bearing physical activity. People normally lose a little bone as they age; about
1 to 2 percent of the minerals in bone
begin to leach out of women’s spines and
hips each year after menopause. But people on bed rest or in the weightlessness of
space may lose 3 to 4 percent of their hip
bone minerals each month, says endocrinologist Peter Vestergaard of Aarhus University Hospital in Denmark. Bed sores
and blood clots also plague the sedentary
human. And yet, come spring, hibernators emerge free of clots and sores and
with their skeletons and muscles intact.
Rita Seger ventures into the backwoods of Maine to learn how American
black bears keep their bones healthy
during hibernation. She and her colleagues from the Maine Department of
Inland Fisheries and Wildlife track bears
right to their dens, thanks to radio collars fitted on the bears in the spring. The
trips usually involve packing a portable
centrifuge and an X-ray machine powered by the battery from a cordless drill.
Comparing X-rays of the paws of tranquilized hibernating bears with X-rays
taken of paws from bears that had been
killed by hunters revealed no differences,
suggesting hibernation doesn’t melt bone
the way bed rest does for humans. Seger
and colleagues also compared blood
samples from the hibernating bears with
blood drawn from bears that were active
in the spring, looking for chemical clues
to how the skeletons are maintained.
Seger, a general internal medical
541
Look ma, no hardening clues from the
blood of seven brown bears may reveal how
they maintain flexible arteries despite levels of
cholesterol, especially during hibernation, that
are far above those desirable for humans.
Total plasma cholesterol in bears
Milligrams/deciliter
387
232
Desirable level for humans
Hibernation
Active
doctor and physiologist at the University of Maine in Orono, and her team
found proteins in the bears’ blood indicating that bone-building cells are less
active in hibernating animals than in
spring bears, reporting the results in the
December Bone. Humans on bed rest also
build less bone than normal, but bone is
torn down at a greater rate, making for
an overall loss. The black bears balance
slower bone building with a reduction in
bone loss, the researchers revealed, keeping the skeleton strong.
Some of the bears’ balancing skills may
be transferable. Already, black bear parathyroid hormone, which helps determine
how much bone is built or reabsorbed,
is in development as a possible treatment for human osteoporosis, says Seth
Donahue, a biomedical engineer at
Colorado State University in Ft. Collins.
Donahue is working with a biotechnology company to test the hormone’s bone-sparing power in rats. When researchers
remove the ovaries from female rats to
simulate menopause, the rats’ bones
become spongy. But rats treated with the
bear parathyroid hormone retain more
bone, preliminary studies show.
Bears may also hold the secret to avoiding blood clots, such as those that develop
when people sit still too long on plane
trips, Fröbert says. He and his colleagues
measured the blood-clotting activity of
cells called platelets taken from humans
and from brown bears shortly after the
bears emerged from their dens in the
spring. The bears’ platelets were about
half as active as the human platelets,
the researchers reported in 2010 in
Thrombosis Journal. Fröbert’s team is
trying to find out why the bear cells have
lower clotting power and how that power
changes during hibernation.
It’s in their blood
Bears may have a lot of helpful information to impart, but they are not easy to
work with. Some scientists have turned
to a smaller hibernating critter that can
be studied in the lab, the 13-lined ground
squirrel. This rat-sized creature may
reveal a strategy to help with another
blood problem: massive blood loss.
On a visit to Ronald Cohn’s lab at Johns
Hopkins University Medical School in
early December, most of the 13-lined
ground squirrels were curled into drowsy,
striped balls, their faces tucked into the
cream-colored fur on their bellies. One
slumbering squirrel’s sides rose and then
fell with a single breath. About 20 seconds
later, the squirrel breathed again.
Once these squirrels go into the refrigerator for the winter, they will enter full
torpor, as scientists call deep hibernation. The animals will take only two to
four breaths and their hearts will beat just
two to four times per minute, Cohn says.
“The theory has been that hiberna-
tors are exquisitely adapted to function
in the cold,” says Sandra Martin, an evo-
lutionary geneticist at the University of
Colorado School of Medicine in Aurora.
“We don’t find that. Our results suggest
that they’re exquisitely adapted to shut
everything down in the cold.… It’s almost
as if the animal is putting itself in an ice
bucket, slowing everything down to save
energy.”
But about every two weeks, the hiber-
nators do something that makes little
energetic sense. They warm up and move
around, just for a bit.
“It’s expensive to be up at high temperatures in the cold,” says Hannah
Carey, a physiologist at the University
of Wisconsin–Madison School of Veterinary Medicine. While the animals are
up and about — about 10 to 12 hours for
ground squirrels — they may urinate and
move around, but generally do not eat or
drink. Bears go the entire winter without
