While in the den they do not eat, drink, urinate or defecate. Pregnant females are usually the first to enter dens in the fall.
These females, with their newborn cubs, are the last to exit dens. Adult males, on the other hand, enter dens later and emerge earlier than most other bears. In northern areas, bears may spend up to 8 months in dens, while in areas with relatively mild winters, such as Kodiak, some male bears stay active all winter.
Brown bears have an exceptionally acute sense of smell, exceeding that of dogs. Contrary to popular belief, bears are not nearsighted. Their eyesight and hearing are comparable to humans. They can run in short bursts up to 40 mph 64 kph and are excellent swimmers. By all indications, bears are extremely intelligent and most have individual personalities. Brown bears Ursus arctos , also known as grizzlies, occur throughout Alaska except on islands south of Frederick Sound in southeast Alaska, west of Unimak in the Aleutian Chain, and Bering Sea islands.
They also occur in Russia, northern China, northern Japan, Europe, western Canada, and in limited portions of the northwestern United States. Seasonal Distribution Spring: In spring, black and brown bears are usually found on low elevation south-facing slopes, and in riparian forests and wetlands for early green vegetation and moose. Summer: During summer, brown and black bears are most commonly found in mid-elevation herbaceous habitats, in low elevation river bottoms and fluvial benches for early berries, and in high elevation burns and openings for berries.
In some areas they also focus on anadromous waters for salmon. Fall: In fall, brown and black bears are most commonly found on large rivers for salmon and associated riparian forest areas for roots, late berries and fruits. In the Interior, bears focus on alpine and subalpine berry patches. Winter: In winter, most brown bears are in hibernation dens in alpine and sub-alpine areas; most black bears are in dens in forests.
Bear populations in Alaska are healthy and productive. Densities vary depending on the quality of the environment. Black bears swim well and often climb trees to feed on buds and fruit.
They have a keen sense of smell, acute hearing, but poor eyesight. They can be seen at any hour of the day, but are most active at night.
When very young, the cubs cry when afraid and hum when contented. Black bears are omnivorous; their diet consists of about 75 percent vegetable matter, 15 percent carrion, and 10 percent insects and small mammals. Their love for honey is well known, and sweet, ripe corn in autumn also attracts them. They have few enemies, but the one they fear the most is the Grizzly. Whenever their territories overlap, the latter is given a wide berth. A Glacier bear is a sub—species of the black bear.
They have been spotted all over Alaska, but most frequently in the Southeast region. Go here for Bear viewing flights! Currently you have JavaScript disabled. In order to post comments, please make sure JavaScript and Cookies are enabled, and reload the page. Nonresident brown bear hunters are required to have a guide or be accompanied by an Alaska resident who is a relative. Brown and grizzly bears are classified as the same species, Ursus arctos.
Brown bears on Kodiak Island are classified as a distinct subspecies from those on the mainland because they are genetically and physically isolated. Like black bears, brown bears vary widely in color.
Brown bears can range from dark brown through light blond. Brown bears are larger than black bears and have a more prominent shoulder hump, less prominent ears, and longer, straighter claws.
Both the shoulder hump and the long claws are adaptations related to feeding. The long claws are useful in digging for roots or excavating burrows of small mammals. As surprised or frightened animals sometimes act according to the motto that attack is the best possible mode of defense, one is strongly advised to walk through the wilderness with eyes wide open and ears pricked, talking or singing loudly in places of low visibility to announce their presence.
As a rule, bears show little interest in people and do not consider humans as potential prey. However, we should never forget that we are guests in bear country, and as such, we should show respect for these animals. Ideally, the impact of our presence should not be felt.
Bears should be able to pursue their activities undisturbed at all times. The best photographic opportunities arise when the bears are relaxed, totally ignoring us. If we act obtrusively, they will become nervous and wander off. Although the majority of bears search for a mate within this period of time, some bruins are occasionally observed much later indulging in the game of love.
In bear populations, both genders are represented in about equal numbers. Females tend to teach their offspring survival secrets for several years, which leaves roughly one-third of them available to the overtures of a male. The consequence of this statistical imbalance is a high competition among males for mating privileges, which, in turn, resulted in the course of evolution in adults—dominant males grow to twice the size of females because larger bears have greater success in passing on their genetic material to the next generation.
Although the mating season spans several months, sows are only in estrous for about three weeks, and they will only allow a male to mount during three to four days at the peak of the cycle. As these animals, for the most part, lead a solitary existence and congregate in a small area only at times when food is available in overabundance, they face the problem of finding a potential mate at the right time. Hence bears, dominant boars in particular, travel far across their home ranges during the breeding season.
To detect a female in heat, males primarily trust their noses—sex hormones are eliminated from the body through urine. With the use of their exceptionally acute sense of smell, males are able to determine the receptiveness of the female by sniffing the soil and grass where she urinated, walked or slept. With their noses close to the ground, they will unwaveringly follow a promising scent for miles. At first, this strong interest on the part of the male appears to disturb the female.
