Polar bear hibernation does not occur like black bear or grizzly bear hibernation. Black and grizzly bears, unlike polar bears, are true hibernators. Polar bears probably do not hibernate in a way other mammals do. They just enter into a specialized winter dormancy. This is little different from natural hibernation. During winter dormancy polar bears can also reduce their heart rate and possibly metabolism and temperature. Let’s see how do polar bears hibernate.
Do Polar Bears Hibernate? – Polar Bear Hibernation
Female bears dig dens in snow and they remain asleep for 4 to 8 months. They become absolutely inactive during this period. Polar bears neither eat nor do they drink in fact they do not even urinate whilst they are hibernating. In spite of all this inactivity females appear to have stable mineral levels with a constant body mass.
Many researchers estimate the level of blood metabolites which is almost fully maintained. This is quite remarkable in that the animal loses half of its weight inside the den while maintaining the metabolite level stable.
During hibernation or dormancy polar bears make up the deficiency of water from fat catabolism. By so doing they are able to maintain stable fluid level within the body.
Read More: Polar Bear Den
What is Hibernation?
Hibernation is an adaptation that allows some species of mammals to store fat when the feeding is good, and then burn it off slowly at a lower metabolic rate while in a deep resting state when food is unavailable, usually, but not always, during winter. True hibernators such as some rodents, bats, or insectivores experience a significant decline in heart rate, their body temperature may approach 0°C (32°F), and it may take some time to arouse them because they have to expend stored energy to restore the body to its normal temperature before it can function properly again. The largest mammals that are true hibernators are marmots. Bears are too large to lower their body temperature to very low levels because they would simply not have enough energy to be able to reheat such a large mass back to its normal operating temperature.
Bears also need to maintain a much higher body temperature than true hibernators because they also still need to be able to maintain the physiological demands of pregnancy, birth, and nursing the young. Although bears sleep soundly, they are easily aroused and can, if necessary, defend themselves.Although most species of bears go into dens during the winter, they have some important differences from the so-called true or deep hibernators. From research done on captive bears, we know that the heart rates of hibernating black and grizzly bears are capable of slowing to 10 to 12 beats per minute, or even lower sometimes, but their body temperature only declines to about 31-35°C (88-93°F). The heart rate of a polar bear held in an artificial den decreased to 27 beats per minute after about a month. The deep body temperature of two female polar bears hibernating in natural dens during the winter ranged between 35 and 37°C (95-98.6°F).
Hibernation in Black and Grizzly Bears
Hibernation and seasonal food scarcity for polar bears differ considerably from those of black and brown bears. Firstly, only gestating female polar bears enter dens during the winter, while the rest of the population remains active. Secondly, for many polar bear populations, the period of greatest food deprivation is the open water phase of late summer and early autumn, just when black and grizzly bears are consuming most extensively in order to accumulate fat reserves with which to endure the upcoming winter.
One of the most interesting chapters in our understanding of the “hibernation-like” state in polar bears, and hence how they have adapted their physiological needs to the vagaries of the arctic environment, originated with the late Ralph ag in Nelson. He wondered how black bears could hibernate through the winter at near-normal body temperatures without eating, drinking, or producing any urine or faeces. When hibernating, the black bear produces all the water it needs by chemical pathways from its stored fat; then it reduces the by-products without producing waste materials. Nelson thought if he could figure out how this was done, there might be enormous benefits to humans with kidney problems.
Nelson found that he could define a bear’s physiological state by the ratio of the concentrations of two chemicals in the blood, urea and creatine. Creatine is produced by normal muscle activity and its level in the blood remains pretty H much the same all the time. However, the amount of urea in the blood goes up when an animal is eating and becomes very low when it stops eating and lives only on its fat.
Walking Hibernation in Polar Bears
After some experimentation, Nelson defined the urea-to-creatine (U/C) ratio (that is, the number of units of urea in a sample of blood divided by the number of units of creatine) of a hibernating black bear as any anything less than 10. He then examined blood samples taken from non-feeding polar bears on the western coast of Hudson Bay during the ice-free period in the late summer and fall. He found that the U/C ratio was at a a similarly very low level in them as well. Thus, even though the bears were not in dens and were still active, they were (in the physiological sense) hibernating. This gave rise to the rather intriguing term “walking hibernation”.
A curious aside to this finding is the observation of an old Inuk hunter on the Labrador coast, passed on to a visiting anthropologist, sometime prior to 1916. He said that old male polar bears hibernate in caves along the coast in the summer when there was no ice. As in Hudson Bay, open water prevails along the coast of Labrador for many months in the summer, so we now know that similar hibernation-like responses must occur in the bears there. It has always intrigued me that an observant Inuk hunter noted the parallel between the summer behaviour of polar bears in caves and the winter behaviour of bears in dens.
Nelson and some of his colleagues then came to Churchill to work with our group so he could learn more about how polar bears evolved to live in the arctic environment. We selected polar bears of various sizes for non-harmful experiments, sometimes while other bears wandered by and gazed curiously at the goings on. By looking at the U/C ratios over the next couple of years, we found that female polar bears coming out of the denning area in the spring were in a similar physiological state to that of hibernating black bears. More interesting though, were the results from polar bears spending the late summer and fall along the Hudson Bay coast. At the same time that non-feeding bears were in “walking hibernation”, polar bears feeding in the dump had the be- same U/C values as non-hibernating (feeding)| black bears. The astonishing conclusion was that members of the same polar bear population could be in completely opposite physiological conditions at the same time and place, depending on whether they were feeding or not feeding. That is something a black bear cannot do.
Andy Derocher, Nelson, and Ian Stirling then conducted some preliminary feeding experiments on several polar bears being held in captivity. They had not been fed for some time and had the low U/C ratios characteristic of the hibernation-like physiological state. When they were fed for a few days. the ratios went up, as we expected. However, once food was no longer available, the U/C ratios dropped again after about a week. Remarkably, these results indicated that polar bears could move the physiological state of a fasting bear, to one of a feeding bear, and back to that of a fasting bear, all within a period of a couple of weeks. The contrast with terrestrial bears is dramatic. If you stop feeding a black bear or a grizzly in summer, it will starve to death. Thus, it appears that one of the most remarkable physiological adaptations of the polar bear to life in the arctic environment, where the availability of food is both variable and unpredictable for much of the year, is its ability to change its metabolic state as needed. This allows the bear to maximise the efficiency of its use of stored energy, and not be limited to changes controlled by photoperiod or other seasonal signals.
No subsequent research on this topic has yet been done, though it could be very informative to repeat and extend those experiments one day. Recently, however, another study on wild polar bears gave some results similar to those obtained from the captive bears. From direct observation, we know that when searching for a breeding female, or when courting one, male polar bears may not actively hunt for extended periods of time, although if one of them catches a seal, or they encounter a carcass, both the male and female may feed on it.
In an analysis of the U/C ratios of polar bears from the Beaufort Sea in spring, Seth Cherry found, again, that bears in the same area at the same time could be in either a hibernation-like or non-hibernation physiological state. In particular, a larger proportion of adult males were not feeding, apparently maximising their time for searching for breeding females and then courting them. They could afford to do this because the major feeding period of the year overlaps, but largely follows, the most active part of the breeding season. Thus, a temporary shortfall in energy intake can be compensated for fairly soon afterward. Most important though, since active hunting may absorb up to a third of a bear’s time under normal conditions, it allows the male to slow its metabolic rate slightly and make maximum use of its stored energy to increase its searching and courting time which in turn will improve it chances of finding a mate and passing on its genes.
