Bear Anatomy and Physiology
Size | Weight | Height | Length | Color | Skulls | Teeth | Paws | Vision | Hearing | Smell | Strength | Odor | Body Temperature | Thermoregulation | Heart Beat | Respiration | Pain | Digestive Tract | Scat (feces)
The size of individual bears has long caused heated discussions and continual misjudgment. A bear's size is normally expressed in terms of weight, which is difficult to judge due to individual variations in height, thickness of fur, and physical stature, as well as the observer's proximity to the bear and particular level of stress. Under calm circumstances a bear's weight is often misjudged, but during a close encounter Accurate weight determination is impossible by nearly all except possibly a seasoned field scientist. "The grizzly's reputation for ferociousness toward people," notes Terry Domico in Bears of the World, "makes the animal seem much larger."
To the untrained eye, all bears are "big", as human perception of weight is most often much greater than an animal's true size. During a survey in Great Smoky Mountains National Park, responses to the weights of American black bears ranged from four hundred to four thousand pounds. The actual weights were ninety-five to one hundred fifteen pounds.
"The boar was small," according to Ben East in Bears, "hardly more than 150 pounds, but still big enough to be a formidable antagonist . . . the men guessed him at four hundred pounds."
In nearly all species of bears, the male is relatively larger than the female, though differences vary. For example, the difference between the sun bear females and males on Borneo is minimal, while on the Malaysian mainland the males may be more than one third larger.
Bear Weights: Males vs Females
|AMERICAN BLACK BEARS||Males 33% larger|
|BROWN BEARS (Kodiak)||Males 40% to 50% larger|
|GRIZZLY BEARS||Males 38% larger|
|POLAR BEARS||Males 25% to 45% larger|
|ASIATIC BLACK BEARS||Males slightly larger|
|GIANT PANDAS||Males 10% to 20% larger|
|SLOTH BEARS||Males slightly larger|
|SUN BEARS||Males 10% to 15% 1arger|
|SPECTACLED BEARS||Males 33% larger|
The Largest Bears
The brown bears and polar bears are without doubt the largest bears. However, there are conflicting and contradicting beliefs and statements concerning the largest individuals or species of these bears.
Weights of bears vary between species, with polar bears and Alaskan brown bears more than ten times heavier than sun bears. Such differences between species, though due in part to genetics, are most often a result of variations in habitat, primarily diet. For example, the Alaskan brown bears of the coastal regions of North America, with a major source of fish and more lush vegetation, are nearly twice the weight of the inland brown bears (grizzly bears).
Causes of individual weight differences between bears of the same species, and sometimes the same habitat, may include individual health, age, the sex of the bear, individual ability to locate food or digest specific foods, and the level of ability to withstand human impacts on the habitat.
Seasonal fluctuations in weights of individual bears is common. Fall (pre-denning) weights are normally much greater than spring (emergence) weights. Weights are affected by seasonally available foods.
American black bears of eastern North America are consistently larger than those of the western states. (Weights are averages from a specific sampling.)
|State||Adult Male||Adult Female|
|New York||273 Pounds||196 Pounds|
|California||216 Pounds||127 Pounds|
Wyoming/Montana grizzly bears are larger than those of the Yukon Territory. (Again, weights are averages from a specific sampling.)
|State||Adult Male||Adult Female|
|Wyoming||539 Pounds||334 Pounds|
|Yukon Territory||315 Pounds||209 Pounds|
Bear weights are obtained when bears are harvested during a hunt, when illegally killed (poached), and when immobilized for management and research. The weights below from some states and provinces are "dressed" weights of bears harvested by hunters or taken during management actions.
Weights of Mature Males
|American Black Bear||250||125-600||803|
Bear weights in Lore and Legend - Lore and legend have provided some very impressive weights of bears:
"Legendary" weights are not uncommon, even today. Ben East, in Bears, relates the comment of a zoo director about such weights, ". . . few grizzlies of record weight come from a part of the country where accurate scales are found." And Adolph Murie, in A Naturalist In Alaska, notes that "a bear a long distance from a scale always weighs most."
The height of a bear is measured from the bottom of its paw flat on the ground to the highest point of the shoulder. What follows are the ranges or average heights for adult males.
|American Black Bear||2.5-3 ft.|
|Brown Bear||3-5 ft.|
|Polar Bear||Up to 5.3 ft.|
In comparison, the height of an American bison is five feet; elephant eight feet; hippopotamus five feet; rhinoceros six feet; and a Siberian tiger three feet.
