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Polar bears are among the largest meat eaters on land. If these two males were to unleash their full power on each other they could inflict serious damage. But this is play fighting a test of physical skill among animals that can grow to nearly nine feet. It's a ritual repeated around the world by the seven other species that make up the family of the bear. From the largest to the smallest bears have been shaped by their varied habitats.
The polar bear, with its heat-absorbing coat lives exclusively in the Arctic Circle. lts closest relative, the brown or grizzly bear lives in the more temperate climates of Europe and North America. The main inhabitant of North America and the most numerous of all is the black bear. The spectacled bear is an adept tree climber in the cloud forests of South America. The sloth bear is unusual snout and claws enables it to root out termites in India and Sri Lanka. The giant panda feeds mainly on bamboo in the inland forests of China. The Asiatic black bear roams the southern part of the continent and has a distinctive shaggy coat with a crescent-shaped marking. And the smallest of all the agile sun bear lives in the lowland forests of Southeast Asia.
For all their differences in appearance bears have many characteristics in common. Like humans, when bears walk their heels and their soles both touch the ground. They can also walk upright on their hind feet able to watch out for danger, look for food and fight. All bears can swim. The brown and polar bears are especially at home in the water where they hunt the rich food supply. But often they seem to spend time in the water for the sheer joy of it. All bears have acute senses. Their sense of smell may be on par with that of a bloodhound. Despite the apparently small size of their external ears they have a keen sense of hearing.
And although it's not known whether all bears have color vision some can distinguish between ripe and unripe berries. Most bear's lips are prehensile not attached to the gums and are therefore flexible enough to grasp and pick small fruit. Some bears have even been known to peel a grape using only their lips. All bears have non-retractable claws in some species, up to six inches long.
Their teeth are among the largest of any carnivore despite the fact that many bears have adapted to a largely vegetarian diet. But all bears, by definition, are carnivores. And brown bears can be relentless hunters hitting speeds of up to thirty miles per hour and showing surprising stamina despite their bulk. Most of all, bears are opportunists. They will eat anything and everything. Garbage dumps are easy pickings. The carcass of huge beached whale provides an unexpected and unusual meal. Even polar bears, the most voracious of meat eaters are opportunists sometimes and will resort to eating straggly grass if there's no meat to be had.
Opportunism aside, all bears need a staple diet on which their survival ultimately depends. A spectacled bear's diet is at least eighty percent vegetarian and a favorite food source are bromeliads that grow high in the trees. This bear will need all its strength and climbing skills to bring its meal down to the ground. Only then can it get to the flesh of this tropical plant. The sloth bear uses it long claws to rake out its favorite food, termites. Once a nest has been exposed the bear uses its mobile lips to blow away the dust. It then closes its nostrils and noisily sucks up the termites.
Black bears will occasionally eat meat but they survive mainly on a diet of fruit, nuts and insects. But these are less nutritious than meat and time consuming to collect forcing the bear to spend much of its day feeding. Soapberries are a favorite food among American black bears. They take great care to select only the fruit and to avoid the less nutritious leaves. Although the sun bear eats insects, fruits and bugs this is a bear that eats honey. It's able to use its long tongue to extract the nectar but pays a price in countless, painful bee stings.
The Asiatic black bears are adept tree climbers and will eat almost anything in their path be it animal or vegetable. Unlike the most highly specialized vegetarian of them all the giant panda whose diet is ninety-nine percent bamboo. Yet even pandas share an evolutionary path with voracious meat eaters like wolves. Around thirty-five million years ago the ancestors of bears branched from the raccoons and the so-called bear dogs. These early bears were small dog-like meat eaters The most ancient of today's bear species is the giant panda.
Studies of its DNA link the giant panda firmly with the bears whereas the red panda is considered to be a raccoon. Pandas became giants when they adapted to a diet of bamboo and grew too lumbering to be hunters. A few million years later the ancestor of all other bears evolved It lived in Asian forests that are the home of today's sun and sloth bears. Between twelve and ten million years ago the running bears emerged They had longer limbs than today's bears which enabled them to hunt fast moving prey and exploit open spaces.
Two million years ago they crossed the Panamanian Land Bridge into South America. Their sole living descendant is the much modified spectacled bear. In Eurasia another branch the Etruscan bears evolved into three separate lineages: Cave bears, brown bears and black bears. The cave bear became extinct but the brown and black bears were able to adapt to the available sources of food. One northern population of brown bears began feeding on seals among the ice floes and so emerged the polar bear. A female polar bear leaves her den after a long winter's sleep.
