Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)
BEARS IN WINTER
While searching for kinglets in the Maine winter woods during mid-December 2000, my students and I found the fresh spoor of a black bear on new snow. We had never seen a bear track so late in the year. When there is little food, the bears den up as early as mid-October. It had, however, been a fall with a heavy crop of both beechnuts and acorns. Wild apples had also been abundant on the old overgrown farms in the surrounding hills.
The bear had passed by only hours before, and we took up the chase, hoping to find its wintering lair. The bear had traveled without stopping to rest, walking past a calf carcass that we had laid out to feed ravens. Normally such prime veal would attract a bear miles away and then cause it to gorge. But the fact that this bear did not stop to feed prior to its long winter fast was not surprising. By December it had probably stopped looking for food and was now searching for a denning place.
The bear started to switch from prehibernation to hibernation physiology. This switch is triggered by chemical signals in the blood. Once in hibernation, bears will stay in their winter dens for as many as five months at a time, conserving their hard-won energy resources (fat) that they have accumulated during their feeding frenzies of the previous fall. Running around searching for food when little is to be found is deficit energy spending. Evolutionary logic dictates that appetite would be suppressed in a bear ready to hibernate, because a hungry bear would continue to be a food-searching bear, and deficit-spending bears become dead bears.
There are two ways to try to beat an energy crunch brought on by winter. One approach, used by the kinglet and human beings alike, is to work harder and harder to try to maintain a profit margin, even as we pay ever-higher heating costs in the face of ever-dwindling resources. There comes a point, however, when it is better to drop out in an effort to save as much energy as possible. The latter is sometimes a matter of necessity and it is common in many animals in winter. It may even occur in humans, given the right circumstances. Here in midwinter at the high latitudes of Vermont and Maine, I start to feel sleepy at about 5 P.M. and I have little trouble curling up in a snug bed as soon as it gets dark. In the summer at that time there are still four more hours of daylight to come, and I would still be running around. Of course my semi-hibernation tendencies are blunted ever so slightly by social pressures. In my culture it is just plain lazy to sleep fourteen hours per day. So, as a result of social conditioning I routinely extend my winter day with artificial light in the evening and with caffeine in the morning. Most important, my natural tendencies may be suppressed because I’m not on a stringent diet that winter would normally impose. My calorie intake is undiminished and sufficient to keep up my energy level. Unlike the bears in the woods, we New Englanders don’t need to go into hibernation in October when the nuts run out.
My students and I did not succeed in tracking our bear to its den. If we had, we might have found it under a brush pile, in a hollow tree, under roots, or upon a heap of branches in a stand of dense young spruces or balsam firs. Bears may even curl up and hibernate in the open with the understanding that they eventually will be buried with snow. Grizzlies dig their dens, moving up to a ton of earth to carve a tunnel into a hillside, their snug hibernation chamber at the end of it. They cover the floor of their chamber with bedding material of branches, grass, or duff scraped from the ground nearby.
Bears may prepare their overwintering den a few days or weeks before entering them. Some wait for a big snowstorm before finally crawling into their prepared sanctuary. They are flexible and individual differences abound. However, most bears engage in a feeding frenzy in late summer and early fall, in which they down about five times their normal food intake, putting on a five-inch layer of fat. By late fall they slowly lose their appetites until they eat nothing and when they leave their dens in the spring, they are no more hungry than before entering. Instead, provided they retain some fat, it takes them a long time to regain their appetite. Appetite suppression during hibernation is probably under the control of leptin, a “satiety” hormone secreted by fat cells that circulates in the blood and affects the appetite centers in the brain (Ormseth et al. 1996). In spring, leptin levels decrease and appetite increases.
When are bears in hibernation, if ever? This common question is not a good scientific question, because the best answer is “it depends.” We often seek precision by pigeonholing through definitions, whether with respect to what is right or wrong, alive or not alive, hibernating or nonhibernating. The problem is that animals don’t stay within such simple boundaries. They don’t obey rules, so any precision that might be gained artificially through a definition is apt to slip away at any moment in any case in point.
