At first glance, bears may seem, well, lazy. They eat, they sleep, they play…and that’s about it. It’s quite a lifestyle they enjoy.
But in reality, bears are magnificent masters of survival. Their bodies are perfectly engineered to endure self-imposed famine with no negative consequences, all while maintaining their top predator status.
Let’s take a closer look at these remarkable animals.
Brown versus grizzly bears
The famed bears of Katmai are brown bears, one of three species of bear found in Alaska (the other two being black and polar bears).
The terms “brown” and “grizzly” are often used interchangeably, but “brown” (Ursus arctos) is the overarching species under which all subspecies fall. Within the Alaskan brown bear species, there is the coastal bear found at Katmai as well as two distinct subspecies, the grizzly (U. a. horribilis) and the Kodiak (U. a. middendorffi). The differences between brown bears and grizzlies are mostly arbitrary and largely pertain to habitat and food sources. Brown bears live along the coast and have access to protein-rich marine foods such as salmon, while grizzlies are found inland and therefore lack access to such foods. As a result, brown bears can be much larger than grizzlies. Kodiak bears, indigenous to Kodiak Island in the Gulf of Alaska, are also coastal bears, but they have enough distinctive physical features, including larger size, to make them a subspecies of brown bear.
In general, brown bears are massive, and the bears found at Katmai are some of the largest in in the world. Adult Katmai males can rival Kodiak bears in size, standing 3-5 feet at the shoulders and 7-10 feet in length. At the end of the salmon season, several well-known males will exceed 1,000 pounds, but the average male weighs 600-900 pounds. Generally, adult females weigh about 1/3 less than the adult males.
Oh, and a randon piece of trivia: Want to know another difference between males and females besides their size? The way they urinate. Males urinate straight down, while females do so with the stream going behind them and at an angle, as was randomly captured by Dale in this video:
Now ya know.
The bears of Katmai eat mucho salmon of course, but that’s not their only source of nutrition. In fact, bears are omnivorous, and up to 75% of their diet comes from plant sources such as roots, grasses, berries, flowers, and herbs. We were surprised to see bears eating grass and other green matter in the midst of all that fish!
The rest of their diet is made up of animals, including fish, insects, and caribou & moose calves as well as small mammals such as Arctic ground squirrels (nicknamed “bear burritos” because they’re a favorite bear snack). Bears also are opportunistic, feeding on whatever carrion they might come across. And because they’re the baddest predator around, they can steal a meal from whatever other, smaller predators might be in the area.
One of the brown bear’s most distinctive biological features, and something that contributes significantly to its status as an omnivore, is its shoulder hump. Brown bears use this massive bit of muscle to dig, turning up everything from roots and insects to bear burritos. The hump also enables them to clear out large winter dens.
The larger bears also dig belly holes, which accommodate their rotund bellies, making it possible for them to lie flat.
They also have much larger claws than either black or polar bears. While these claws look terrifying, their main purpose is for digging, not mauling. They’re long and relatively straight and average 2-4 inches in length, around twice as long as that of black and polar bears.
The size of a bear’s home range is determined by the amount of food available. The estimated range for the average adult male brown bear is a little over 500 square miles, but in areas where food sources are more abundant, bears may require only about 10 square miles. The food in Katmai is often so plentiful that the bears tolerate one another in very close proximity.
With winter always looming in Alaska, the challenge for these bears is to eat a year’s worth of food in about 6 months. In order to accomplish this, the larger bears will consume between 80-90 pounds of food daily. With the help of the calorie-rich salmon, these bears can gain 3-6 pounds of fat every single day! See this post for a look at the various strategies that brown bears utilize when fishing for salmon.
Mature adults mate in late spring and summer, usually between May and August. The resulting embryo won’t implant into the mother’s uterine wall until she dens in the late fall, and if she doesn’t acquire enough fat reserves in the summer, it won’t implant at all, instead being reabsorbed by the body. Cubs will be born in January or February, about 8 weeks after their mother dens and the embryo implants. Mothers typically have 1-3 cubs per litter, although a litter of four is sometimes seen.
At birth, cubs usually weigh under a pound and are blind, hairless and helpless, but instinctually they find their mother, who remains in hibernation, to nurse. They grow quickly on the rich milk (20% fat), and when they emerge from the den, usually in April or May, they’ll weigh between 4-8 pounds.
Newly born bears are known as either spring cubs or cubs-of-the-year (COY’s); in their second year they are known as yearlings. Generally, they’ll be with their mother for the first 2.5 years of their lives, spending two winters denning with her, but a mom might sometimes let her cubs stay with her through as many as three winters.
When it’s time for the cubs to leave the nest (or den, as it were), the mother will use aggression and threats to chase them away. In the video above, it appears that this particular mother is weaning her kiddos off of breast feeding, and they’re none too happy about it, hence the groaning and complaining.
Once they’re on their own, cubs become known as subadults, falling under this category until they reach maturity at about 4-6 years old. Subadult bears will frequently stay with their siblings until they reach adulthood.
In the wild, the average lifespan of a brown bear is 20 years, with the longest-lived known specimen being a male that lived to about 35. Currently at Katmai, the oldest bear is the female 410, shown in photos above napping and digging. It’s thought that she was a Spring 1989 cub.
Then there’s Otis (Bear 480), who, at 20+ years of age, is another of Katmai’s oldest bears. Otis is also one of its celebrities, a star of the Explore.org Bear Cam. Because of his age, he’s not as dominant as he once was, and now he mainly sits in his “office,” a pool at the far end of Brooks Falls, waiting for the salmon to come to him!
The feeding binge that occurs in the summer and fall is a prelude to hibernation, which can last for 5-8 months. Bears will seek out the coldest part of their range when choosing where to den. In data gathered from radio collar studies in the 1970’s, Katmai bears tend to den in nearby mountains at between 500-1500 feet in elevation and on steep, heavily vegetated slopes. It appears that these bears dig a new den every year.
In the Katmai region, bears usually enter their dens in October and November, with pregnant females and mothers with cubs denning earliest. Generally, the single females and subadults go next, with the adult males going into hibernation last, sometimes as late as December. Scientists don’t know the exact mechanism that triggers hibernation in bears, but it’s probably a dwindling of high-calorie food sources combined with seasonal hormonal changes.
While hibernating, a bear goes through several physiological adaptations. First, its body temperature drops, although comparatively less so than with other mammals that hibernate. Normal body temperature for a bear is 100 degrees F, and it generally doesn’t drop below 88 degrees while in the den. In contrast, smaller mammals’ temperatures will drop to near freezing during hibernation. Bears’ heart and respiratory rates do decrease considerably, though, with heart rates averaging 8-10 beats per minute and breathing dropping to just one breath per minute.
Bears will not eat, drink, urinate, or defecate while hibernating, and they live off fat metabolism. Despite their inactivity, they can still burn 4000 calories or more each day just keeping warm, which is why the binge eating is literally a life-or-death act for them. When they exit the den in the spring, they will weigh on average about 33% less than when they started hibernation, with lactating females losing even more. But thanks to special adaptations, that weight loss comes almost exclusively from fat, and healthy bears have no loss of bone density or muscle mass. The bears’ kidneys almost completely shut down, and the toxic urea produced from fat metabolism is recycled into proteins to protect muscle mass and organs. As a result, bears can survive winter and famine with no hunger or thirst and no negative physical consequences. Remarkable.
Up next, we’ll talk about the specialized techniques that Katmai’s bears use to catch all those slippery, fast-moving salmon!