Bears in the Neighborhood – Part 2

By Jeff Keay

Photo by Jeff Keay

Denali National Park

This is Part 2 of a three part series.

May 15, 1991 dawned cool in Denali National Park, Alaska with temperatures in the 50s, which actually felt pleasant after the long, cold winter. Patches of snow gleamed bright in the sunlight. This was day five of my first grizzly bear capture effort and I had become comfortable with the routine. Ken Butters deftly maneuvered the helicopter close behind a medium-sized grizzly bear, adult female 117. Leaning out of the door and hard against my safety harness, I squeezed the trigger of the capture rifle. A small pop and I saw the dart land squarely in the bear’s hind end. Ken flew the helicopter a short distance away so the bear could go to sleep in peace. Her three small cubs scampered off when the helicopter returned to drop me off near the bear.

Long guard hairs in the fur of bears recently emerged from winter dens makes them look bulky and healthy. You can imagine my shock when I felt between each rib and the deep contours of her pelvis. I had never handled a bear this skinny.

After weighing 117, we put on a collar fitted with a radio transmitter, then left her to wake up peacefully. As we flew away, her three little cubs scurried over and nestled against her. How would they fare in this land of long winters and short summers, I wondered.

I radio-tracked bear 117 every week that summer, along with six other adult females, each of which had dependent cubs or yearlings. Before July 1, all three of 117’s newborn cubs disappeared. Five of the other six adult females lost complete or partial litters. The next spring 117 emerged from her winter den with another set of triplets, and once again all of them disappeared by July 1.

Denali’s skinny female grizzly bears and the high cub mortality was so opposite to my experience in Yosemite that I just had to learn why. For eight years we captured and tracked grizzly bears on the north slope of the Alaska Range in Denali National Park. Radio-collars were placed on male and female bears two years old and older. We documented the number of dependent cubs, the timing of cub mortality or dispersal, and the dates of den entrance and exit. The patterns that emerged taught us a great deal about the characteristics of naturally regulated grizzly bears.

My study area explored the remote regions of Denali National Park, reaching 70 miles southwest of the Eielson Visitor Center, along the north slope of then Mt. McKinley (now Denali). It received very few human visitors and the bears hadn’t been hunted for at least 80 years. This was a population totally uninfluenced by human activity or foods.

Adult females emerged from winter dens, on average, on May 1, and entered dens in the fall around October 6, making for almost seven months tucked away in winter dens. Add to that the physiological slowdown and time digging dens prior to entrance and the lethargy and food shortages post-emergence, and their fasting exceeded seven months. That left less than five months to secure a year’s worth of nutrition.

Denali female grizzly bears survived on a plant-based, low-protein diet with little access to terrestrial meat and no access to salmon. Spring foods included digging roots and eating emergent grasses and succulent forbs. Denali boasted abundant crops of blueberries and buffalo berries most years along with several other high carbohydrate fruits. Berries could ripen as early as mid-July on the best years but were sometimes delayed until mid-August. They gorged on berries from first ripening until snow buried the fruit in late September or early October. Large male bears dominated caribou and moose carcasses, giving adult females very little access to animal protein. Ground squirrels, and possibly a few spring caribou calves, did provide females some protein.

Winters were long and hard on adult female grizzly bears. Pregnant females entered dens averaging 366 pounds, and lost 43% of their body weight over winter. Of the females that produced new litters, those that successfully kept their cubs alive all summer averaged 34% body fat the previous fall, while those that lost complete or partial litters averaged only 28% fall body fat. So, the fatter the bear, the better the chance of producing cubs and keeping them alive.

Christy Welch, a Ph.D. student at Washington State University, and her major professor, Charlie Robbins, accompanied me in a government pickup as we drove out the gravel Denali Park Road. The sky hosted a few high clouds on August 24, 1994; the winds were calm, and the snow level had dropped to 3,000 feet. Christy came to Denali to observe the foraging behavior of grizzly bears with a special interest in berry consumption rates. She had completed feeding trials on captive bears at the University and on wild bears in Glacier National Park, Montana.