She's lost up to forty percent of her weight during a four to six month hibernation and the birth of her two cubs. She will spend up to ten days near the den to allow her cubs to adapt to the freezing climate. At night they will take shelter inside where the temperature can be as high as sixty-eight degrees. Eventually, she and her cubs have to leave the den to search for food. Ring seals are the polar bear's favorite food. They live beneath the ice, but must surface to breathe. Doing so, however, puts them in danger.
A polar bear will sit motionless for many hours waiting for a seal to surface. Seals can also surface at breathing holes that they create by gouging out solid ice with their teeth and claws. But a polar bear can smell a seal under three feet of snow over half a mile away. A polar bear owes its powerful sense of smell to a structure in its nose known as the turbinate bone. lts folds and scrolls create an enormous surface area that is packed with olfactory receptors.
In turn, a seal can detect movement above the ice and will swim to alternative holes if it senses danger. Polar bears will also stalk their prey. This three year old male has just made a kill. But is chased off by a larger male who seems more interested in challenging the young bear. Meanwhile a mother and her cub pick up the scent and head......cautiously towards the seal, not knowing it is dead. The mother paddles towards her prey through channels of melt water. If needed she can swim like this for more than sixty miles without resting. She has large webbed fore claws which provide propulsion. Her hind legs act as a rudder and stabilizer.
The two to four years that a cub follows its mother is an intense learning period. Though this seal turns out to be dead the cub now understands the stealth needed to hunt. In times of plenty polar bears will eat only the more nutritious fat and skin but for the mother and cub this is a rare and welcome feast. Getting to a food supply often requires dexterity and problem-solving abilities.
This young black bear has learned that there is usually an abundant meal beneath a rock. With great care and strength it is able to remove the rock and expose the nest of ants beneath. But what if the food itself is protected...by a tough and inedible cover. The panda is able to feed almost exclusively on bamboo only because it has a sixth digit. This so called thumb is actually an extension of the wrist bone. This allows it to grasp bamboo stems firmly in one paw. The panda can strip the coat from the bamboo using its two prominent front teeth and crush the stems with huge molars to get to the soft tissue inside.
The panda's skull is wider and deeper than other bears with a crest that runs the length of the head providing a large surface area for the attachment of its necessarily powerful jaw muscles. But pandas still have the simple stomach and intestines of a carnivore which means that they can only digest about twenty percent of their bamboo diet. Unlike herbivore stomachs which contain microorganisms to dissolve plant matter pandas have evolved a tough gullet and mucous cells that coat the stomach so it won be torn by the shards of chewed bamboo. They also have a very thick stomach wall effective in squeezing and churning large quantities of tough vegetation.
To maintain their energy levels they must eat constantly spending up to fourteen hours a day doing nothing else. In fact, the panda can only afford to sleep about four hours a day. Adaptation is the key to survival in a cold climate. Despite their bulk, polar bears are quite sure footed on ice, usually. lts feet are larger in proportion to its body than those of other bears. Add to that the hairs and suction pads on the soles of their feet and the polar bears can get a good grip on a slippery surface.
Polar bears have evolved a thick layer of fat and a dense under fur. This is so effective that even when temperatures drop to around minus thirty-five degrees their body temperature
remains unchanged Despite appearances, polar bear hairs are not white. Instead, they are colorless and hollow like fiber optic wires. They conduct the sun's ultra violent rays down to the black skin where they are absorbed. This can be so effective that polar bears may actually overheat.
And so to cool down, they spread themselves out on the ice. Polar bears can store immense amounts of fat over four inches thick. But this fat layer is not simply for insulation. When food becomes scarce the bear can live off this store for months at a time. This means that a polar bear's body weight can fluctuate enormously in one recorded case, by as much as nine hundred pounds.
When forced to live on this fat alone they can enter a state known as winter sleep. After around fourteen days without food they will switch into winter sleep regardless of the time of year. They don't urinate and have no need to drink because they recycle their waste while barely using their kidneys. Hibernation is not unique to polar bears. Brown bears will dig dens for themselves or hole up in caves or hollow logs. When you're about to sleep, comfort is a top priority
Dr. Lynn Rogers and his wife, Donna began studying bears in their dens thirty years ago by tracking black bears to their winter homes in the forests of Northern Minnesota.
"I got into bear studies back in 1967 and I was excited about that...because I thought it was, ah, working with dangerous animals. And then I got into it and I saw that they're not the dangerous animals. I had always thought they were and there was an awful lot that wasn't known about them. I became intensely curious about how they live."
