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Black Bears and Polar Bears—Still Metabolic Marvels

  • Ralph A. Nelson
    Correspondence
    Address reprint requests to Dr. R. A. Nelson, Department of Research, Carle Foundation Hospital, 611 West Park, Urbana, IL 61801
    Affiliations
    Director, Department of Research, Carle Foundation, Chairman, Department of Medicine, University of Illinois College of Medicine at Urbana-Champaign, Urbana, Illinois
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      Black Bears.

      Webster's definition of “editorialize” includes “to express personal opinion when reporting facts.” Few scientific subjects generate more personal opinions than facts about the black bear's amazing adaptation to existing for months with no food or water. Consider that the black bear does not eat, drink, urinate, or defecate for 5 months or more. Yet, its body temperature is normal. The bear burns up to 4,000 kcal/day. While the female bear is denning, cubs are born and nursed, and they weigh 4.5 kg or more before leaving the den in April.
      • Nelson RA
      Winter sleep in the black bear: a physiologic and metabolic marvel.
      A closer look at the black bear's adaptation reveals some startling facts. The female bear is able to starve (in the sense of having no food or water intake) but carries out successful pregnancy and lactation. This situation combines three usually incompatible states: starvation, pregnancy, and lactation. Can other mammals do this? A human mother would lose her fetus if food were withheld for a few days, and she certainly cannot nurse a baby while starving.
      Startling calculations based on winter data show that black bears have a small net increase in lean body mass during denning.
      • Nelson RA
      • Steiger DL
      • Beck TDI
      Neuroendocrine and metabolic interactions in the hibernating black bear.
      Recent studies of both male and female bears show that this net increase in body protein is lifesaving; bears who cannot increase lean body mass become uremic and die.
      • Nelson RA
      Protein and fat metabolism in hibernating bears.
      In successful denning, blood urea declines and serum creatinine increases. The net effect is a substantial decrease in the ratio of blood urea to creatinine. This ratio decreases from 20 or more in summer to 10 or less in winter.
      • Nelson RA
      • Beck TDI
      • Steiger DL
      Ratio of serum urea to serum creatinine in wild black bears.
      This indicator of successful adaptation to no food or water has been found in bears in captivity and in wild bears from Colorado, Maine, Pennsylvania, and Minnesota.
      • Ensrud ER
      • Nelson RA
      • Alt G
      • Beck T
      • Matula G
      • Rogers L
      Seasonal variations in the ratio of serum urea to creatinine (U/C) in black bears (abstract).
      Net formation of protein and prevention of uremia result from at least two mechanisms that shuttle nitrogen into protein. One shuttle combines nitrogen with glycerol; thus, amino acids such as alanine and serine are produced. These amino acids are then incorporated into plasma proteins.
      • Ahlquist DA
      • Nelson RA
      • Steiger DL
      • Jones JD
      • Ellefson RD
      Glycerol metabolism in the hibernating black bear.
      Preliminary studies reveal another, somewhat surprising shuttle. Injection of doubly labeled [15N]urea in denning bears shows that urea is quickly degraded. Nitrogen from it appears in the amino acids ornithine, arginine, glycine, tyrosine, phenylalanine, and threonine.
      • Wolfe RR
      • Nelson RA
      • Stein TP
      • Rogers L
      • Wolfe MH
      Urea nitrogen reutilization in hibernating bears (abstract).
      • Wolfe RR
      • Nelson RA
      • Wolfe MH
      • Rogers L
      Nitrogen cycling in hibernating bears (abstract).
      All these amino acids are either neurotransmitters or precursors. Similar to alanine and serine, however, these amino acids also enter synthetic pathways and produce plasma proteins such as fibrinogen and albumin. Over winter, the net effect of these nitrogen shuttles is the incorporation of about 3 g of urea nitrogen into about 21 g of plasma proteins.
      • Nelson RA
      • Steiger DL
      • Beck TDI
      Neuroendocrine and metabolic interactions in the hibernating black bear.
      In this respect, the male bear is “pregnant”; it makes approximately the amount of protein found in one cub. The net effect of prolonged starvation is that the bear loses only fat while gaining a little protein. Although male king penguins, while fasting, make keratin for feathers from body protein, the net effect is loss of both body fat and protein Oust as in other mammals).
      • Groscolas R
      Study of molt fasting followed by an experimental forced fasting in the emperor penguin Aptenodytes forsteri: relationship between feather growth, body weight loss, body temperature and plasma fuel levels.
      • Le Maho Y
      • Delclitte P
      • Chatonnet J
      Thermoregulation in fasting emperor penguins under natural conditions.
      • Stonehouse B
      The general biology and thermal balances of penguins.
      Many questions remain. Does the bear degrade urea within its body? Is the essential amino acid threonine transaminated in the bear? Can the bear synthesize essential amino acids? From a personal standpoint, the answer to all these questions most likely is yes because no animal, to date, can survive during starvation like bears. Answers to these questions may be forthcoming from studies now under way at the Carle Foundation Bear Research Station located in Champaign County, Illinois.