The length of a bear is measured from the tip of the nose to the tip of the tail. Adult male average lengths (ranges) are listed below.
|American Black Bear||6 ft.|
|Brown Bear||7-10 ft.|
|Polar Bear||8.4 ft.|
|Asiatic Black Bear||5-7 ft.|
|Giant Panda||5 ft.|
|Sloth Bear||5-6 ft.|
|Spectacled Bear||7 ft.|
In comparison, the length of an American bison is nine feet; elephant eleven feet; killer whale thirty feet; mountain lion eight feet; and a Siberian tiger thirteen feet.
The coloration of bears is quite variable between species and within species. Color changes are not uncommon, due to maturation or seasonal fading and shedding in individual bears, or with the angle and intensity of the natural light of the moment. Variations may include totally different color or different shades of a color. (The underfur color normally remains the same, while the guard hairs change.)
American black bear cubs of the same litter may be different colors, and they may change as they mature from brown to black--or the opposite may occur. They may change before they reach one year old, or at two and three years.
A bear's underfur may be brown while the outer, guard hairs are tipped in black, and some bears are entirely of a single color. Several species of bears have yellowish or whitish chest markings on many individuals, while the chest mark, or medallion, is found on all members of the tropical bears--sloth, sun, and spectacled. The markings vary shape and size.
Albinism: Albinism, though extremely rare, occurs in bear species. A "partial" albino American black bear, with white breast and white front feet, was observed in Wyoming in 1948. There is record of a whitish American black bear with four cubs: one brown, two black, and one true albino. In Oregon, an American black bear had a light chocolate brown head and feet with the rest of the body a dirty white (not a true albino).
Generally, the skulls of bears are massive, typically long, wide across the forehead with prominent eyebrow ridges, a large jawbone hinge, and with heavy jaw muscles and broad nostrils. Combined with dentition, the structure of bears' skulls are very much carnivorous, though with omnivore modifications.
The skull may be the most important feature of an animal, housing the brain, providing a major protective and nutritional feature (mouth with teeth), and containing sensory-communication features. "Bear skulls undergo a series of changes from early life to old age, and in most species do not attain their mature form until seven or more years of age," observed C. H. Merriam in North American Fauna, Biological Survey, 1918.
Diet and other eating habits have influenced the individual development of the heads and skulls of each species. "Head shape and size . . . are influenced by dentition and jaw muscles," write Paul Shepard and Barry Sanders in The Sacred Paw. ". . . [skulls] are shaped to anchor the appropriate muscles. Because of the heavy jaw muscles it [spectacled bear] uses for crushing palm nuts, its skull shape is unusual, rather resembling that of the giant panda, which has massive molars for grinding bamboo shoots."
Brown bears normally do not bite to kill, but have grinding, crunching teeth with the massive muscles to accomplish the task. Polar bears are more carnivorous than other bears, and do bite to kill; their skulls are specifically shaped for the appropriate teeth and muscles to hold, chop, and slash their prey. Each of the eight bear species has its own distinctive skull shape and size.
|American Black Bear||Broad, narrow muzzle; large jaw hinge; female head may be more slender and pointed|
|Brown Bear||Massive; heavily constructed; large in proportion to body; high forehead (steeply rising); concave (dished face); domed head; long muzzle; flat nose tip; ears barely observed as bumps; eyes tiny|
|Polar Bear||Large; small in proportion to body; long; snout long (warms air); Roman nose; large eyes|
|Asiatic Black Bear||Large; sloping forehead|
|Giant Panda||Massive; wide; zygomatic arches widely spread; constructed for attachment of powerful jaw muscles; short muzzle|
|Sloth Bear||Thick; long muzzle; small jaws; bulbous snout; wide nostrils|
|Sunbear||Wide and flat (unbearlike); short muzzle.|
|Spectacled Bear||Wide; short muzzle; lower jaw shorter than upper (overbite); unusual skull shape; resembles giant panda; young and female skulls narrow and long|
Animal classification is primarily based on skulls (". . .details of skull and leg bones are the usual criteria for the biologists," note Shepard and Sanders) and in part led to the "splitting" of the bear species. Skull size is also the criteria for the "record size" bears of North America.
A bear's teeth, combined with paws and claws, are its first-line tools for defense and obtaining food. The teeth are large, and though originally carnivorous, are adapted to an omnivorous diet of both meat and plant materials. The major difference between carnivore and omnivore dentition are the molars, which in bears are broad and flat. Dentition-- the size, shape and use of the teeth--and jaw muscles influence the size and shape of a bear's head.
