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.
Nursing bouts usually last from six to eight minutes when the cubs are still small. Later, when they are older, stronger, and more efficient at suckling, nursing bouts are shorter, lasting from three to four minutes. Its body size far from the adult maximum, such a young boar stands no chance in a confrontation with a fully grown rival. Females mature sexually one or two years earlier, and some sows experience the joys of motherhood at the young age of 5 years.
However, these first attempts at raising offspring are rarely blessed with success. An adult brown bear lives in a world with no natural enemies. The mortality rates among fully grown animals are low, at about 5 percent per year.
By contrast, the chances of survival for bears in their first years of life look rather grim. Almost one-third of cubs do not live to see a second summer. Between 10 and 20 percent of the yearlings disappear, and 25 to 30 percent of the sub-adults released into independence by their mother never attain sexual maturity.
Most deaths are caused either by another bear or by malnutrition. Lactating females can lose even more weight. Surviving a winter without food or water requires fuel, and a bear fuels its body on the fat reserves it acquired during the previous summer and fall. Bears do not eat, drink, urinate, or defecate while in the den. Fat is metabolized to produce water and food, but instead of defecating or urinating to eliminate waste, bears recycle it.
Without the ability to recycle urea, ammonia would build up to toxic levels and poison the animal. Since they are living off of their stored body fat, bears also have very high levels of cholesterol in their blood.
Remarkably, healthy bears emerge from hibernation in the spring without losing muscle mass and bone density, or suffering from hardening of the arteries. But even the older bears are looking extra-big, she said, citing as an example a year-old called Walker.
The connection between Bristol Bay and the fish-fattened bears of Katmai is not lost on opponents of a proposal to build a massive copper and gold mine downstream from the park. The planned project, known as Pebble Mine, would threaten the survival of the salmon that sustain the park's bruins, they say.
The Biden administration said earlier this month that it intended to resurrect an Obama-era policy that could bar development of a Pebble-type mine in the Bristol Bay watershed. Subscribe for our daily curated newsletter to receive the latest exclusive Reuters coverage delivered to your inbox.
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