Three-month-old nonhibernating cub climbing on hibernating mother.
Overwintering bears have many physiological and behavioral characteristics, but they were for a long time not considered to hibernate, simply because their body temperature showed only modest drops and hibernation was defined in terms of low body temperature. A bear’s body temperature during hibernation remains near 35°C, only slightly lower than the 37° to 38°C or so when active. At a body temperature of 35°C a bear may be slightly sluggish, but it is by no means unresponsive to disturbance, especially from human researchers who would dare to enter its lair to take its temperature with a rectal thermometer, or stick it with a syringe to draw its blood in trying to track down the marvels of the bear’s hibernation physiology. It turns out, however, that the key to a bear’s hibernation is not to be found in the temperature of its rectum. Instead, diagnostic characteristics are discerned through the bear’s appetite physiology, waste metabolism, water balance, and bone retention despite lack of exercise. Indeed, the marvels of hibernation concern many medical matters of acute practical relevance to humans, especially as regards aging, space flight, and osteoporosis.
One of the first issues of hibernating bears to be studied was how, despite maintaining a high metabolic rate (high body temperature) the bear still does not need to drink or urinate all winter. We can, like bears, also go without food for a long time, provided we have body fat. But we can’t get along without water. If we were to spend considerable time in a bear’s den in winter we would, even without sweating, quickly dehydrate due to urination. But if we shut off our kidneys, then our metabolic waste, principally urea, would pile up in our blood until it poisoned us. Urea is our vehicle for getting rid of nitrogen, which becomes a waste product after we digest protein or nucleic acids. A bear does not urinate all winter. Thus the question is: Does urea not poison the bears or don’t they produce urea? To find out, physicians Ralph A. Nelson and Dianne L. Steiger from the Carle Foundation Hospital at the University of Illinois teamed up with game biologist Thomas I. Beck from the Division of Wildlife in Colorado to try to examine the urea content in the blood of hibernating bears. But how to get the blood? Bears in their winter dens are alert enough to be intolerant of people with hypodermic syringes. In part to get more compliant subjects for their project, the researchers did the next most difficult thing, they live-trapped bears in the fall and equipped them with radio transmitters that could be used to track down their subjects later when they were denned up. There, the bears were tranquilized with chemicals (Rompun) from a dart gun that made then more tractable and the task of taking their blood easier. A total of 76 blood samples from 48 bears were collected and analyzed.
Since hibernating bears metabolize mostly fat, they do not accumulate huge amounts of urea in their blood. What small amounts that they do produce they convert into creatine, which is nontoxic. Additionally, instead of becoming a toxic waste, the nitrogen wastes in hibernating bears are biochemically recycled back into protein; hence no loss of muscle mass is experienced even as they don’t exercise. Thus a hibernating bear never needs to get up to take a drink or go take a leak all winter. Water is conserved because none is needed to flush out toxic wastes, and the animals stay in shape. But that alone does not make the bear the ultimate enviable couch potato that it is. Other physiological wonders of its fitness continue to be elucidated.
We require mechanical stress of exercise on our skeleton to maintain bone structure and function, as was dramatically illustrated during weightlessness experience in space (Johnson 1998). Bone mineral content loss, depending on specific bones, was 3.4 percent to 13 percent, even on a 17-day space mission. Muscle volume decreased at similar rates, raising concerns for the effect of extended space travel on astronauts’ health (White and Avener 2001). Paraplegic accident victims suffer similar bone-density loss, with 30 percent lower-body bone mass depletion within six months of losing the use of their legs. Neither space nor weightlessness as such is therefore the real cause of debilitation. The real culprit of osteoporosis and muscle mass loss is physical inactivity, and to counter these effects, the Soviet space program, which included a record-setting mission of 366 days, emphasized intensive exercise. Bears do without the exercise and suffer no ill effects. In their long evolutionary history, those that could not tolerate the rigors of prolonged inactivity were weeded out.
