A change of heart keeps bears healthy while hibernating

Feb 07, 2011
Grizzly Bear and Cub
Grizzly bear and cub feed on a dandelion. Credit: University of Alberta

Hibernating, it turns out, is much more complicated than one might think.

Research published in the latest issue of the journal Physiological and Biochemical Zoology illustrates a complex series of changes that occur in grizzly bears' hearts as they hibernate. The changes guard against complications that could arise from greatly reduced activity.

A grizzly hibernates five to six months of the year. During that time, its slows drastically from around 84 beats per minute when active to around 19. "If a human heart were to slow down like this, you'd see very detrimental things happening," said Bryan Rourke, a professor at Cal State Long Beach who worked on the research with his graduate student, Nathan Barrows.

Such a slow beat causes blood to pool in the heart's four chambers. In a human, the increased pressure would cause the chambers to stretch out. The dilated muscle would be weaker and less efficient, leading ultimately to congestive .

"Bears are able to avoid this," Rourke said, "and we're interested in how they do it."

Barrows and Rourke worked with Lynne Nelson and Charles Robbins, researchers at Washington State University who have been studying bears for years. They operate a facility at Washington State where grizzlies have been raised since birth and acclimated to echocardiogram testing. Research at the facility is providing crucial insight into the mysteries of the hibernating heart.

Nelson and Robbins had previously shown that, during hibernation, the muscle of a bear's left ventricle stiffens to prevent it from stretching as blood accumulates. But the stiffening of the ventricle presents another problem. The left atrium, which pushes blood into the , must then work against greater resistance.

"The atrium is pushing against a brick wall," Rourke said. "We thought there must be some kind of mechanism to keep the atrial muscle from wearing itself out."

Using echocardiogram data from the captive bears at Washington State and tissue samples from wild bears, the researchers found that the atrium protects itself by weakening its beat.

Muscle contractions in the heart are controlled by a protein called myosin heavy chain. The protein comes in two varieties, alpha and beta. The alpha version produces a quicker but slightly weaker contraction compared to the beta. "We found that the muscle in the left atrium produces more alpha protein during hibernation, which results in a slightly weaker beat," Rourke said. "The lower force keeps the atrium from being damaged as it pushes against the stiffer ventricle."

When the bears emerge from hibernation, the protein ratio switches back and the atrial contraction returns to its original strength.

The researchers believe this is the first study to show a shift in myosin ratio in bears, and they hope the information might someday have an application for humans.

" aren't a perfect model for humans, but the way in which a bear's heart can change could be helpful in understanding human disease," Rourke said. "It's a really interesting physiology."

Explore further: Rare new species of plant: Stachys caroliniana

More information: N. D. Barrows, O. L. Nelson, C. T. Robbins, and B. C. Rourke, "Increased Cardiac Alpha-Myosin Heavy Chain in Left Atria and Decreased Myocardial Insulin-Like Growth Factor (IGF-I) Expression Accompany Low Heart Rate in Hibernating Grizzly Bears." Physiological and Biochemical Zoology 84:1 (January/February 2011).

Related Stories

Female Cantabrian bears and their young do not hibernate

Oct 06, 2010

A team of Spanish scientists followed the brown bear population through the mountains of the Cantabrian Cordillera between 1998 and 2007 in order to find out about their hibernation habits, which had been ...

Recommended for you

Rare new species of plant: Stachys caroliniana

Nov 21, 2014

The exclusive club of explorers who have discovered a rare new species of life isn't restricted to globetrotters traveling to remote locations like the Amazon rainforests, Madagascar or the woodlands of the ...

Mysterious glowworm found in Peruvian rainforest

Nov 21, 2014

(Phys.org) —Wildlife photographer Jeff Cremer has discovered what appears to be a new type of bioluminescent larvae. He told members of the press recently that he was walking near a camp in the Peruvian ...

The unknown crocodiles

Nov 21, 2014

Just a few years ago, crocodilians – crocodiles, alligators and their less-known relatives – were mostly thought of as slow, lazy, and outright stupid animals. You may have thought something like that ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.