Bears have forty-two teeth, except the sloth bear which has only forty. Permanent teeth are normally in place by the time a bear is approximately two and a half years old. For each species the characteristics of the four kinds of teeth--incisors, canines, premolars, and molars--vary depending on diet and habitat.
|American Black Bear||Premolars and molars for grinding|
|Brown Bear||Flat and broad crowns on molars; premolars and molars for grinding|
|Polar Bear||Canines larger and longer than for other bear molars smaller than those of land bears; molars more for shearing; premolars more for biting than grinding|
A bear's paws are important in locomotion (walking, running, climbing, swimming), killing, feeding, digging, lifting, raking, pulling, turning, sensing, and defense. Bears walk plantigrade like humans, paws with durable pads down flat on the ground, and pigeon-toed, forepaws turning inward. A bear's heat loss (thermoregulation) is primarily through its paws. "All the pads [paw soles] are surfaced with tough, cornified epidermis over a substantial mass of resistant connective tissue," describe Tracy Storer and Lloyd Tevis in California Grizzly. "This coverage of the foot is the sturdy, self-renewing shoe. "
Bears have relatively flat feet (paws) with five toes, except the giant panda, which has six. Hind paws are larger than forepaws and resemble the feet of humans, except the "big toe" is located on the outside of the paw. Bears are renowned for their forepaw dexterity; they can pick pine nuts from cones, unscrew jar lids, and delicately manipulate other small objects. "The grizzly, though apparently awkward and lumbering, is really one of the most agile of beasts," noted Enos Mills in The Spell of the Rockies. "I constantly marvel at . . . [the] bear's lightness of touch, or the deftness of movement of his fore paws."
Claws are curved, longer on the hind paws than the forepaws, and unlike a cat's, non-retractable.
The eyesight of bears has long been thought to be generally poor. However, more recent studies have shown it to be reasonably good, though there is still much to be learned of the visual capabilities of each species.
Generally, bears' eyes are various shades of brown, small (except those of polar bears), have round pupils (except giant pandas' which are vertical slits), and are widely spaced and face forward. They are important and useful feeding tools, and are reflective and mirror the faintest glow of the moon.
Bears approach objects due to nearsightedness and stand upright to increase their sight distance. Polar bears may have the most specialized eyes, providing very adaptable and excellent vision that exceeds that of other species of bears. They are large--almost as large as human eyes--and have an extra eyelid to filter snow glare. Depth perception is excellent and they are capable of good under water vision due to nictitating membranes that protect the eyes and serve as lenses. Polar bears' eyes adapt to a wide range of light conditions, including darkness for hunting at night or during the dark winter. "The polar bear's visual world is marked by intense, glaring sunlight, contrasted by long, dark polar nights," relates Thomas Koch in The Year Of The Polar Bear. "Days are often punctuated with blizzards, sleet, and the constant, driving wind. With these factors present, the bear's vision is rarely given optimum conditions to view his surroundings. When traveling on the ice during good conditions, polar bears are able to identify immobile objects lying on the ice as far as one mile away."
However, a whaler's journal describing a blind polar bear demonstrates that good vision may not always be necessary. "From the appearance of the bear's eyes, the men surmised that the bear had been blind for a considerable period of time," relates Koch. "Even though the bear was blind, he was still fat, indicating that he hunted successfully, using only his hearing and smelling senses."
The ability to distinguish color, and activity at all levels of light (day and night) are excellent indicators of good vision. Some biologists believe the vision of bears is at least average, and at least two have expressed the thought that though bears act as if they have poor eyesight, it just may be they do not trust their eyes as well as their trustworthy noses. "Much of the anecdotal information on bear vision," according to Paul Shepard and Barry Sanders in The Sacred Paw, "assumes that the animal approaches strange objects because it does not see them well at a distance, but crows and coyotes do the same thing and nobody doubts their visual acuity."
The ears of bears vary between species, both in size and in their location on the head. They range from large and floppy to small and hardly visible, and from those located well forward on the head to low and to the rear.
In general, a bear's hearing is fair to moderately good. "Hearing in bears is probably good," explains Stephen Herrero in Bear Attacks, "although most of the evidence is anecdotal."
Bears, he also notes, ". . . probably hear in the ultrasonic range of 16-20 megahertz, perhaps higher." "The grizzly's sense of hearing is far more sensitive than man's," writes Thomas McNamee in Grizzly Bear, "and it is undoubtedly an important aid in the pursuit of such subterranean prey as gophers, ground squirrels, mice, and voles, which grizzlies locate blindly and pounce on with noteworthy accuracy."