As our vehicle crossed Sable Pass, we spotted a solitary grizzly bear feeding on blueberries. Blueberry bushes in interior Alaska are short and their branches extend vertically allowing bears to strip the branch by lifting their head vertically, often taking leaves along with the berries. Like oil wells in high gear, this bear bobbed its heads up and down taking a mouthful of blueberries with each bite. Charlie worked the stop-watch while Christy and I counted bites.

After a half hour of bite counting, Christy exclaimed, her voice elevated with excitement, “Jeff, I can’t believe this. This bear is taking 95 bites every minute, that’s twice as fast as captive bears and wild Montana bears!”

“Really?” I puzzled. “And what’s more, a recent study here showed they do this continually for over 14 hours every day.”

We watched several other bears over the next few days, eating both blueberries and buffalo berries, all with the same result. Denali grizzly bears consumed ripe berries twice as fast as bears in lower latitudes. So serious were they about berry eating, that we often saw grooves in the teeth of older bears worn there by stripping branches.

In many healthy increasing grizzly bear populations, with abundant foods, moms kick their cubs out at two years of age, and sometimes as yearlings. Female grizzly bears in Denali typically kept their young until age three, and sometimes age four. Also, females produced their first litter somewhere between age six and 10. Denali’s late maturing females and older age of young at family breakup, were typical of declining grizzly bear populations, populations under nutritional stress. We believe the reason is because Denali female grizzly bears experienced nutritional stress, and that their dependent young were protein deficient resulting in slower growth rates and later maturation.

The typical litter size of Denali females was 2 cubs, but occasionally they had triplets and less frequently singletons. They also maintained a high reproductive rate, producing cubs as often as possible. The large litter size and high reproductive rate were characteristic of a healthy, growing population, populations with abundant food resources. This suggests Denali females were not nutritionally stressed as much as declining bear populations.

Producing cubs every year, only to lose them, as was the case for Bear 117, seems like waisted energy and not ecologically sound. Not knowing what next year’s forage crop will sustain, it actually does make ecological sense to invest a small amount in producing cubs to see if they can make a go of it next year. Thus seems to be the harsh reality of life in a highly variable and unpredictable environment. Bear 117 slipped her collar that second summer. When we captured her again a couple years later she was accompanied by a single yearling. So this ecological approach apparently worked for her.

Survival rates for independent bears were very high, and averaged 97%. Dependent bears, on the other hand, didn’t do nearly so well. First year cubs had only a 34% survival rate, and those that did survive had only a 46% chance of surviving as yearlings. Two-year-olds fared a bit better with 79% surviving. I lacked good quantitative data on the causes of dependent bear deaths, but did observe deaths from predation by other bears, accidents, and starvation.

The age distribution of adult females struck me as odd. We had a lot of older females and a number of younger females, but nothing in the middle. This bi-modal age structure suggested that recruitment of young females into the population occurred in pulses, and not consistently over time. This likely resulted from the high mortality rate of dependent young, related to nutritional shortages. During good forage years, more young would be expected to survive to independence. My research started right at the end of one recruitment pulse and continued through a non-recruitment phase.

Combining reproductive and survival rates, we calculated that the Denali grizzly bear population was declining at a rate of two percent per year. However, we did not observe dependent bear survival rates that would have resulted in the female recruitment pulse we detected during the study. We suspect the grizzly bear population increased and decreased slightly over time, depending on nutrient availability.

Grizzly bears in Denali occurred at a fairly low density. We measured the density of bears in Denali at 35 bears per 1,000 square kilometers (386 square miles). Bears in Katmai National Park, that gorged themselves on abundant salmon, averaged 412 bears per 386 square miles.

