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Study links arctic ground squirrels to human obesity

Kathleen McCoy
Erin Hooley

Squirrels seem so ordinary, so every day. Go for a hike in the mountains and you'll hear them chattering in the trees or scurrying away from your dog's curious nose. But they don't make an impression, nothing like walking up on a bear.

Still, new science suggests we rethink the lowly status we assign to these squat, bushy-tailed mammals. The National Institutes of Health think so; it has provided new research dollars to microbial ecologist Khrys Duddleston at the University of Alaska Anchorage to learn more.

Surprisingly, the reason has to do with our human obesity epidemic.

Arctic ground squirrels are the Olympians of hibernation, opting to sleep through the harshest weather of the year (just the time we humans would prefer a trip to Hawaii).

In torpor, they perch on the very brink of life and death at temperatures that would freeze water and blood; even a small cut on a paw can endanger their careful balance and tip them toward death.

As a survival mechanism, they shiver themselves to warmth on a fairly precise schedule, warming up to 35 degrees for 15 to 24 hours, then dipping back into near-death at minus 2.9 degrees Celsius for 20 more days. Repeat until spring, when they "wake up" to a brief three months for mating, giving birth and weaning, and fattening up again for their next big sleep.

These metabolic gymnastics piqued Duddleston's scientific curiosity. She and her collaborators, biologists Loren Buck and Fred Rainey of UAA and Hannah Carey of the University of Wisconsin-Madison, wonder how squirrels survive starvation seven to nine months of the year under brutal winter conditions, then suddenly revive with a fully functioning digestive system, ready to go.

The answer may come from a symbiotic relationship mammals cultivate with bacteria in their gut. Scientists refer to these bugs as the gut microbial community. Hosts give them a warm place to live, and they reciprocate by extracting nutrition from food and even making vitamins.

Now here's where the obesity link gets interesting.

Amazingly, arctic ground squirrels manage to pack on 50 percent of their body weight in fat in just three weeks every fall before they resume hibernating. Anyone who's put on a few pounds over the holidays knows it can happen quickly, but we certainly don't go from 100 to 150 pounds in three weeks.

Still, the mechanism that allows this fat deposition to happen so fast in squirrels might help us understand the human capacity to store fat.

Scientists are deciphering the sophisticated relationship between gut microbes and the body. They know some microbes send signals to the host to turn energy sources into fat deposits. Could this be happening in humans too?

Duddleston describes her current phase of research as "discovery." She and graduate student Tim Stevenson are literally sketching out the arctic ground squirrel's gut microbial community for the first time. Which bacteria live in their guts? What happens to their numbers and types when the squirrel hibernates? How about when it's rushing to fatten up?

Stevenson's work, guided by Duddleston, is providing some insight into those questions.

To generalize, 90 percent of the gut micro-flora in all mammals belongs to one of two groups: Firmicutes or Bacteroidetes.

In his work, Stevenson has found that when squirrels are fat later in summer, the Firmicutes are more numerous. When they are at low weight during hibernation and early in the summer, Bacteroidetes dominate. That suggests the Firmicutes may be most efficient in extracting energy from food sources and signaling the body to store calories as fat -- NOW!

Obesity research really took off in 2005, Stevenson says, when scientists connected findings in humans to what they were seeing in mice. Over the past seven years, research papers showing host/gut connections related to disease states have increased dramatically, pointing to effects on diabetes and heart disease, or even inflammatory ailments like irritable bowel syndrome. Gut microbes are even suspected of influencing hormone levels and blood status; one study draws a neurological link to anxiety, he said.

What Duddleston and Stevenson are doing is adding another animal model to the body of work investigating human obesity. Next, UAA's scientists will manipulate the squirrel gut flora with antibiotics and diet changes to see how that influences the host's physiology.

Could the day come when a doctor will prescribe an antibiotic to fight obesity? More likely, Duddleston thinks, a probiotic might introduce gut flora for a specific known effect, like weight loss. That's all in the future, and there is much more work to be done.

Still, the next time you see a nattering, chattering squirrel on the trail, give it a little more respect; it might just show the way to trimming our expanding waistlines.

 

Kathleen McCoy is an electronic media specialist at UAA, where she highlights campus life through social and online media.

 

 


Kathleen McCoy
Hometown U