Adaptive Traits of the
Polar Bear
(Ursus maritimus)
3/14/08
Throughout life’s long history evolution has been demonstrated over and over again. Competition for resources and geographic and climatic changes have lead to natural selection, which since the beginning of time has molded our world into what it is. The polar bear, Ursus maritimus, is an excellent example of a natural survivor. Since divergence from their most closely related extant ancestor during the mid-Pleistocene Epoch (300 400 TYA), the brown bear (Ursus arctos), (Talbot and Shields, 1996) polar bears have acquired several adaptive traits. By what is believed to be 100,000 years ago the polar bear came into existence. They finally had diverged enough from brown bears to be classified as there own species, although in captivity the two currently produce fertile offspring (Rosing, N., 1996). It has even been stated that polar bears have “been shown to possess the most advanced evolutionary adaptation of any previously studied hibernating mammal to conditions of no food or water” (Talbot and Shields, 1996). These adaptive traits not only include unique hibernation patterns, but also physiological and morphological advantages, and behavioral radiation. Unfortunately, not all adaptive traits remain beneficial throughout the course of life’s history, and as the biosphere changes an organism must continuously adapt as well. Whether prior adaptations can be reversed or continue to be modified is a question not easily answered (I. B. Weiner., et al., 2003). For instance, will global warming lead to the end of the polar bear?
An inference made from the study of mitochondrial DNA strongly implies that polar bears have diverged from the same linage as brown bears currently living on the islands of southeast Alaska. Both extant groups are believed to have descended from brown bears in the proximity of Siberia that had expanded into North America (Talbot and Shields, 1996). Since their initial divergence they have come to inhabit such areas as Canada, Greenland, Norway, Russia, and the USA (Howell-Skalla, L. A., et al., 2002). Despite geographical differences experiments have shown little genetic variation among these populations indicating minute independent adaptations (Paetkau, D., et al., 1999), which can be complimented with the fact that polar bears are a relatively new species; the last descendant of the Ursine evolutionary linage (Struzek, Ed, 2003, Rosing, N., 1996). Since other bears, especially modern brown bears most likely share a common ancestor with that of polar bears, brown bears are a great model to use in comparison due to their similarities. Many traits unique to polar bears may be due to the initial adaptations that were gradually introduced by their ancestors as their habitat shifted closer and closer to that which polar bears now occupy. Modern brown bears have proven to be capable of living very close to the harsh environment of the winter arctic, which basically means that the first generations of the evolving white bears started with the same good genes.
As top carnivore of the arctic it is no surprise that polar bears are the largest living, some are estimated to weigh as much as 800 kg, and are even known as the “King of the Arctic”, and globally hold the record as largest terrestrial carnivore. They can easily kill a 450 pound walrus with a single blow of its paw. These great white bears may also be considered a keystone species as they help reduce the population of seals, and provide carcass remains for arctic foxes, ravens, and glaucous gulls who also call the arctic their home. The males live an average of 25 years, where as females live slightly longer (Struzek, Ed, 2003; Rosing, N., 1996).
Their physical appearance is similar to a brown bear, but polar bears lack a shoulder hump, have a longer neck, smaller ears and a short tail to reduce heat loss (Demaster and Stirling, 1981; Kolenosky, G. B., et al., 1992; Gunderson, A., 2007). The polar bears specialized ears contain a network of blood vessels selected to bring extra heat to one of their only exposed regions where they experience heat loss (Rosing, N., 1996 ). They also have larger and sharper teeth allowing them to tear up seals efficiently (Struzek, Ed, 2003). Polar bears are the only bear that is primarily carnivorous with a digestive system more adapted to processing meat than plant matter, although they are still omnivores. Measuring isotopes of a polar bears breathe has proven that they actually eat berries, which happen to be high in fat. In Canada during the months of August and September they have also been observed consuming berries as they start their journey northward along the coast until the Hudson Bay freezes over; an example of their intelligence and survival skills in the absence of prey.