"At 300 meters [328 yards]," write Shepard and Sanders, "the bear can detect human conversation, and it responds to the click of a camera shutter or a gun being cocked at 50 meters [54.7 yards]."
"The use of hearing by bears is not as obvious as that of sight and smell," notes Adolph Murie in The Grizzlies Of Mount McKinley. "Even though it may not play a prominent role in their activities, I believe grizzlies do have an acute sense of hearing."
Whether low to the ground or held high in the wind, the nose of a bear is its key to its surroundings. "Smell," writes Herrero, "is the fundamental and most important sense a bear has. A bear's nose is its window into the world just as our eyes are."
The keen sense of smell--the olfactory awareness--of bears is excellent. No animal has more acuteness of smell; it allows the location of mates, the avoidance of humans and other bears, the identification of cubs and the location of food sources. ". . . the nose provides the leading sense in the search for nourishment," notes Paul Schullery in The Bears of Yellowstone. The nose of the bear is somewhat "pig-like," with a pad extending a short distance in front of the snout.
A bear has been known to detect a human scent more than fourteen hours after the person passed along a trail. "The olfactory sense of the bears ranks among the keenest in the animal world," according to George Laycock in The Wild Bears. "A black bear in northern California was once seen to travel upwind three miles in a straight line to reach the carcass of a dead deer."
The sense of smell of polar bears may be the finest--able to detect a seal several miles away--and, as Domico relates, ". . . male polar bears march in a straight line, over the tops of pressure ridges of uplifted ice . . . up to 40 miles to reach a prey animal they had detected."
An old, and much related, Indian saying may best describe the olfactory awareness of bears. "A pine needle fell in the forest. The eagle saw it. The deer heard it. The bear smelled it."
Bears possess enormous strength, regardless of species or size. The strength of a bear is difficult to measure, but observations of bears moving rocks, carrying animal carcasses, removing large logs from the side of a cabin, and digging cavernous holes are all indicative of enormous power. No animal of equal size is as powerful. A bear may kill a moose, elk, or deer by a single blow to the neck with a powerful foreleg, then lift the carcass in its mouth and carry it for great distances.
"The strength . . . is in keeping with his size," describes Ben East in Bears. "He is a very powerfully built, a heavy skeleton overlaid with thick layers of muscle as strong as rawhide rope. He can hook his long, grizzly-like front claws under a slab of rock that three grown men could not lift, and flip it over almost effortlessly...." "... a brown [bear] ... took a thousand-pound steer a half mile up an almost vertical mountain, much of the way through alder tangles with trunks three or four inches thick."
Strength and power are not only the attributes of large bears but also of the young. The author observed a yearling American black bear, while searching for insects, turn over a flat-shaped rock (between 310 and 325 pounds) "backhanded" with a single foreleg. The bear was captured the following day in a management action and weighed 120 pounds.
Bears have a definite odor, as do other animals, including humans. However, the odor of a bear is quite pronounced, though not necessarily repugnant (depending on the individual nose), and is considered by many hunters as the easiest for a dog to track. The Eskimos often located polar bear dens by the scent emitting from the den vent hole.
The American black bear has a somewhat different odor from that of the grizzly bear which, according to one bear biologist, smells musky and musty. Scientists, naturalists, hunters, and others who have experienced the odor of a bear agree that for them it could never again go unrecognized.
The normal body temperature of bears is approximately ninety-eight to ninety-nine degrees Fahrenheit. The temperatures vary, as do those of other mammals, based on individual differences and levels of activity. Temperatures are normally taken while the bears are immobilized (for obvious reasons) and under physical and psychological stress, resulting in elevated temperatures and the near impossibility of determining a "normal" temperature. However, two adult, male grizzly bears in a captive situation recently had their temperatures taken under "normal" circumstances. They each swallowed a tiny temperature-sensitive radio transmitter placed in their food. Their recorded body temperatures ranged between 98.5 and 99 degrees Fahrenheit, with a mean temperature of 98.9. Interestingly, while the transmitters were still in their stomachs (before being passed with other feces), they were fed frozen fish at which time the "stomach" temperatures dropped to the low eighties.
A bear's temperature may drop a few degrees when the animal is sleeping at night or resting on a snowbank or in a cool day bed. A hibernating bear's temperature drops in relationship with the outside and den temperatures, but appears to have a safety mechanism, as it does not drop below approximately eighty-nine degrees Fahrenheit.