Lynn Rogers first had to learn to sedate bears in order to get them out of the den which can be difficult when they weigh as much as a quarter of a ton And he didn't know how they'd react.
"I goofed up. A lot of the bears that were awake they greeted me by lunging at the entrance kind of like <blowing> and ah, it was so explosive. I couldn't help but jump back. But after seeing that hundreds of times then I realized it's more of a ritualized display than any serious threat. In fact, I wondered one time... wonder what would happen if I would call this bear's bluff and when it bluffs, I crawl, I crawled in And I did, and she bluffed from the back of the den then kept her distance from me......and eventually turned her head to the side hid in the corner and left her cubs exposed to me."
This is the time of the year when we can get the answers to the questions that we're wondered about the whole year. Also, we can find out all at once with all the bears in the dens how many cubs they had how fat they got, what kind of a year it was for them. He also began radio tracking the movements of black bears throughout the rest of the year. What we've learned from this, ah from the radios as we've flown over bears and just got glimpses of them little segments of their life through the year we've learned how big an area bears need to survive. Ah, all the travels they take the habitats they're used and the kind of the general things that bears do to make a living.
Bears live in home ranges which are determined by the availability of food A male black bear's range can be up to forty square miles and can include anything from verdant valley floors to towering rocky cliffs. Throughout their lives bears acquire a detailed knowledge of their home ranges and clearly have good memories. Bears have a tremendous ability to remember feeding sites and to find their way in the wild. They can build a mental map that's superior to anything that we could come up with.
A little cub a few month old cub toddling along behind its mother and she leads it on a trek forty miles outside of her usual territory and shows it, ah, a nice oak stand. When that cub grew up he remembered that oak stand and returned to that thing. Another bear made a trip a hundred and twenty-six miles kind of a circular trip, it went, went out there. Then it was time to come back and hibernate. It came straight home as if it had constructed a mental map on the way back way out and knew which way was home. It traveled only at night, and most of the time no vision of the stars and moon because it was cloudy under a forest canopy, and acted like it had a mental compass. lt, didn't take any easy paths. It just went straight in the direction towards home no matter what the barriers.
All bears have a compass sense. But how it works is unknown. The most striking example comes from polar bears whose home ranges are not only the widest but are literally shifting. Their compass sense allows them to navigate accurately over more than one hundred thousand square miles of moving ice floes and still find their way back to familiar ground. Even when the ice floes are turning their compass sense coupled with a powerful homing instinct will still take them home.
From the great Smoky Mountains of Tennessee scientists are trying to redistribute black bears to other areas that can support them. But first, they must overcome the bear's homing instinct. Though some mechanism that we're not... fully ah, that we don't fully understand, they're they're very capable of, of homing in on, on where they originated from. And we've had bears in this study do that. We've had bears that have traveled, over a hundred miles. And they had one animal that, that, went from Big South Fork on the almost Kentucky line to almost the Georgia line right there at Chattanooga, Tennessee, so you can see the the extent and the capability of, of these animals to, to move.
Joe Clark's team is trying to relocate female black bears to areas of the Southeastern United States which have not had black bear populations for over one hundred years. Today, they are tracking a female bear tagged over a year ago. Once they have located her den she will be tranquilized and prepared for the one hundred mile journey to what they hope will be her new home range. We're trying to target females with cubs. They give birth in the winter time while they're hibernating in those dens during, during January. Here it is March, and the cubs will be you know six weeks, eight weeks old. We've going to immobilize her, move her up to Big South Fork about a hundred miles from here place her in a den that we've chosen for her.
She will wake up and she will wonder obviously you know... where am I? But she's basically stuck there because she's got these young cubs that she has to care for. So, when she emerges from that den in mid-April or May she'll have these very young, very immobile cubs that will not be able to follow her back to the Smoky Mountains. So she's basically stuck there Clark's team has already found that the female bear they are targeting today has four cubs all of which will have to be moved with the mother to their new home. She and her cubs are sleeping in the hollow below a fallen tree. It's the end of the hibernation season and the team has to approach the den with great care or the mother may awaken and bolt, abandoning her cubs.
By early May, the family is settled in its new territory. The reintroduction effort has been successful because the mother's maternal instinct has overridden her homing instinct. The cubs are now following their instincts making a mental map of their new home range Kodiak Island off the Alaskan coast is home to one of the world's largest concentrations of brown bears. It is February, and deep inside a well protected den a female has given birth to four cubs. Brown bears produce litters only once every three to five years usually consisting of just two cubs. Even the healthiest mother is unlikely to give birth to more than eight cubs in her lifetime. Because of this low birth rate mothers will go to great lengths to ensure the survival of their offspring.
