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ECOLOGY

SEASONAL FASTING BY POLAR BEARS IS MORE COSTLY THAN HIBERNATION
Polar bears in some regions are forced on land during the summer-fall months when sea ice melts. While waiting for the ice to re-freeze in the fall, these bears typically go without significant food sources for months at a time. This period has been referred to as “walking hibernation.” However, researchers were skeptical of whether a fasting bear’s metabolism is similar to hibernating bears. This is of special concern because the amount of time polar bears have been spending on land fasting has lengthened over time, and it is projected to continue to expand. This increases the amount of energy bears need to store as body fat to sustain them through the fast, and it also leaves less time during the year for bears to hunt and obtain those energy stores. Our research showed that the energy expenditure of fasting polar bears was higher than hibernating bears, with fasting polar bears losing 2.4-3.2 times the mass of equal-sized hibernating brown bears. Therefore, the idea that polar bears are able to enter “walking hibernation” implies greater energy and protein conservation than actually occurs. This information helps researchers better predict the energy stores necessary for fasting, as well as whether bears will survive during longer fasts.
MEASURING BEAR BODY FAT PROVIDES IMPORTANT INFORMATION TO RESEARCHERS
People can measure their body fat content by stepping on battery-powered bathroom scales. Bear researchers have a similar capability to measure the body fat content of bears, either by using bioelectrical impedance analysis or by measuring total body water in either anesthetized or unanesthetized bears. These techniques have been critical to understanding how a bear’s size, fatness, and cub production are related to its food resources. For example, we now know that leaner females produce cubs later and that those cubs grow more slowly and are less likely to survive than those produced by fatter females. Similarly, biologists in Yellowstone National Park have learned that “problem bears” that get too close to people or kill livestock are leaner than the rest of the bear population, which may explain why those bears are willing to take a chance and seek high-risk foods.
MATERNAL BODY FAT CONTENT AFFECTS CUB PRODUCTION AND SUCCESS
The number, size, and survival of bear cubs emerging from winter dens depend on maternal condition prior to entering the den. Bears at the Bear Center were able to vary both the birth date and growth rate of their cubs in response to maternal fat stores. We also found that females with under 20% body fat failed to produce cubs even though breeding had occurred. Females in poor condition near the entrance of hibernation would likely not be able to support the energetic cost of cub production and lactation while hibernating and therefore fail to implant embryos. This apparent threshold value gives researchers an important key to understanding the productivity of wild females and the future health of populations based on maternal fat stores.
HAIR OR BONE CAN TELL US WHAT BEARS ARE EATING
Bears eat a wide variety of foods. However, rarely have we been able to determine the relative contribution of those foods to growth, reproduction, and the well-being of wild bears. A relatively new technique called stable isotope analysis does allow us to use a few hairs, a drop of blood, or a flake of bone to quantify which foods are being used by a particular bear. This technique can be used on both modern bears and on bears that died thousands of years ago. For example, we’ve learned that grizzly bears currently living in Yellowstone National Park have about the same diet as their ancestors had a thousand years ago. However, things have changed in other ecosystems. For example, salmon provided 58% of the nourishment for grizzly bears living 100 years ago in Idaho, Washington, and Oregon. This level of salmon consumption is about the same as that occurring today in many populations of Alaskan grizzly bears with access to abundant salmon. Unfortunately, no grizzly bear in Idaho, Washington, or Oregon currently consumes salmon.
BEAR VIEWING CHANGES BEAR BEHAVIOR AND ACCESS TO FOOD RESOURCES
Grizzly bears that eat a lot of salmon are 80% larger, produce 25% more cubs, and live in populations that are up to 50 times denser than grizzly bears that fatten primarily on fall berries. Thus, human activities that disrupt foraging by bears on salmon or other important seasonal food resources could seriously harm individual bears or their populations. Bear viewing has become very popular in many areas of Alaska. We’ve learned that bear viewing affects both the number of bears and the age and sex ratio of bears willing to visit salmon streams while people are present. Managers can reduce these impacts by minimizing the number of trips people make into viewing areas, standardizing the timing of those trips, confining people to well-defined viewing areas, and providing significant blocks of time when people are not present.
