How Hibernating Squirrels Survive Eight Months Without Water

Analysis by Dr. Karen Shaw Becker

13-lined ground squirrel

Story at-a-glance -

  • During hibernation, a squirrel species known as the 13-lined ground squirrel of the Midwest can suppress its thirst and need for food for as long as eight months
  • Like woodchucks, bears, prairie dogs and other hibernators, when 13-lined ground squirrels hibernate, their whole system, including their body temperature, heart rate and metabolism, lowers so drastically that the sensation of thirst is nonexistent
  • From regular body function, the hibernating squirrels’ metabolism drops to a state called torpor, which is very low, then regulates on and off to more median levels called interbout arousal, or IBA, for the duration of the hibernation period
  • Typically, animals with a high blood serum concentration become thirsty, as do humans; one way deep-sleeping squirrels are able to sleep so long is because their blood serum concentration is low
  • The squirrels’ blood serum concentration is regulated when their body chemicals (such as glucose urea and electrolytes such as sodium) are “redistributed” to other areas of the body, such as their bladders

There are innumerable ways nature amazes us, and animals are a large part of that, continuing to fascinate us on many levels. The charismatic creature known as the squirrel has its own share of quirks and foibles that scientists are still learning about.

While their more common cousins gather up their winter stores and hunker down in their nests during the cold spells rather than hibernating, the oddly named 13-lined ground squirrel (Ictidomys tridecemlineatus) takes winter hibernation to a whole new level by snoozing for as long as eight months with no food and no water.

Found in the Midwest, these tiny squirrels share a skill with woodchucks, bears, prairie dogs and others — when they hibernate, their whole system, including their body temperature, heart rate and metabolism, drops so drastically that the sensation of thirst is nonexistent.

In the pursuit to determine how these squirrels were able to suppress the thirst response that’s powerful enough to wake them, scientists divided the blood serum — blood fluid — of dozens of squirrels and separated them into three groups:

  • Active squirrels
  • Squirrels in a deep hibernation state called torpor
  • Squirrels in a drowsy but still hibernating state

Typically, animals with a high blood serum concentration become thirsty, as do humans. That’s one way deep-sleeping squirrels are able to sleep so long; their blood serum concentration is low.

Low Blood Serum Facilitates Hibernation

Even when the researchers tried to get the squirrels to wake up and take a drink, they remained basically comatose; they just weren’t interested. However, when the scientists upped their blood serum concentrations artificially, that changed. According to the study, published in Current Biology:

“To understand how squirrels cope with months of water deprivation, we characterized changes in hydration status by measuring serum osmolality in active, prehibernation-torpor, torpor, and interbout arousal (IBA) states … Blood osmolality is tightly linked to fluid homeostasis and is regulated by a physiological circuit that includes the subfornical organ (SFO), hypothalamus, pituitary gland, and kidney ...

In mice, multiple hours of water deprivation can increase serum osmolality by 1% –2%. SFO neurons detect this increase and trigger both water-seeking behavior and the release of antidiuretic hormones to stimulate water retention by the kidney.

We found that, despite not having access to water over the period of several months, squirrels undergo state-dependent changes in serum osmolality ... Compared to active animals, torpid squirrels experience a significant (10%) drop in serum osmolality.”1

The researchers then needed to find out how and why the squirrels’ blood serum levels dipped so drastically. One obvious supposition was the possibility that drinking a lot of water prior to hibernation drastically diluted their blood serum levels, but that possibility was tossed out when the scientists filmed the squirrels’ activity and found they actually drank less water than normal, not more.

As Science2 explains, testing on the squirrels revealed that their blood concentration is regulated when their body chemicals (such as glucose urea and electrolytes such as sodium) are “redistributed” to other areas of the body, such as their bladders.

In addition, hibernating squirrels “undergo cyclic changes between normal and increased hydration throughout the entire period of hibernation by relying exclusively on reversible internal mechanisms,”3 the study explains.

Squirrel Hibernation Brings on Drastic Body Functions

In a video in Current Biology,4 Slav Bagriantsev, associate professor at Yale School of Medicine’s department of cellular and molecular physiology, notes that during the hibernation that starts in late summer or early fall, squirrels’ metabolism allows these dramatic bodily changes:

  • Their heart rate drops from 300 beats per minute (bpm) to 19 bpm
  • Their respiration decreases from 150 breaths per minute to only 5
  • Their temperature changes from 30 degrees Celsius (C) to just 4 C

This is the torpor state, which lasts about two weeks, Bagriantsev explains, but interbout arousal, or IBA, begins when those factors change. The squirrels’ breathing levels off to a more median 63 breaths per minute, their body temperature rises slightly to 18 C, and their heart rate becomes around 131 bpm.

IBA lasts only 24 hours until the squirrels’ body functions return to torpor mode, but the cycle repeats itself “over and over again during the entire period of hibernation until they become active again in the spring,”5 Bagriantsev says. According to Ni Feng, postdoctoral associate at Yale:

“The goals of this study are to identify how fundamental psychological processes that are so important for survival are changed and adapted in hibernators to allow hibernation to happen, and specifically for this study, we focused on the fluid homeostasis pathway (or water balance).”6

Maintaining fluid homeostasis is actually a lot of work, says Maddy Jenkins,7 a graduate student and colleague at Yale. We’re always losing water because we sweat, urinate and breathe, but we also consume salty foods, she maintains. Additionally, drinking water helps to maintain a proper balance of fluids and ions. Failing to hydrate can lead to dehydration, which can be dangerous.

How Can Squirrel Hibernation Help Humans?

This knowledge may be able to help humans with serious conditions like diabetes. It may also help astronauts on long flights into space. We have specialized brain regions in charge of monitoring our hydration levels. Even a 1% deviation can make us feel thirsty, Jenkins adds. A sensation of thirst is healthy for us because it’s an urge that tells us we need to get our fluid levels back to normal.

According to Yale colleague and associate professor Elena Gracheva,8 understanding how these squirrels can survive so long without eating or drinking may help scientists figure out how people can survive prolonged periods without resources needed to live. Science notes, “Unfortunately, even if people can figure out how to drop their serum concentrations, it’s unlikely they’ll ever be as cute as sleeping squirrels.”9

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