Driven by hunger the mother gathers her cubs and leaves the den. She has lost nearly fifty percent of her body weight. But thanks to her highly developed memory and compass sense she knows that there is a source of food over this mountain. First, she introduces the cubs to the cold conditions on the mountain. The cubs eventually get the courage to venture onto the unfamiliar snow. But once they are settled, they become playful reluctant to continue the climb. At the mountain summit, the mother and her cubs rest. But while three of her cubs appear healthy the smallest is already showing signs of weakness.
Bears are normally solitary animals. But in September, when the salmon are running the concentrated abundance of food brings them together in groups of up to fifty. This is when young bears get a lesson in hunting their own food. They will remember this site for the remainder of their lives and come back year after year. Finally, the mother and three of her cubs arrive at the stream. Her smallest has died on the mountain. And now they face a new threat. A large male blocks their path to the river. A female with cubs is not sexually receptive. But if the male can kill the cubs it's possible that she'll eventually come into season and mate with him.
The mother tries to warn the male off but he continues to stalk the cubs. The mother charges the male with a display of great ferocity. She may be smaller than he is but her maternal instinct gives her the upper hand. Eventually, the male leaves. Whenever bear cubs are vulnerable these confrontations will be played out again and again. Aggressive black bears can be thwarted by mothers signaling their cubs to climb out of danger.
Black bears can communicate not just warnings but a surprisingly sophisticated range of emotions. It's easy to read the emotions of bears because that's their main communication......not so much communicating detail but in communicating their emotions about a situation. And, they, they seem to have all the emotions that we have. And there's, three levels of communication that they use. There's the grunts that mothers use to their cubs and that they use to, to friendly bears and sometimes to people.
There's, a next higher level of intensity where they're expelling air sometimes quite sharply in different ways. And many of these are expressions of fear. And then they have deep resonant human-like voice that's used to experience a range of of emotions of high intensity. This can be, great pleasure, great fear, pain, anger. They have, as far as I can see they've got all the same emotions that we do. Dan, you'll drop off two people and then come down and pick up Chip. And then thirty-three is probably going to be in this canyon right up in here so we want to move her up front.
Low birth rates and the ever present threat of predators makes the survival rates of bears an important issue for scientists. This team, led by Vic Barnes is studying bears using electronic collars. These devices must be replaced every three years. The bears are targeted with a tranquilizing drug giving researchers a few minutes to fit new collars. Barnes has discovered that sixty percent of bears die before the end of their third year.
A lot of information on habitat use critical feeding areas critical denning areas, their home ranges. We also get an awful lot of good information on productivity and survival. We've had some animals we've been marking animals for ten to eleven years. Some animals we have a good ten year history on them. We've followed them through two and three litters. We know the survival of the litters. Right now about, only about forty percent of the of the cubs that we identify in the spring survive to be weaned at two and three years old. Given that so few bears reach their maximum life span of thirty years it is surprising they haven't evolved a more prolific breeding strategy. Only one in three brown bear females will breed in any given year.
So for a male to successfully mate each year he must travel up to four hundred fifty square miles to encompass the home range of at least three females. When he eventually finds a receptive female he mates with her many times a day for up to a week to ensure fertilization. The reproductive stakes get even higher when it comes to pandas. Females are receptive for only two to three days a year between March and May and produce on average only one cub every two years. Female giant pandas travel within a range of little more than a square mile.
Males scent mark commonly used panda trails with a form of perfumed grease. But scent is only one mating signal and a relatively temporary one at that.Biting trees and stripping their bark lasts longer. And all of these elaborate efforts are for nothing if the female is not in heat. This male appears to be making very little progress. When a female panda gives birth she can only look after one cub at a time and leaves any other newborn to die.
At the Wolong Giant Panda Research Station in China's Szechwan Province researchers are nursing the cubs that their mothers cannot care for. This rejected three month old cub was one of twins but this represents only a small success. The giant pandas remain firmly on the endangered species list. American black bears have survived best of all to the present day with a population that may be as large as three quarters of a million. But the numbers of brown bears have fallen dramatically to no more than one hundred fifty thousand. There are no firm estimates for the Asiatic black bear.
There may be forty thousand polar bears but less than two thousand spectacled bears. There are between seven hundred and one thousand giant pandas and between one thousand and ten thousand sloth bears. As for the sun bears, no one really knows. Now more than ever, these bears must depend upon the adaptability that has brought them this far to ensure their continued survival.
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