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Decoding the Polar Bear Paradox
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Little is known about the unusual metabolic and morphological adaptations polar bears have evolved to cope with some of the most extreme fasts of any mammal. Douglas Page profiles two scientists who endure extremes of their own while providing some surprising answers about the world’s largest land predators.

by Douglas Page, 1999

The screaming Bell 206B Jet Ranger begins its pounding descent out of the High Arctic sky, closing on the lumbering polar bear from behind and to the left.

"100 m...50 m...10 m," the pilot shouts, while the shooter steadies the rifle in a special window in the helicopter’s right side door. The target is a 100 cm2 region in the bear’s neck and upper shoulder. Expert pilots and marksmen are required. At approximately 3 m over the stampeding bear’s left shoulder a dart containing a tranquilizer drug called Telazol is fired.

It takes about 5 minutes for the bear (Ursus maritimus) to drop, during which time the pilot maintains control over the bear’s movements, remaining close enough to herd the dazed beast away from open water or unsafe Arctic terrain but far enough away to minimize stress on the animal. The more stressed the bear becomes, the more resistant it is to the anesthetic. Once the bear collapses the researchers begin their sampling and tagging procedures.

For polar bears, alien abduction is no tabloid fantasy.

The Bear Facts

Male polar bears, 2.5 to 3 m in length, can weigh over 650 kg. Females, normally shorter and weighing less than a third of that, are no less ferocious. Some have been seen to rear up and leap at helicopters carrying scientists.

"Unmarked bears are marked with ear tags and lip tattoos for identification," says University of Saskatchewan vertebrate ecologist Malcolm Ramsay, who has conducted about 40 4-6 week Arctic field seasons in 20 years. "A vestigial first premolar tooth is extracted for age determination." As polar bears age, each year a thin layer of cementum is added to the outside of each tooth. Age can be estimated by examining a slice of tooth and counting the layers. Polar bears have been known to live 20 to 30 years in the wild, but most live only 15 to 18 years.

Body mass is determined by weighing the bears with an electronic load cell. Other procedures vary depending on the objectives for each field season, and have included the attachment of radio telemetry to monitor the condition and range of individuals; intravenous administration of deuterium (2H2O) to calculate lean body and fat mass of each individual; expired air collection to determine respiratory exchange ratio; serial collection of blood, serum, and plasma to trace administered labels, to determine hematological and biochemical differences between seasons and sexes; intravenous glucose tolerance testing to determine differences in insulin and glucagon responses between feeding and fasting bears; and collection of milk samples and adipose tissue biopsies for determination of lipid soluble contaminants.

The procedures last 2 to 3 hours, or until the bear begins to stir, at which point the researchers collect their gear, ensure the bear is safe, then begin the search for the next bear. In the past five years Ramsay and his graduate students have captured and sampled more than 700 individual free-ranging polar bears in the Arctic wilderness on Cornwallis Island near Resolute, Nunavut (formerly part of the Northwest Territories), 300 miles from the North Magnetic Pole.

Pervasive Peril

For those working in such harsh, isolated conditions, peril is as pervasive as the cold.

"There are numerous safety issues that concern helicopters, firearms, the anesthesia of large carnivores, and life in a remote field camp," says Ramsay colleague, comparative physiology and biochemist Mark Cattet, also of the University of Saskatchewan. "Specifically, on the topic of anesthetizing polar bears, there have been some close calls in past years. Nevertheless, they are rare and usually result from carelessness on our part, and not as a consequence of the nature of polar bears."

The greatest danger the researchers face is having a catastrophic helicopter accident, says Ramsay. "The weather in the Arctic can change quickly and we can suddenly find ourselves in blowing snow with zero visibility."

With ambient temperatures well below zero and the nearest help hundreds of kilometers away, the researchers are then as alone as it’s possible to be on earth.

"The worst situations we’ve found ourselves in is being grounded in some remote location by terrible weather or because of a mechanical failure in the helicopter," says Ramsay. "The longest we’ve ever been stranded, however, is overnight."

The crews carry gear to survive the hopeless cold, but nothing to warm the wandering mind left stranded in uncertainty on the tundra. This is the same chilblain wilderness where John Franklin’s third voyage to find a Northwest Passage disappeared in 1845. The entire expedition of 129 men was never heard from again.

The scientists use GPS navigation now, but before that they often had only a crude idea of where they were, which made locating critical supplies like fuel caches problematic.

The bears themselves are, of course, dangerous and all researchers carry handguns and rifles for protection. The forepaws of a polar bear, measure up to 30 cm in diameter. "Contrary to public perceptions, however, polar bears are not greatly aggressive and we have never been in a serious altercation with a bear," Ramsay says.

There are suggestions the bears, however, may suffer long-term consequences from their encounters with researchers. According to Polar Bears Alive, a non-profit, international organization dedicated to the worldwide protection of the polar bear, studies have shown that darted and tagged female bears consistently produce smaller litters and lighter cubs. If tagged in the den area, pregnant females may abandon the site.