      Meredith M, Nelson RA, Baker DH: Urea/creatinine ratios in black bears. Int Union Physiol Sci, 1986, abstract no. 465.13

      Of interest, one of these investigations, done in conjunction with other faculty members from the University of Illinois, has pursued the study of threonine metabolism in bears on the same campus where Will Rose demonstrated threonine as an essential amino acid for animals.
      Taking a closer look at the natural history of black bears generates another personal opinion that bears must control metabolism of calcium and phosphorus in a unique manner also. Dens have been estimated as too small to permit weight-bearing positions. Visual observations have confirmed the impression that bears (both in the wild and in captivity) lie down in their dens. Yet, bears do not have the increased losses of calcium in urine and feces that cause so many problems in bedridden human beings and astronauts in outer space.
      • West JB
      Man in space.
      In fact, during denning, both in wild and in captive bears, serum calcium remains within normal limits, serum phosphorus decreases slightly, the calcium-phosphorus product remains normal, and no calcium is lost in either urine or feces.

      Alt G, Matula GJ, Nelson RA, Storm GL: Blood chemistry of Pennsylvania black bears during winter dormancy (submitted for publication)

      David LM: Study of calcium, phosphorus and hydroxyproline in black bears. Thesis, University of Illinois, Urbana, 1987

      What is the pattern of food intake of black bears? In late summer and early fall, the black bear becomes obese because of hyperphagia. Closer observations, however, reveal that this is not an uncontrolled state but a marvelously regulated short-term intake of food. During the hyperphagic period, the bear eats almost to a calorie the amount of energy required for denning. In fact, some bears store sufficient energy for denning by late summer or early fall, even while food is available.
      • Nelson RA
      • Beck TDI
      • Steiger DL
      Ratio of serum urea to serum creatinine in wild black bears.
      When this occurs, bears leave fall ranges, where food is ample, to roam in summer ranges, where no food is available. This behavior is seen in pregnant and solitary female bears as well as in male bears. Apparently, the bear is prepared for denning long before cold weather or considerable decreases in periods of sunlight occur. Coincident with this finding, the ratio of serum urea to serum creatinine is low, approximating values found early in denning. This low ratio reveals that the biochemistry for successful denning is operative.
      • Nelson RA
      • Beck TDI
      • Steiger DL
      Ratio of serum urea to serum creatinine in wild black bears.
      The biochemical adaptation of denning persists into springtime after bears leave their dens.
      • Nelson RA
      Protein and fat metabolism in hibernating bears.
      Bears studied in captivity and in the wild demonstrate, for about 3 weeks, continuation of anorexia and refraining from drinking water. Thus, the period of adaptation that permits independence from need for food and water begins before denning and persists afterward. Apparently, cold weather is not necessary for denning, because bears den in Arizona, Kentucky, Tennessee, Louisiana, and Florida, where ambient temperatures are mild.
      Thus far, the bear has eluded attempts to relate stages of hyperphagia and denning to changes in activities of hormones. Hypothalamic hypothyroidism occurs in denning bears as it does in humans with anorexia nervosa.
      • Azizi F
      • Mannix JE
      • Howard D
      • Nelson RA
      Effect of winter sleep on pituitary-thyroid axis in American black bear.
      Cortisol, insulin, and glucagon do not seem to be factors in effecting these adaptations.
      • Palumbo PJ
      • Wellik DL
      • Bagley NA
      • Nelson RA
      Insulin and glucagon responses in the hibernating black bear.
      The changes in sex hormones, although showing seasonal differences, do not occur in synchronization with the denning adaptation.
      • Palmer SS
      • Nelson RA
      • Bahr JM
      Circannual serum concentrations of testosterone (T) in male and progesterone (P) and estradiol (E) in female black and polar bears (abstract).