She tries to evade him, as the much larger boar is a potentially dangerous threat. Often, the pair is observed for several days as they cross meadows, amble through brush , and travel along streams at an unvarying distance. In time, the female will allow the boar to come closer; however, it may take as much as one week for her to lose her fear and become approachable.
The animals soon graze, play and rest in close proximity. After several days of this intimate togetherness, the female finally permits the boar to mount, and the copulation usually lasts about 45 minutes. Ovulation in mammals occurs either spontaneously without any external trigger, as in humans, or is induced by the male, as in bears. Spontaneous ovulation harbors the risk that the ovum dies before conception is achieved.
The chance that no breeding partner is encountered at the time of ovulation or soon after is small in animals living in herds or family groups. Not so in bears, which, as a rule, spend much of their adult life in voluntary separation from their fellow bruin.
Thus, to guarantee that a fertile egg is available at the time of copulation, ovulation occurs only upon appropriate mechanical stimulation. Characteristics of mammals that practice induced ovulation are a long copulation period and a bone called a baculum in the penis. In a large male brown bear, the baculum is slightly longer than a pen, and twice as thick.
Brown bears usually have two or three cubs per litter. Occasionally, a sow may even give birth to four young.
The record to date is six cubs. However, such a large family is about as rare as naturally conceived quintuplets in humans.
As the cubs are not identical multiple offspring, and as each ovulation produces only one ovum, several copulations are required to produce and fertilize the eggs. Consequently, the bears often remain inseparable for three to four days, with intervals of copulation throughout. Then the female loses interest in such close contact, appears to grow uncomfortable in the proximity of the male, and wanders away.
The boar then uses his regained independence to search for further prospective mates. On the Alaska Peninsula, such an uncomplicated, comparatively peaceful courtship and mating process is the exception rather than the rule.
On average, along the Katmai coast, seven bears live per 4 square miles of land. In June, at the peak of the mating season, the bruins start to congregate along salmon streams. Along rivers where the fishing is excellent, as many as ten bears are found per square kilometer. In such crowded conditions, boars are forced to defend their breeding privileges against rivals. Smaller individuals give way to dominant boars, which may mate with four or five, if not more, different females in the course of the breeding season.
In confrontations over the right to mate, boars are sometimes injured. Gaping lacerations on their heads, shoulders and front legs, as well as broken jaws and broken canine teeth, attest to the seriousness with which the battles are fought. The high concentration of bears along salmon streams also results in females mating with several males, a situation that primarily arises when a subordinate boar is chased off by a more dominant male. As bears are induced ovulators , it thus may happen that the cubs of one litter have several fathers if the sow was intimate with more than one mate.
However, implantation of the embryo into the uterus wall does not yet occur. Instead, the development ceases—a phenomenon called delayed implantation or embryonic diapause. The blastocyst finally implants into the wall of the uterus in November, after the female has entered the den and is in hibernation. However, should the bodily reserves of the sow be insufficient to sustain both her and her young until spring, the pregnancy is terminated.
Delayed implantation of the blastocyst prevents the bear cubs from being born in fall at the onset of winter. Also, as an added benefit, the female invests little in the embryo unless her nutritional condition is ideal and the expenditure of her bodily reserves is likely to produce results. A mid-winter time of birth is clearly preferable to a fall delivery, although it is still a far cry from ideal. A later date is impossible due to the restrictions imposed by hibernation.
A fasting mammal supports its bodily processes primarily through fatty acids released from stored fat. As hibernation is, in principle, a prolonged fasting period, bears depend on stored fat as energy during winter sleep. The problem arising in this context stems from the fact that mammalian fetuses are unable to use free fatty acids to meet their nutritional requirements. Thus, the pregnant female is only able to sustain the demands of her unborn young through the breakdown of her own body protein.
Ultimately, this would put her life at risk by inevitably reducing her muscle mass. So, the period of gestation is shortened and the cubs are born in a premature stage. They are raised on the rich milk produced by their mother, which contains up to 40 percent fat in some bear species.
This is possible as the cubs, in contrast to developing fetuses, are able to utilize free fatty acids to meet their energy demands. The blind, newborn brown bear cubs are virtually naked, measure 7 to 9 inches in length, and weigh 14 to 18 ounces. Of all higher mammals, bears give birth to the smallest young in comparison to the size of their mother.
A female brown bear has six nipples—four on the chest and two on her lower abdomen. The newborn offspring are able to locate them by migrating toward her body heat. In the ensuing months, the cubs gain about a 1 every two weeks. In mid-May when they emerge with their mother from their winter home, the young weigh 11 to 13 pounds. By the end of their first summer, they have multiplied their weight yet again. As a rule, the cubs remain with their mother for two and a half years. At the start of their third summer, the sow, often quite suddenly, quits tolerating her cubs around her, chasing them off.
As the female is frequently observed soon after in the company of a male, it is assumed that the aggressive behavior on the part of the mother toward her own offspring, which leads to the severance of family ties, is the result of hormonal change. About 15 percent of females keep their progeny with them for a third year.
Occasionally, some cubs remain four years under the protective maternal wing. The sow nurses her cubs for at least two years, although in their first summer the young supplement their diet with grasses, roots, herbs and fish.
0コメント