Fast and Loose

Ramsay chose to work in the Arctic because the ecosystems seemed relatively simple. "If one is interested in predator-prey relationships - one predator (polar bears), one prey (ringed seals primarily), what could be simpler?" he asks rhetorically. "Unfortunately, after almost 20 years of study I feel I know less about polar bears than I thought I knew when I began."

Ramsay is currently studying the long-term physiological and ecological effects of intermittent feeding patterns on vertebrates, primarily polar bears, and to a lesser degree the narwhal (Monodon monoceros) and bluntnose sixgill sharks (Hexanchus grisus). Polar bears go without food for up to eight months, yet remain active, give birth, and nurse young while fasting, making them excellent models for Ramsay’s research.

Polar bears, feeding predominantly on seals, reside not only at the top of the world but at the top of the Arctic food chain. Alone among bears, the polar bear is considered a marine mammal.

"Late spring and early summer is their peak feeding period," says Ramsay. "When food is abundant they preferentially consume the blubber. The body mass of individual polar bears can more than triple over this period of hyperphagia on a high fat diet and their bodies may consist of more than 50 percent adipose tissue (vesicular cells filled with fat)." The bear's blubber layer can measure 11.4 cm thick.

For most of the remaining year polar bears appear to feed little. At the end of a fast, adipose tissue depots may be reduced to less than 10 percent of body mass. Gestation and early lactation, the most energy-costly periods of the mammalian reproductive cycle, are undertaken by polar bears entirely while fasting. At parturition, a female polar bear will have been without food and water for several months and will continue to fast for many weeks more while the cubs nurse.

"Our working assumption was that although the underlying metabolic adaptations of polar bears to fasting were unknown, it seemed logical to assume that they would be similar to the closely related and more intensively studied black bear," he says. Instead, he found profound differences in the metabolism of fasting between these species. Ramsay now hypothesizes that polar bears maintain themselves throughout the year on a lipid (fat)-based economy. When feeding, the fat comes from seals. When fasting, it comes from endogenous stores. In contrast, most mammal diets are high in carbohydrates or proteins.

"In most vertebrates there is a switch in energy substrates between the feeding (carbohydrate-based) and fasting (lipid-based) states," he says.

The Toxic Arctic

Because they feed almost exclusively on marine mammals, polar bears are exposed to relatively high levels of bioaccumulating environmental contaminants. Researchers have found mean levels of polychlorinated biphenyls (PCBs) recorded in the adipose tissue of polar bears similar to the threshold level of PCB concentration in the blubber of seals at which deleterious reproductive effects are evident.

Because polar bear cubs are nourished for many weeks on fat-rich milk derived from their mothers’ adipose stores, their diet is consequently high in organochlorine contaminants.

The unique annual dietary regimen of polar bears coupled with their high trophic status offered Ramsay a change to investigate the kinetics of organochlorine contaminants under long-term fasting.

The most significant findings from this longitudinal toxicological sampling of free-ranging carnivores include the first characterization of cytochrome P450 (an enzyme that gives an organism the ability to metabolize foreign chemicals) in polar bears; the first determinations of the body burdens of several PCBs in polar bears before and after a lengthy fast; and the first determination of the transfer rates of these contaminants from mother polar bears to her cubs.

Metabolic Derangements

In a separate but related study in the same barren reaches of the Arctic, Mark Cattet is also trying to understand polar bear physiology and biochemistry. If human diabetes is ever understood and cured we may have scientists like Cattet and Ramsay to thank.

In humans, excessive consumption of dietary fat, obesity, and prolonged fasting are well known risk factors for diseases such as coronary heart disease, type II diabetes mellitus and anorexia nervosa. Interestingly, these dietary extremes are not only consistent with normal health in polar bears, their survival may mandate them. Therefore, the question Cattet is trying to address is, how do polar bears maintain normal health in the face of a dietary lifestyle characterized by high-fat consumption, obesity, and prolonged fasting?

Three steps are required to address this question, he says. First, determine the physiological and biochemical response of polar bears to variation in food availability (from feed to fast) and to variation in body condition (the availability of body fat and muscle stores).

Second, identify the points of departure in the physiological and biochemical response of humans and polar bears. In other words, where exactly do these two species differ in their responses to food availability and body condition?

Third, determine what mechanisms are responsible for regulating the physiological and biochemical response of polar bears.

"Our supposition is that many of the differences between polar bears and humans will prove to be quantitative (similar mechanisms, but different degrees of expression), rather than qualitative (entirely different mechanisms) in nature," Cattet says. "We believe that concentrating our research efforts on the points of departure between these species may ultimately provide new insight into both the energy metabolism of polar bears and the pathogenesis and treatment of human coronary heart disease and type II diabetes mellitus."

So far, many of the results have been surprising.

"There are numerous features in the physiology and biochemistry of polar bears that, when viewed in isolation and from a human health perspective, are suggestive of significant metabolic derangements," Cattet says. "Nevertheless, we find no evidence of metabolic disease in polar bears. Instead, these features appear to represent components of a tightly regulated energy metabolism that is sensitive both to the availability of food and the availability of body energy stores."