Another adaptive trait is their large thickly furred, webbed feet with rough foot pads, and short claws that are immune to frostbite. These features allow them to travel on ice and swim much more efficiently then other bears (Struzek, Ed, 2003; Rosing, N., 1996). Only visible around the eyes and nose a polar bears skin color is black, and their fur usually appears white due to a refraction of sunlight, but the hairs are actually clear. Colors such as yellow, brown, and gray have also been observed in a polar bears fur, associated with light conditions dependent upon the season. (Gunderson, A., 2007). Short insulating fur, and long, hollow, oily and water repellent guard hairs combined with large stores of fat, aka blubber, anabolized (created) during hyperphagia (a state of increased appetite and consumption) act as an insulation that is vital to the endothermic regulation of this great white viviparous (give birth to live young) mammal. This insulation is so efficient that aerial photographers are unable to use infrared technology to spot polar bears in the wild. The only heat emissions observable came from their breathe. They release so little heat that the snow on their back does not even melt! (Rosing, N., 1996).
Daniel Koon conducted an experiment to determine if the clear, hollow hairs on a polar bear conducted ultraviolet light fiber optically. The test disproved this widely accepted theory. Instead, it is the keratin protein of the hair that is absorbing the energy into the bears dark skin (Ned Rozell, 1998), and has led scientists to understand just how well polar bears have become adapted to the harsh and volatile arctic. This is good news for these endothermic mammals as they can utilize energy emitted from the sun to supplement their efforts of obtaining and conserving energy as a secondary consumer in order to maintain their internal temperatures during the day, but it does not necessarily help them at night when there is no sunlight providing any means of heat energy. Experiments using Kirchoff’s laws, and Beer’s law, both being methods to test the transmittance and absorptivity of energy, have shown that polar bear hair is a also a good absorber of infrared light, which helps greatly in obtaining heat energy in the dark when temperatures are significantly lower (Preciado, J. A., et al., 2002). Amazingly, moonlight supplies a steady transmission of the infrared light wavelength. A physics term labeled as “black body radiation” is used to describe the effects of the sun’s energy being absorbed and reflected off of the moon, therefore supplying earth with relatively long wavelengths of infrared light. In places such as Alaska it is dark during some of the coldest winter months, and the full spectrum of light is unavailable to be absorbed through the bears dark skin making the animals ultraviolet and infrared absorbing abilities more useful than just at night. Human hair also possesses this amazing trait, however, polar bear hair absorbs 4 % more infrared than human hair. If you multiply that by the much larger amount of hairs per square centimeter a polar bear has it is at a much greater advantage (Preciado, J. A., et al., 2002).
Ironically, in addition to the success of a polar bears hair composition it’s white appearance may have increased it’s success at catching prey. Seals which are their main food source have monochromatic vision, meaning they see only black and white. Even though white bears appear camouflage even in trichromatic vision (full color), it may be even more difficult for a seal to detect a predator. These large, white creatures display a good deal of intelligence in their methods of hunting for food. They have been observed taking advantage of their camouflage to sneak up on unsuspecting seals in their dens. The bears would actually scratch away the snow slowly as they lay right on top of the lair imitating the ice and snow above the lair. In addition, the bears will lay quietly on a sheet of ice with only their head projecting (resembling a protrusion of the platform) until a seal came close enough to snatch up with a single paw (Matthews, D., 1993).