      Polar Bears.

      The polar bear, it is now suspected, differs from black and grizzly bears almost as much as the latter differ from deep hibernators such as ground squirrels, wood-chucks, and marmots. In an evolutionary sense, polar bears in their adaptation to the hostile and stressful environment of the Arctic may be the most advanced of any animal as they cope with variant supplies of food, extremes of weather, and lack of fresh water.
      Preliminary data suggest that the polar bear assumes a hibernation-like metabolism during the summer and fall while maintaining physical activity. Studies of polar bears in the southern Hudson Bay area have shown that when ice breaks up on the bay, all bears come ashore on the coast or on islands. At Cape Churchill in Manitoba, polar bears are observed walking along the water, swimming, or sitting by Hudson Bay. There is little evidence that vegetation is eaten. During summer, polar bears ignore food;

      Stirling I, Jonkel C, Smith P, Robertson R, Cross D: The ecology of the polar bears (Ursus maritimus) along the western coast of Hudson Bay. Can Wildl Serv Occ Pap No. 33, 1977

      they survive by using their fat reserves.
      During summer and fall, in association with the lack of eating and drinking, polar bears have ratios of serum urea to creatinine of 10 or less.
      • Nelson RA
      • Folk Jr, GE
      • Pfeiffer EW
      • Craighead JJ
      • Jonkel CJ
      • Steiger DL
      Behavior, biochemistry, and hibernation in black, grizzly, and polar bears.
      • Ramsay MA
      • Nelson RA
      • Stirling I
      Ratio of serum urea to serum creatinine in wild polar bears (abstract).
      The conclusion is that these animals are in a biochemical state of hibernation during summer similar to that found in black bears during winter. Thus, when their favorite food (seals) is not available, polar bears do not eat other food and only rarely have been noted to drink water.
      In November, the polar bears return to the ice of Hudson Bay. They roam long distances. Arctic activity exacts a high price in energy. The adult polar bears meet these needs by restricting food consumption primarily to fat in the form of seal blubber. They meet energy requirements by killing a seal approximately once every 6 to 8 days and consuming about 50,000 kcal of blubber.
      • Stirling I
      • McEwan EH
      The caloric value of whole ringed seals (Phoca hispida) in relation to polar bear (Ursus maritimus) ecology and hunting behavior.
      Once again, a personal opinion is generated from these behavioral observations. If adult polar bears were forced to eat meat, their intake of water would have to be increased to meet the demands for excretion of urea. Thermal regulation would be affected because ice and snow consumed at an ambient air temperature as low as −60°C might require up to a third of the daily metabolism merely to be warmed to the 37°C body temperature of the bear. Consumption of ice and snow would cause a severe drain on fat stores in the body of the bear and would seriously affect its ability to survive. Although consumption of meat might disrupt the metabolic adaptation of the polar bear, consumption of fat would not. The end products of fat combustion are carbon dioxide and water, which are excreted by respiratory exchange. Therefore, no need exists for increased consumption of water.
      Polar bears that require protein (lactating females, cubs, yearlings, and subadults) can feed on the protein part of the seal to supply growth needs. Once the polar bear reaches adult size, however, its protein intake expressed in grams per kilogram of body weight must be the lowest of any mammal.
      On one occasion, when I expressed my opinion to zoo veterinarians on how to feed bears in captivity similar to their eating pattern in the wild, it was not greeted with enthusiasm. No food or water need be given between July and November. The rest of the year, fat could be fed about once a week!

      Speculation.