Scientists at the NASA Ames Research Center have studied the effects of inactivity in healthy young volunteers who got well paid to lie flat on their backs in bed for a few days to a few weeks at a time (Miller 1995). Since 1971 more than 500 participants at the Ames Center have proven the dramatic implications of sedentary lifestyle to humans. There is not only bone loss and weakened muscles, but also slower absorption from the gut and prediabetic resistance to insulin. The scientists concluded that the physical stresses placed on the body during space flight are virtually identical to those of prolonged bed rest, or those of a hibernating bear. Again the question was: How does the bear’s body stave off bone loss?
Most of the American population subjects itself to the physical stress of inactivity. One in three Americans over fifty is completely sedentary. Therefore, our muscles, deprived of exercise, become resistant to insulin that normally promotes the absorption of glucose; blood sugar reaches dangerously high levels whenever we consume sugar-containing products and so we risk the onset of adult (Type II) diabetes. Our bodies are not adapted to inactivity. In our evolutionary history, in contrast to bears, exercise was a constant, and we’re not made to tolerate being idle for long. We’re adapted as long-distance endurance predators, as I’ve elaborated on in Why We Run: A Natural History. Inactivity adversely affects every organ system in the body, at least so long as we continue to eat. However, I suspect that a caloric surplus could be a relevant variable, too, since that is often the result of inactivity.
Ralf Paffenbarger, a physician at Stanford University, studied the effects of the lifestyles of 17, 000 Harvard students for twenty-five years after they graduated from college and concluded that exercise is a prime variable for health and longevity. That is, the stresses of inactivity mimic the aging response. Every hour of vigorous exercise as an adult was repaid with two hours of additional life span. There is, obviously, a limit to the benefits of human exercise or else more exercise could make us immortal. Instead, too much exercising increases the aging process as well. I suspect the debate of optimum exercise for maximum longevity may relate less to how much exercise we get than to how many calories we take in versus how many we burn off. There is a correlation between eating less and having a longer lifespan. But of course starvation shortens life, and there is thus also a correlation between eating more and having a longer lifespan. The difference is in the range of food intake versus the amount of exercise.
Rest does not as a rule speed up the deteriorating effects of aging, since hibernators that enter torpor have a longer life span than nonhibernators. That is not surprising, because during hibernation physiological functions are put on hold, presumably those that result in degradation as well as those of regeneration. Thus, the long period of low body temperatures characteristic of hibernation are like death to the animal, so that in effect its life span may be extended, even if the number of hours it spends living (as we define eating, defecating, moving, and sleeping) are curtailed. (It would be of interest to know if, subtracting the hibernation time, hibernators have the same or different life spans than their nonhibernating congeners.)
Hibernating bears accomplish metabolic feats that, if we knew their secrets, would likely lead to cures for many human ills. They have the secrets of how to survive lack of exercise, and then, after five months of resting, of how to get up and walk up a mountain. In all of those months of what amounts to bed rest, they suffer no bed sores. They have marginal loss of muscle mass and no change in muscle fiber type. Despite their non-weight-bearing position for months at a time, they do not suffer from bone loss or osteoporosis. After burning fat for fuel for months during which their cholesterol levels become double those of humans and those they have in the summer, yet they still don’t suffer from hardening of the arteries or gallstones, conditions resulting from high cholesterol levels in us. Most of the enigmas that have been revealed in hibernating bears have not been solved, maybe because bears just can’t be studied as conveniently as lab rats. We can be reasonably certain, however, that once we understand how bears hibernate through the winter, we will also have a larger window into ourselves. We inadvertently simulate a hibernation-like state of inactivity in our modern environment, a new state of nature to which we are not well adapted.