Bears, like all mammals, must regulate their body heat. A bear's fur is an extremely effective insulation during the winter, maintaining body heat while absorbing heat from the sun. However, it does not allow adequate cooling during warm weather. As they don't have sweat glands, bears must cool themselves through several unique methods, shared by dogs.
Polar bears are faced with overheating like the other bears, but they also require additional heating during the subzero temperatures of the arctic winter. They have three to four inches of subcutaneous fat on their rumps and backs that provide additional insulation. However, they primarily bask in the sun and their outer fur functions as a unique system of heat transmission. Polar bear hairs, according to Charles Feazel in White Bear, have ". . . an empty core in the center of each strand. Each hair functions as a light trap, a conduit that takes the sun's rays . . . the last few inches to his dark skin. Polar bear skin is one of nature's most efficient UV [ultraviolet] absorbers. Ultraviolet light penetrates clouds, so Nanook's efficient solar collection system works even on overcast days." Also, a polar bear's long snout warms the cool arctic air as it inhales.
Giant pandas, according to George Schaller et al., have a ". . . short, thick coat [that] provides excellent insulation; the animal readily sleeps on snow. The density and oily texture of the hairs probably prevent moisture from penetrating to the skin, an important adaptation in a damp, cool environment. And the hairs have a springy quality; they are resistant to compaction, which reduces heat loss when the panda lies on snow or cold ground."
A sloth bear, with its belly and underlegs nearly bare, is quite tolerant of heat.
A normal heart rate for bears is ninety-eight beats per minute while awake and while walking, but it will increase with activity, as well as drop to forty to forty-five beats per minute during night sleep. The heart rates of some bears have slowed to eight to ten beats per minute when resting in a snow bank.
The lungs of bears are relatively large and their breathing rate is six to ten breaths per minute while resting, forty to eighty when hot and panting, and sometimes over one hundred breaths per minute during extreme exertion. The oxygen intake (resting) is reduced by approximately one half during hibernation.
Bears have sensory end organs and experience pain stress from internal and external sources. Bear pain should not necessarily be compared with that of humans, which is possibly more complex. Generally, they do not appear to display obvious reactions, as humans do. Bears have numerous injuries due to the nature of their existence, and have been compared to professional football players who "live in a world of constant pain."
Persistent pain produces irritability and many "problem" bears, which display their discomfort by aggressive actions toward humans and other bears. They are found to have abscessed teeth or other wounds. Their wounds, or other problems, may be from natural or human sources:
"Although the bears may cry in pain when stung by angry bees, they will persist until all the honeycomb has bee eaten," describes Terry Domico in Bears Of The World.
Bears have a simple intestinal tract, of which the colon is the primary site of fermentation. They have a long gut for digesting grass, but do not digest starches well. Their small intestine is longer than that of the true carnivores, and the digestive tract lacks the features of the true herbivores.
The barrel-shaped body of a bear is considered an indication of a long intestine. The brown bears' intestinal length (total and small) is greater than that of the American black bear's and giant panda's. Polar bears have the longest intestine.
The short intestine of giant pandas results in poor digestion efficiency. Only twenty to twenty-five percent of what they consume is digested; thus they must eat enormous amounts--twenty-two to forty pounds of leaves and stems daily--to gain minimal energy. They produce considerable feces, mostly undigested bamboo, passing it in only five to eight hours.
The alimentary system of a sun bear cub must, for the first several weeks following birth, be externally stimulated for the urination and defecation processes to take place. The sow licks the cubs to provide this simulation. The American black bear must also at times perform this function.
Scat, or feces, is the excrement of animals. Scatology is the scientific study of scat; scientists collect and thoroughly analyze bear feces to determine many things about bears, including what they have been eating, how much of each type of food, and during what season of the year they were eating the specific food. The information provides more knowledge about the bears' requirements and activities and assists in the appropriate management of their habitat.
Bear scat is also beneficial to the land. It scatters and fertilizes seeds of the plants the bear has consumed and provides humus that enriches the soil.
For the traveler in bear country, the observation of bear scat triggers excitement and anxiety: it is exciting to find an indication of a bear, but at the same time being unaware of its exact location provides anxious moments. However, bear scat may answer questions, too--how long has it been since the bear was here, how big is the bear and what has it been eating?
Next: Bear Behavior & Habits. ALSO Species Habitat Requirements & Preferences with special attention to Diet.. In the next article of this series we will review the behavior, habits, habitat requirements, and diet of these four subspecies of bear.
Until Then Good Luck and God Bless.......Stu Keck
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