Do black bears hibernate?
BPart 1: Does recent research settle the debate?
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lack bear boars sows without cubs may already have emerged from winter dens here in Virginia, and sows with new cubs should soon follow. Bear experts have long debated whether bears truly hibernate while in their winter dens, but research over the past few decades seems to be bringing that discussion to a close .
Experts have long agreed that some mammals go into torpor in the winter, reducing some of their vital functions — including metabolism, temperature and heart rate — and becoming sluggish, even totally inactive (see my Jan. 9 column). Hibernation was generally thought of as being extreme torpor, the depth of which was determined by an animal’s internal body temperature. Years of research on hibernation in small mammals, particularly squirrels, indicated that their temperature and metabolism rates were in sync, so it was assumed that this was the case for all mammals.
Doing research on bears in their dens has been problematic, but the data that has been collected showed bears, especially sows with cubs, experience only a slight drop in temperature, leading many experts to conclude they weren’t truly hibernating. As research expanded, thanks in part to new technology, the discussion about the very nature of hibernation grew more complex.
Eric C. Hellgren studied captive black bears at Virginia Tech’s Black Bear Research Center (BBRC), formerly known as the Center for Bear Research. BBRC was established in 1988 in partnership with the Virginia Department of Game and Inland Fisheries (VDGIF). In a 1998 article in Ursus, the journal of the International Association for Bear Research and Management (bearbiology. com), Hellgren pointed out that the physiology of bears and squirrels was not the same:
“Bears in hibernation exhibit several characteristics distinct from the deep hibernation of rodents, such as a lesser reduction in body temperature, protein conservation, lack of defecation and urination, and normal bone activity.” He went on to argue that 30 years of previous studies by “have led investigators to unequivocally state that hibernation is the fitting term for the dormant or torpid state of bears during denning.”
A few years ago, the hibernation discussion evolved further because of research coming out of the University of Alaska, which also uses captive black bears to study hibernation. Biologist Øivind Tøien and his colleagues there reported in the Feb. 18, 2011, issue of Science that their ursine subjects could suppress metabolism by 75 percent while regulating temperature from near normal (100 to 101 degrees) down to only about 20 percent of normal, in multiday cycles. When the bears’ body temperatures dropped to 86 degrees, they shivered to warm up. In the spring, after returning to normal body temperature and emerging from dens, the bears maintained a reduced metabolic rate for up to three weeks.
The results suggested, the researchers concluded, that “the majority of metabolic suppression during hibernation is independent of lowered body temperature.” In theorizing about how bears can do this, the researchers suggested that it may be due to their large size, which protects their internal organs from the cold better than in rodents. But why did bears evolve this decoupling capability?
Bernardo Mesa, a veterinarian from Columbia currently pursuing a masters degree in conservation physiology of large mammals at the BBRC and the Smithsonian Conservation Biology Institute, recently discussed bear hibernation and reproduction with me, acknowledging that both are complex issues that are not well understood.
Taking a broader view of the term he sees hibernation as simply “a series of changes in behavior and metabolism that allow an animal to limit its activity to survive under periods of extreme environmental conditions, such as cold weather and lack of sufficient food resources.” He added that at last year’s annual meeting of the International Association for Bear Research and Management, which focused on bear physiology, it was assumed that bears hibernate.
When I asked Mesa why bears might have evolved to decouple metabolism from temperature during hibernation, as the Alaska study showed, he suggested that it may be linked to reproduction.
Sows breed during the spring and summer, but delay implantation of their eggs until late fall when they head for dens, usually ahead of other black bears. In Virginia, cubs are born in the den in January or February, usually two to three to a litter. Small at birth — just 6 to 12 ounces — they develop slowly during the winter.
According to the American Bear Association (americanbear.org), the black bear is the only mammal known to be able to lactate for as long as three months without eating. While the cubs’ small size means the demand for milk is lower than in many mammals, she still needs to care for them, which requires maintaining a near-normal temperature without burning too many calories — in other words, decoupling temperature from metabolism.
Sows may replenish their resources lost from nursing in another way, Mesa suggests. The bear mom licks cubs to stimulate them into excreting waste. She then consumes the waste, likely recycling nutrients and water the cubs have not absorbed.
Why and when bears den up in winter is another mystery, says Mesa. In northern regions, black bears are more likely to go into dens early and stay there, but in Virginia boars and sows without cubs often emerge to feed when food is available. When food is scarce, such as this past year in Virginia, they are more likely to go into dens early, stay there longer and emerge less often.
As in Alaska, the bears studied at BBRC are “nuisance” sows brought to the center by VDGIF in the fall. They are provided with dens, with some pregnant sows also serving as foster moms for orphaned cubs rescued by VDGIF. The bears are immobilized every 10 days so blood samples and ultrasounds can be taken to check for egg implantation and to see how fetuses grow throughout gestation. All the bears are released in April or May.
In the fall, pregnant sows will just stop eating, Mesa says, “even if there is food or the weather is beautiful,” Some internal cue may tell them that they are pregnant and drive them to look for a den, he theorized. Those without cubs may emerge during the winter because they know that sustained high temperatures outside can make some foods available. The nutritional value of what bears find may also influence when or if they go back into dens.
It will likely take many more years of research to unravel all of the complexities surrounding hibernation, but rapidly advancing technology is helping, and what is learned may have implications for human medicine. In the meantime, the discussion of whether bears hibernate seems to be coming to a close. As the North American Bear Center (bear.org) puts it:
“People have called black and grizzly bear hibernation torpor, winter sleep, dormancy and carnivorean lethargy. The leading physiologists now simply call it hibernation.”
Next week: Technology advances bear-hibernation research, and the implications for human medicine.