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A Guide to the Fascinating Alaskan Brown Bear Life Cycle

Brown Bear life cycles follow the seasons and the demands of their lifestyle—particularly for those living in the Alaskan wilderness who must cope with cold and snowy winters. The average life span for a Brown Bear in the wild is about 20 years. However, the oldest known Brown Bear lived for 35 years in the wild. 

From being born in a den in the depths of winter to maturing into master salmon catchers, this article will dive into the fascinating patterns of Brown Bear life cycles. 

The Birth of Brown Bears

Alaskan Brown Bears give birth to their cubs in their winter dens during January and February. Typically, a female will have a litter of between one and three cubs, although occasionally litters of four can occur.

The cubs are born tiny, hairless, and sometimes weigh less than half a pound. Like most newborn mammals, they depend completely on their mothers for survival. They spend their first few months in the cozy den with their mother, relying on her warmth and protection for the rest of the winter. They nurse on her milk to help them gain weight in preparation for leaving the den in the spring. 

The first stage of a Brown Bear life cycle is spending their first few years under the care of their mother. Brown Bear sow and cubs, Lake Clark National Park, Alaska

The Stages of the Brown Bear Life Cycle

Once spring has arrived, around April and May, Brown Bears emerge from their winter slumber and leave their dens in search of food. Mother bears are very protective of their young cubs; the family stays together for the first two or three years of the cub’s lives. Male grizzlies have no part in raising cubs and can sometimes pose a threat to them.

If you go on a trip to see bears, you might see cubs of different ages and sizes being referred to as “spring cubs” or “yearlings”—we’ll explain what that means next. 

Spring Cubs

Spring cubs is a term used for Brown Bears that were born earlier that year—they are cubs that first emerged this spring. When spring arrives a few months after they are born, the cubs will have grown to weigh anywhere between four and eight pounds. Spring cubs are also sometimes called ‘COYs’, which stands for ‘Cubs of the Year’.

Once they leave the den, cubs begin to explore the world around them. They stay close to their mothers, learning essential survival skills like fishing and foraging. They still feed on their mother’s milk but also start eating solid food, including berries and other plants. As they grow, the cubs explore their surroundings under the watchful eye of their protective mother bears. 

 

Yearling Cubs

Brown Bear cubs stay with their mothers for the first few years of their lives. After their first hibernation, a cub is referred to as a yearling—they are one year old and this is their second year of life. The cubs have grown significantly, but they continue learning from their mother, gradually becoming more independent and preparing for their lives as adults. Yearlings will den with their mother for at least one more winter. 

 

Second and Third Year Cubs

Second-year cubs are even bigger than yearlings while third-year cubs are almost full-grown! By now, they are well on their journey towards independence, becoming more self-reliant and confident.

An important step in the Brown Bear life cycle is when they become independent. The age a Brown Bear cub becomes independent from its mother can vary, but in Katmai National Park, Alaska, cubs will generally stay with their mothers for two and a half years. Katmai bears generally separate from their cubs in May or June of a cub’s third summer. However, some females will keep their cubs through a third summer and hibernation, before separating during the next spring.

 

Subadults and Adults 

When Brown Bears have become independent of their mothers, but have not yet reached sexual maturity, they are known as subadults. Subadult Brown Bears are typically between two and a half and five and a half years old.

Once Brown Bears reach five to six years old they are usually classified as adults. The distinction between a subadult and an adult bear is defined by the point they reach sexual maturity. Like in many animals, there is no set age when this happens but in a Brown Bear’s life cycle, it generally occurs around the bear’s sixth year.

 

The Salmon Run

Alaskan Brown Bears are opportunistic eaters and will eat almost anything. Their diet consists of berries, grasses, flowers, roots, carcasses, small mammals and, famously, salmon.

A significant part of a Brown Bear’s life cycle is the Salmon Run. From June – September each year, salmon swim upriver to reach their spawning sites—a phenomenon known as the Salmon Run. Alaskan Brown Bears gather around rivers in large numbers to feed on these salmon.

Alaskan Brown Bears require a high caloric intake of food, they can eat 80 to 90 pounds of food per day. This is necessary for them to gain weight and store fat for their winter hibernation. A large, dominant male bear sometimes catches and eats more than 30 fish daily. 

 

Hibernation

Hibernation is a big part of the Brown Bear life cycle. Hibernation is state of dormancy that allows animals to avoid periods of intense cold and food shortages, such as the cold Alaskan winter. For Brown Bears, hibernation can last between five and eight months, and they spend it in a den, typically dug into a hillside.

In Katmai National Park, Alaska, bears have not been seen using the same den for multiple winters, and they’re not known to den in sites like caves or crevasses. Most bears in Alaska go to their den and begin hibernation in October and November. This is most likely triggered by hormonal changes and a shortage of food.

Surviving a winter without food or water requires lots of fat reserves. Alaska Brown Bears spend the summer months preparing for their hibernation by gorging on food, such as fish during the Salmon Run, to increase their fat stores.

The mechanics of Brown Bear hibernation are fascinating. During hibernation, a bear’s body temperature remains around 88˚F, only slightly lower than their normal body temperature of 100˚F. Their heart and respiratory rates drop dramatically—they average one breath per minute and have a heart rate of eight to ten beats per minute.

Brown Bears do not eat, drink, or defecate at all during hibernation. Their body fat is metabolized to produce water and food, which they recycle instead of defecating. Their kidneys almost shut down completely and urea (a major component of urine) is recycled into proteins. These proteins maintain the bear’s muscle mass and body tissues. 

When they emerge from their dens in the spring, most bears have lost up to 33 per cent of their body weight from using up their fat reserves. Lactating females can lose even more weight due to the energy they use to produce milk for their cubs. Remarkably, healthy bears still emerge from hibernation in the spring without losing any muscle mass and bone density—just with a bit less body fat than before!

 

Mating Brown Bears

Female Brown Bears do not mate until they are at least four to six years of age. Mating season occurs from mid-May to mid-July and bears will mate with multiple partners during the season. Male Brown Bears compete for the attention of females, fighting to establish dominance. 

Female Brown Bears who have previously had cubs are not ready to mate until they are no longer lactating and their last litter of cubs has become independent—typically within two to three years after birth. When the time comes, female Brown Bears will sever the bond with their cubs, using threats of aggression to encourage the young to disperse. This can be exacerbated by the presence of a male Brown Bear—a potential father for her next litter.

If a female Brown Bear has successfully mated, the embryo will start to develop once she enters her den in the fall – a process called delayed implantation. After about eight weeks, the cubs are born in January or February, and the Brown Bear life cycle begins again.

 

Grizzly Brown Bears in a river feeding on salmon at Brooks Falls, Katmai National Park, Alaska

Alaskan Brown Bears are truly remarkable creatures, with each stage of the Brown Bear life cycle offering a glimpse into their world and its challenges. Their capacity to survive in such an extreme environment as the Alaskan wilderness results from masterful adaptations and the ability to really eat! 

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