Squirrels rise from hibernation

True hibernators, such as ground squirrels, marmots and chipmunks, regularly wake themselves from deep sleep. It didn't seem to make sense because this uses up huge amounts of valuable stored energy. But the answer seems to be they do it to fire up their immune systems and carry out a systems check for parasites and pathogens.

Small mammals hibernate to conserve energy when food is scarce during long, cold winters. While larger creatures such as bears and badgers go into a state of torpor, in which their body temperature drops for short periods, true hibernators cool right down. Their temperature often drops as low as 5 C for weeks on end.

California's golden-mantled ground squirrel (Spermophilus lateralis) is a champion hibernator. For five to six months each winter it spends most of the time with its heart ticking over at just two beats a minute. But roughly once a week, it wakes up and for 12 to 16 hours its body temperature rises to 37 C. The wake-up periods use up to 80 per cent of the animal's winter energy budget.

Researchers wondered if the animals wake to clear waste from their body, but that seemed unlikely given the huge energy costs. To investigate, Brian Prendergast of Ohio State University at Columbus and his team took 31 squirrels into the lab and implanted them with radio transmitters that recorded body temperature every 5 seconds.

When the squirrels went into hibernation, the researchers injected some of them with lipopolysaccharide (LPS), which makes up the dead outer cell walls of bacteria. LPS normally triggers a high fever in the squirrels, raising their body temperature 1 to 2 C for up to 8 hours. However, in the hibernating animals nothing happened, suggesting that their immune system has shut down.

But several days later, when the squirrels began to rouse naturally, the temperatures of these animals skyrocketed as if they had just been injected with LPS. "Animals may arouse from hibernation t

Contact: Claire Bowles
New Scientist

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