood oxygen Free

Disney would have you believe that hibernation is one long sleep, but that couldn't be further from the truth. The body temperatures of hibernating animals – such as 13-lined ground squirrels –tumble, and they seem to stop breathing and appear almost dead. And instead of remaining dormant for an entire season, most hibernating animals arouse periodically, warming up and pottering about for a few hours before resuming hibernation. ‘During rewarming, energy consumption increases more rapidly than heart rate. This, coupled with poor oxygen delivery to tissues, creates a mismatch in oxygen supply and demand’, says Brynne Duffy from the University of Western Ontario, Canada. But no one really knew what happened to the oxygen levels in 13-lined ground squirrels’ bodies each time they emerge briefly from hibernation. Everyone presumed that oxygen surged back into their bodies, but no one had checked. Duffy, Catie Ivy and Jim Staples (also from University of Western Ontario) fitted hibernating 13-lined ground squirrels with collars containing tiny blood oximeters – which measure the amount of oxygen carried in the blood – to find out how their oxygen levels vary during hibernation and each time they emerge briefly.

Recreating the chilly conditions inside a hibernating ground squirrel's burrow, the team gradually dropped the temperature in each animal's enclosure to 4°C and turned out the lights to send the animals into hibernation. But the team ran into a technical snag when they fitted the blood oximiters to each hibernating animal's neck as they removed the animals from their chilly homes to bring them out of hibernation. ‘You need a strong pulse for the blood oximeter to work, but the hibernating squirrels’ heart beats were just too infrequent to get reliable oxygen measurements’, says Duffy. Fortunately, the oximeter began picking up readings within 15 min as each ground squirrel warmed and their heart rates rocketed from ∼4 beats min⁻¹ to ∼100 beats min⁻¹, eventually reaching ∼370 beats min⁻¹ when their body temperature reached 37°C.

However, the team were astonished as they tracked the ground squirrels’ blood oxygen levels. ‘The initial arterial oxygen saturation values were extremely low, just 45% oxygen’, says Duffy. ‘This is lower than any other hibernator reported to date’, exclaims Duffy; and definitely fatal for us. But the team was even more surprised when they noticed the ground squirrels’ oxygen levels plummet even further as the chilly animals continued to emerge briefly from hibernation, declining eventually to 34%. ‘This decline suggested that the early stages of arousal from hibernation represented a phase where demand for oxygen outpaced oxygen delivery’, says Duffy, explaining that the ground squirrels were probably using more oxygen than their cold blood could supply as they worked hard to return their body temperature to 37°C. And she suggests that the oxygen levels of hibernating ground squirrels in natural burrows – where the concentration of oxygen is likely to be lower than normal (20.9%) – could fall even further.

But the extraordinarily low blood oxygen levels experienced by hibernating 13-lined ground squirrels are only half of the challenge. When oxygen surges back into their bodies, it produces toxic byproducts – such as peroxide and superoxide – which damage cells, similar to the injuries suffered by humans immediately after a stroke or heart attack. But Duffy and colleagues suspect that ground squirrels may be protected from the ill effects by looking after their hearts. They suggest that the animals’ hearts may depend less on oxygen and more on glycogen-fuelled anaerobic respiration when leaving hibernation, preventing them from producing the oxygen toxins that could cause harm as they emerge repeatedly from hibernation over the course of a long winter.