      Black bears and polar bears may not be hibernators. They do not undergo periodic arousals as do deep hibernators. They are alert during denning. Furthermore, polar bears have the winter biochemical adaptation of black bears year around. In my opinion, polar bears must represent the most advanced evolutionary adaptation of any mammal to conditions of no food or water. They may be able to revert to the protein-conserving state at any time when food is not readily available.
      The initial purpose for studying bears was to identify information with possible applications to human diseases. After approximately 18 years, accumulation and interpretation of data have not changed that aim. Data from the studies of bears have helped formulate diets for treatment of patients with chronic renal disease and end-stage renal failure.
      • Nelson RA
      • Anderson CF
      • Hunt JC
      • Margie J
      Nutritional management of chronic renal failure for two purposes: postponing onset and reducing frequency of dialysis.
      In chronic renal disease, successful nutritional management produces a reduction in the urea to creatinine ratio in the blood.
      • Nelson RA
      • Anderson CF
      • Hunt JC
      • Margie J
      Nutritional management of chronic renal failure for two purposes: postponing onset and reducing frequency of dialysis.
      Remember that this change is a necessity for bears to survive when they are not eating or drinking.
      The data from studies of bears have numerous other possible applications to human conditions—for example, improved management of chronic renal failure, prevention and treatment of osteoporosis, treatment of obesity and anorexia nervosa, and increased duration of healthy space travel. A quote from a recent issue of Science summarizes the importance of bear data: “This paradigm of fuel efficiency, absence of toxic wastes, budget balancing, and productivity is one from which individuals, governments, and businesses have much to learn.”
      • Koshland Jr, DE
      New Year's resolutions and the black bear (editorial).

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        Protein and fat metabolism in hibernating bears.
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        Ratio of serum urea to serum creatinine in wild black bears.
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        Seasonal variations in the ratio of serum urea to creatinine (U/C) in black bears (abstract).
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        Nitrogen cycling in hibernating bears (abstract).
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        Thermoregulation in fasting emperor penguins under natural conditions.
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      1. Meredith M, Nelson RA, Baker DH: Urea/creatinine ratios in black bears. Int Union Physiol Sci, 1986, abstract no. 465.13

        • West JB
        Man in space.
        News Physiol Sci. 1986; 1: 189-192
      2. Alt G, Matula GJ, Nelson RA, Storm GL: Blood chemistry of Pennsylvania black bears during winter dormancy (submitted for publication)

      3. David LM: Study of calcium, phosphorus and hydroxyproline in black bears. Thesis, University of Illinois, Urbana, 1987

        • Azizi F
        • Mannix JE
        • Howard D
        • Nelson RA
        Effect of winter sleep on pituitary-thyroid axis in American black bear.
        Am J Physiol. 1979; 237: E227-E230
        • Palumbo PJ
        • Wellik DL
        • Bagley NA
        • Nelson RA
        Insulin and glucagon responses in the hibernating black bear.
        Int Conf Bear Res Manage. 1983; 5: 291-296
        • Palmer SS
        • Nelson RA
        • Bahr JM
        Circannual serum concentrations of testosterone (T) in male and progesterone (P) and estradiol (E) in female black and polar bears (abstract).
        Biol Reprod. 1986; 1: 68
      4. Stirling I, Jonkel C, Smith P, Robertson R, Cross D: The ecology of the polar bears (Ursus maritimus) along the western coast of Hudson Bay. Can Wildl Serv Occ Pap No. 33, 1977

        • Nelson RA
        • Folk Jr, GE
        • Pfeiffer EW
        • Craighead JJ
        • Jonkel CJ
        • Steiger DL
        Behavior, biochemistry, and hibernation in black, grizzly, and polar bears.
        Int Conf Bear Res Manage. 1983; 5: 284-290
        • Ramsay MA
        • Nelson RA
        • Stirling I
        Ratio of serum urea to serum creatinine in wild polar bears (abstract).
        Fed Proc. 1985; 44: 1045
        • Stirling I
        • McEwan EH
        The caloric value of whole ringed seals (Phoca hispida) in relation to polar bear (Ursus maritimus) ecology and hunting behavior.
        Can J Zool. 1975; 53: 1021-1027
        • Nelson RA
        • Anderson CF
        • Hunt JC
        • Margie J
        Nutritional management of chronic renal failure for two purposes: postponing onset and reducing frequency of dialysis.
        in: Cummings NB Klahr S Chronic Renal Disease: Causes, Complications, and Treatment. Plenum Medical Book Company, New York1985: 573-585
        • Koshland Jr, DE
        New Year's resolutions and the black bear (editorial).
        Science. 1986; 234: 1481