Ursus maritimus have become known as “walking hibernators” due to their unique physiology allowing them to survive through long periods of starvation. This is accomplished much like hibernation seen in black bears (Ursus americanus) and brown bears, however, polar bears stay motile in search for food while urea nitrogen is converted into plasma proteins to avoid build up of toxic wastes and aid in the reutilization of nutrients (Lennox, A. R. and A. E. Goodship, 2008). This is made possible by the re-absorption of urea through the bladder wall into the blood stream, where it is metabolized into useful proteins resulting in no net increase in toxic waste (Nelson, R. A., et al., 1980). Only pregnant polar bears den like their sister species in order to provide shelter for their under-developed newborns (Rosing, N., 1996). Their ability to roam during a hibernative state may be due to the very low temperatures in the arctic region, limiting and slowing down the life processes that call for more energy. During hibernation most bear temperatures decrease a few degrees, which in turn slows their metabolism. This process is called torpor. Torpor patterns are not predictable in sea bears like they have been noted in black bears most likely due to the unpredictable availability of food sources (Palmer, S. S., et al., 1988) . As food sources can be few and far between, it is no surprise that polar bears have adopted random torpor patterns, which can be complimented with random sleep patterns. Based on their necessity to survive the bears have been known to take many short naps throughout the day in between their almost never ending search for food (Matthews, D., 1993).
Great white bears have hearing comparable to that of humans, a great sense of smell, and good vision which tends to be farsighted. It’s olfactory senses are so good, “It can detect young seals hiding in a snow cave buried under three feet of snow nearly a mile away” (Rosing, N., 1996). It has also been said that they can smell only a few molecules at distances as great 20 miles away (Matthews, D., 1993). Dichromatic vision enables them to see blue and green colors in addition to black and white, which can put them at an advantage when hunting in their monochrome world.
Polar bears sexually mature at the age of four or five, but males don’t breed until 8 years old (Struzek, Ed, 2003). Like other bears, females are induced ovulators (W. Boone, 2003.; Rosing, N., 1996) and are usually fertilized in early spring when seal pups are plentiful. The following egg implantation is delayed for about 6 months until the fall when the mother begins to dig out a den where she will stay for as long as 9 months. From the time she enters the den it only takes from one to two months before she gives birth (Drew, L., and M. Hoshino, 1996/1997) to 2 or 3 cubs no larger than a 2 pound tree squirrel (Rosing, N., 1996). The young are completely helpless at birth and may require the care of mother for approximately another 6 months, however, the new family usually remains together as the cubs continue to nurse from1-1/2 to 2-1/2 years. The cubs will usually stay together until the females mature enough to start their own families around the age of 5. At birth the cubs are not able to see, but they do have long, pointed claws useful in reaching nipples hidden in their mothers thick fur. Unique to polar bears, the mother’s milk contains 40% fat providing more essential fat than any other bear helping them to develop more quickly (Rosing, N., 1996). Interestingly, 50% of female polar bears den in offshore glaciers. A Russian biologist by the name of Nikita Ovsyanikov believes that this is done purposely due to a mothers need to find food for her young when she digs out of the den in spring. By drifting on glaciers the bears can travel several hundred kilometers possibly increasing their chances of finding a plentiful food supply (Drew, L., and M. Hoshino, 1996/1997).
Polar bears, called Nanook by the Inuit, are excellent swimmers spending as much as 2 hours in water traveling lengths as great as 60 miles. However, this is unusual and usually a sign of the negative affects of global warming. A temperature increase of only 2-3 degrees during the summer, since 1950, has been enough to melt ice so dramatically that people have been finding an increased amount of bear carcasses due to drowning (Iredale, W., 2005). Some polar bears will actually walk entirely across the Hudson Bay from Manitoba to Quebec (500 miles) in a single winter (Rosing, N., 1996) showing the dangers they may face during the warming months of spring. If rates of global warming continue to proceed it is quite apparent that these sea bears just may not make it back to shore on time.
Polar bears have also been known to be social and playfully interactive (Drew, L., and M. Hoshino, 1996/1997; Rosing, N., 1996) and even express their love of play throughout adult-hood. Photographer Norbert Rosing was actually a witness to an adult bear in Manitoba, Canada playing with an old tire, and was able to snap some really amazing pictures. Some have suggested these behaviors are to refine social behavior, although most bear species typically live solitary lifestyles. For the younger bears, these play sessions will aid in the development of skills that will increase their chances for success as they get older. Intelligence is an important aspect in the survival of these bears, and walking on ice is a great example of this. Walking on melting ice can be risky business if one is not careful. Polar bears run the chance of falling through the ice and never surfacing again, and it is clear they know this if you ever have the chance to watch one. They walk very carefully making great use of their giant paws and spreading their legs far apart, and if they sense the ice is going to break they actually may drop to the ground, limbs spread wide, in an attempt to slide forward (Rosing, N., 1996).