September 25, 1997 found me in Brooks Camp, Katmai National Park, Alaska to help a friend and colleague, Tom Smith, with his grizzly bear research project. The sky held a high overcast with intermittent light rain; temperatures reached into the 50s. A large grizzly bear passed close to our cabin while I showered in the morning. Another bear fished the upper end of the Brooks River, not too far from our cabin. Red, King, and Silver salmon run up this river and provide abundant food for bears. Consequently, large numbers of bears concentrate here each fall for the feast. I saw quite a few large craters in the ground, about two feet across and a foot or so deep. Tom identified them as belly holes; bears dig them to place their gigantic bellies in when they lay down. I can assure you, there are no grizzly bear belly holes in Denali.

Bears populations are self-regulated. They have no predators, except other bears, so they have some mechanism that keeps their population within certain bounds. The actual mechanism for the regulation of bear populations has not been confirmed, but we believe adult male aggression is a major factor. Our data indicates that Denali grizzly bears are food limited. That is, the constraint on population size is the amount of food that is available. If you increase available food, the population should increase. Highly abundant nutritious food produces high density bear populations, like in Katmai. Our data also suggests that the Denali population is regulated below a level that would result in reduced reproductive rates or serious starvation events. Otherwise, females wouldn’t have been able to consistently produce cubs every year, even years when nutrition was in short supply. Thus, we believe that the Denali population was self-regulated, and maintained at or near carrying capacity.

Carrying capacity refers to a theoretical cap on how many animals a habitat can sustain over the long term. It’s a theoretical cap, because it’s really hard to put a number on it. It’s a concept that means animal populations can only increase until they run out of some crucial resource, be it food, water, or other necessity. For Denali grizzly bears, we believe food is that critical limiting resource.

To summarize, adult females will work hard to secure enough groceries to produce cubs every chance they get. To successfully produce and raise twin cubs, they need over 30% body fat prior to entering dens in the fall. Populations below carrying capacity will have sufficient natural foods to allow females to meet their nutritional needs. At carrying capacity, there will be periods of natural food shortages, such as during years of drought or other environmental perturbations, like wildfire. During years of natural food shortages, we would expect bears to actively search out and exploit any potential new food sources they can find.

August 5, 1992
Don Glaser revved the engine while I settled into the back seat of Piper Super Cub 27 Yankee. Letting go of the brake, he launched the small two-seater down the Denali National Park airstrip. The Super Cub’s narrow fuselage, high wing, and slow speed capability made it ideal for radio-tracking grizzly bears. Antennas mounted on left and right wing struts told us the direction of the signal from a radio-collared bear. The clear sky and calm winds promised good visibility for seeing our subjects. Once in the air we headed west along the Park road toward my study area.

Just after 7:00 a.m. my scanner picked up the first beep of a radio transmitter, bear number 656, a three-year-old male. Don steered the plane so the signal strength was the same from both antennas. As the signal got louder, we dropped in altitude. I searched out the window, my lap filled with data sheets and maps.

“There he is, Don, on the toe of the Muldrow Glacier. I’ll run the scanner. Oops, here’s the next one already, this will be 648, an adult female.”

The Muldrow, a massive glacier, begins high on the eastern flank of Mount McKinley. At the lower end, the toe, it’s covered with rocks, soil and has vegetation growing over the glacial ice. Buffalo berry bushes are thick on the toe of the glacier.

“Sounds like this one is right here, too.” Don said matter-of-factly.

After a moment searching, I responded, “Yup, right there, also on the toe of the Glacier.”

As was the next one, and the next one, and two more.

“Don, that’s crazy, six bears all feeding on ripe buffalo berries on the toe of the Muldrow Glacier.”

“Oh! Look, Don, there’s a black bear eating berries right in the middle of the grizzlies.

When the berries were ripe and abundant, social disputes quelled and tolerance for each other ensued. Serious carbohydrate consumption topped the agenda for both black and grizzly bears.