Simanaitis Says

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I USED to believe that bears were the typical hibernators. However, “Squirrels With a Rainy Day Fund,” by Mitch Leslie, in AAAS Science magazine, March 2, 2018, encouraged a bit of Internet sleuthing on my part. Here are some tidbits on hibernation.

Hibernation, Not Just Sleep. Hibernation is characterized by an induced lowering of body temperature and an accompanying reduction in heart and respiration rate. Depending on the species, this state of reduced metabolic inactivity can last for several days, weeks, or months.

Hibernation differs from normal sleep, which exhibits less pronounced reduction of respiration and heart rate. In fact, many hibernators experience periodic arousals with metabolism returning closer to normal. Reasons for this are not clear: Some researchers say periodic returns to higher body temperatures benefit immune response. Others say hibernators must occasionally warm up to get some proper sleep.

Squirrels Do It; Bears, Only Sorta. Squirrels and other rodents are the most typical of what are termed deep hibernators. Science reports that the 13-lined ground squirrel, Ictidomys tridecemlineastus, reduces its hibernating body temperature in its native South Dakota to just above freezing. The rodent’s heart rate drops from a normal 350 to 400 beats/minute to as low as five beats/minute.

By October or so, the animal enters an especially deep portion of its burrow and rolls itself into a tight ball. Its respiration drops from around 150 breaths/minute to one every five minutes. Hibernation lasts until March or early April.

A hibernating 13-lined ground squirrel. Image from Science March 2, 2018.

By contrast, the body temperature of a hibernating bear drops only a few degrees, from around 99 degrees Fahrenheit to an average 91. The bear’s metabolic rate is reduced by 75 percent, versus a squirrel’s 90-percent shutdown.

Hibernation Prep. According to AAAS Science writer Mitch Leslie, “By the time a squirrel holes up to hibernate, its weight will have soared by about 40 percent, thanks to extra fat that will tide the creature over until spring…. It can gain more than 2 percent of its body weight in a single day as it gorges on seeds, grasshoppers, and other delicacies.”

There are downsides to a ground squirrel’s evolutionary tactic: Leslie notes, “A roly-poly rodent is easier prey for a hawk or coyote…. Packing on the fat requires metabolic and behavioral adjustments.”

Food bingeing gives the squirrel some of the metabolic defects of Type 2 Diabetes. “But somehow,” Leslie notes, “the squirrel dodges the health problems that plague obese people…. And by spring, it is lean and spry and ready to begin the cycle again.”

In between, though, the squirrel may roll over every few weeks. I would. ds

© Dennis Simanaitis,, 2018

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