For instance, polar bears accumulate glycogen (a storage form of glucose) in their livers while fasting for many months. In contrast, humans deplete their liver glycogen within 1-2 days of fasting.

Another surprise was finding that the blood plasma concentration of ketone bodies (liver exports that build up in human blood because of starvation or uncontrolled diabetes) remains at barely detectable levels in polar bears even after months of fasting. In humans, by contrast, ketone bodies are a very important fuel source and, as a consequence, the blood plasma concentration of these compounds increases dramatically during fasting and reaches pathological levels (ketoacidosis) in uncontrolled diabetes.

"Our results, however, appear to indicate that ketone bodies are not an important fuel source to polar bears, whether fasting or feeding," Cattet says.

The biggest surprise, however, was the finding that polar bears develop significant resistance to the effects of insulin as their body fat stores increase and they become obese.

In human adipose tissue and skeletal muscle, insulin increases glucose uptake. Insulin resistance occurs when, for an average level of insulin in healthy patient, these tissue and muscle do not react normally. It is more difficult for glucose to enter the cells and, to compensate for this defect, more insulin is produced. It forces the glucose into the muscle and adipose tissue. As a result, the insulin resistant patient shows both hyperinsulinaemia, due to pancreas overproduction, and hyperglycaemia, caused by the low glucose uptake in tissues. As long as the pancreas is able to produce enough insulin, blood glucose levels are under control and insulin resistance is balanced. Otherwise, glycemia increases and type II diabetes, a disease characterized by abnormally high sugar levels in the blood, may appear.

Cattet administered glucose by intravenous injection to anesthetized polar bears. Serial blood samples were then collected over a two-and-a-half hour period following the glucose injection. By measuring the concentrations of glucose, insulin, and glucagon in the blood plasma, Cattet was able to determine the response of the pancreas to a glucose challenge and the effectiveness of insulin in returning the blood glucose concentration back to baseline values.

"We found that the plasma insulin concentration in obese polar bears increased approximately eight-fold in response to a glucose injection, whereas the increase in the plasma insulin concentration of leaner polar bears was approximately three-fold," Cattet says. "Yet, despite a markedly greater plasma insulin response in obese bears, the plasma glucose concentration returned to normal levels sooner in the lean polar bears (60-90 minutes versus 120-150 minutes in obese bears). Thus, obese polar bears can be described as having a significant degree of insulin resistance or the inability of body tissues to respond to insulin."

In humans, obesity is also associated with insulin resistance, but often progresses to diabetes. The complications of untreated diabetes include the progressive destruction of the circulatory and nervous systems, the eyes, and the kidneys. Diabetes is the fourth leading cause of death by disease in the United States. Worldwide, 130 million cases were reported in 1997 and 300 million cases are predicted by the year 2025.

"In polar bears, we see no evidence of diabetes," Cattet says. "Instead we see insulin resistance as being a tightly regulated process that is highly sensitive to body condition. It’s present when obese, but disappears when lean. If we can determine how polar bears are able to accomplish this, then perhaps we can determine why insulin resistance progresses to a disease state in humans."

BEARING FRUIT

Polar bears range throughout the Arctic, clawing across the ice cap from Russia to Alaska, Canada to Greenland, and on to Norway's Svalbard archipelago. Scientists estimate the world’s polar bear population somewhere between 20,000 and 40,000.

Scientists attempting to understand polar bears face an exhausting challenge. Not only are the bears difficult to locate in the vast, bleak Arctic, once they’re found the researchers must contend with the world’s largest land carnivore and some of the world's worst weather.

Vertebrate ecologist Malcolm Ramsay’s polar bear work on Cornwallis Island, Nunavut, has been to develop innovative methods to determine basic life-history parameters. In the past four years his research has produced several ‘firsts’, including:

o the discovery that the metabolic responses of polar bears to fasting are in stark contrast to black bears (U. americanus) and, perhaps, all other mammals;

o the first-ever determinations of milk yield and transfer rates in polar bears and the growth dynamics of cubs;

o the fat content (hence energetic value) of polar bear milk was found to be sensitive to maternal body fat stores;

o mothers in poorer condition produce milk of significantly lower nutritional value;

o the determination of body composition in free-ranging bears showing when polar bears fast, lipids from adipose tissue depots meet the majority of metabolic energy requirements. Body protein is catabolized during long fasts, albeit at a significantly lower rate than most terrestrial mammals, and the efficiency of protein sparing is inversely proportional to relative fatness at the start of the fast;

o body fat was found to be critically important for reproductive success. Offspring weight at emergence from the overwinter den was strongly correlated with the size of maternal fat stores immediately prior to entering the den;

o and the determination that growth in early life influences adult body size of females, but not males. "This finding, while robust, was unexpected and likely has profound implications for maternal investment strategies in the species and for the dynamics of their mating system," Ramsay says.

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Comments? Questions? Corrections? Assignments? douglaspage@earthlink.net

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