Even though polar bears received little impact from human development as of 1984 and currently hold the highest proportion of their original range compared to other large carnivores (Derocher, A. E., et al., 2004) they are still in danger from global warming and pollution. One population, in particular, located on the southwestern Hudson Bay coast is used commonly as an example of the loss of an ice habitat due to global warming. This specific population happens to be among the southern-most where the affect of global warming would be most visible (Dyck, M. G., et al., 2007). Bears may drown in the ocean when ice thaws out and cannot find refuge, or over heat on shore during the warm months. Usually migration during climatic changes can aid in the survival of this species, but if the current rates of global warming remain steady the positive effects of migration will not be feasible. Possibly due to global warming, grizzly bears may even replace polar bears in the arctic as they have already been seen on the ice tundra. On the other hand, both bears produce fertile hybrids which may be the key to positive gene flow enabling the genetic variation needed to aid in the polar bears survival. Despite the fact that this intermingling has yet to be seen in nature, you never know how these animals may react when put into an actual life or death situation.
In one unfortunate case in 1991, Biologist Mitch Taylor observed the remains of several polar bear pups that had been eaten by a grizzly bear in Viscount Melville Sound. Grizzly bears had never before been seen as far as 600 km’s from the main land, but since then have been spotted more and more. Experiments are underway to find out exactly what these grizzlies are doing there, and if this is something they have always done, but is most likely for the same reasons the polar bears first entered the arctic a new, more abundant food source with limited competition. The brown bears habitat is also decreasing in size as the human population pushes them to their limits, and forces them to find a new place to live. A government wildlife officer of Nunavut, Joe Niego, says, “…a lot of animals are moving east toward Hudson Bay because of global warming….we’ve seen marten, which are never found this far east, a few river otters, and a lot more grizzlies than usual….I saw a pack of wolverines just west of Baker Lake. I’ve never seen that before.” (Struzek, Ed, 2003).
Environmental pollution resulting in high levels of organochlorine contaminants becoming stored in the adipose cells of sea bears during hyperphagia is of large concern to scientists as well, because these pollutants found largely in pesticides can later become antagonistic to hormone-dependent processes (C. Sonne et al., 2005). This pollution is inescapable and affects all animals in the arctic regions with polar bears on top of the food chain pyramid experiencing the effects 100 fold compared to fish, for example, at the bottom of the food chain. Canadian scientists have observed that present-day polar bears are smaller in size, and produce less offspring then they did just 25 years ago. High PCB (polychlorinated biphenyls another pollutant) levels have been observed in North American populations and sadly the concentration in populations of sea bears in Norway and Russia have reached dangerously high levels. Fewer polar bears are being recorded in these areas with over 50 % of the metapopulation residing in Canadian Territory! In addition, current laws have relaxed the polar bears protection policies and nearly all countries serving as their home now allow hunting (Rosing, N., 1996; worsleyschool.net, 2001). Interestingly, polar bears living in captivity appear to live as much as 1.6 times longer than in the wild with captive bears recorded to live as old as 40 years! This may be due to the absence of pollutants in their new habitat.
Despite all of the adaptive traits that have, so far, put Ursus maritimus on the top of the Arctic food chain there are no adaptive traits that seem to help against humans and their negative effects on the environment and these massively, wondrous mammals. Only time will tell if polar bears will survive and/or adapt to the increasing threats of the modern world. It is really up to us, the only extant humanid species, to preserve our environment and to attempt to reverse the negative affects of our actions in order to increase the probability of success for all species in the only inhabitable world, thus far, we know.
References
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