SOME FOODS ARE ESPECIALLY IMPORTANT TO BEARS, AND CHANGES TO ECOSYSTEMS AFFECT THEIR AVAILABILITY
Although grizzly bears are frequently identified as large carnivores, whitebark pine nuts are one of the more important foods for grizzly bears in Yellowstone National Park. During years of good pine cone production, two-thirds of the bears derive over half of their nourishment from pine nuts. Surprisingly, bears depend on red squirrels to harvest the cones and store them in their middens, where bears can efficiently excavate and consume the energy and protein-rich nuts. Fortunately, the squirrels also gather other cones that grizzly bears don’t use, so there are food resources left for the squirrels. Unfortunately, whitebark pines are being attacked and killed by white pine blister rust, a non-native fungus. Another important food for Yellowstone grizzly bears is cutthroat trout that migrate out of Yellowstone Lake each spring and summer to spawn in at least 59 tributary streams, much like spawning salmon in Alaska. However, illegal introductions of lake trout have led to a dramatic reduction in cutthroat trout. While a study conducted in the 1980s, prior to the crash in the cutthroat trout population, found that female bears ate more fish than did males and that 90% of their diet came from cutthroat trout, a later study by us in the late 1990s found that male grizzly bears were the only ones making extensive use of the remaining cutthroat trout and the entire population of bears consumed less than 10% of what had been eaten in the 1980s.
BEARS ARE NUTRIENT TRANSPORTERS
In large-scale global nutrient cycles, nutrients from the land are leached into freshwater and ultimately flow via rivers into oceans. However, salmon that return from the oceans to spawn in streams and lakes are one way that nutrients can return from oceans to land. While dying salmon provide the nutrients necessary to nourish the next generation of salmon, bears also move the nutrients that salmon consumed in the ocean back onto the land. For example, each adult female grizzly bear on the Kenai Peninsula in Alaska eats about 3,000 lbs of salmon per year. While bears retain the energy from the salmon as fat, much of the nitrogen, phosphorus, and other excess nutrients used by plants are excreted in urine and feces as bears walk through surrounding areas. Spruce trees growing along Alaskan streams with healthy salmon runs get almost 20% of their nitrogen from salmon, and over 80% of that nitrogen passed through a grizzly bear. Thus, grizzly bears are extremely important in moving nutrients from the ocean back to the land.
FOOD SOURCES FOR BEARS ARE HIGHLY VARIABLE AND INFLUENCE BEAR BEHAVIOR, BODY SIZE, AND CUB PRODUCTION
Most people do not understand the link between the types of foods that occur in an area and the number, size, and productivity of resident bears. For example, developers wanting to build homes on salmon streams, thereby preventing bears from accessing salmon, have argued that the grizzly bears that currently live along those streams will learn to feed on berries and other dispersed, low-quality foods that exist in the surrounding forests. After all, other bears are already feeding on those food resources. However, the very large size of salmon-feeding bears means they also have very large daily energy and nutrient requirements. These big bears will find it impossible to physically gather and process enough berries and vegetation on a daily basis to meet even their minimum requirements. While they can lose some excess fat, their large skeletons and muscles are not shrinkable to the size of a bear that can live on lower quality, dispersed foods. Because there is no physical solution to this imbalance, these bears are destined to become “problem bears” as they try to regain access to salmon or starve. Although the foods are different, the same problem can occur when long-term, open-pit garbage dumps are closed or abundant livestock carrion are removed as food resources for grizzly bears.

PHYSIOLOGY

BEAR HEARTS CHANGE DURING HIBERNATION
During their annual 4-6 month hibernation, bears minimize activity and do not eat, drink, or urinate. As part of that process, they develop extremely slow heart rates (profound bradycardia), yet the heart remains healthy. For example, active bears have heart rates of 80-90 beats per minute, but hibernating bears have average heart rates of 15-18 beats per minute, with some as low as 5 beats per minute. In people, heart rates this low would cause congestion and heart failure within a matter of weeks, but the bears show no signs of congestion even after 5 months. Studies have shown that the heart muscle becomes stiffer during hibernation and that the bears can “turn off” two of the four heart chambers. These studies are continuing in an effort to understand these natural adaptations and how they might benefit people or pets with heart disease.
BEAR MUSCLES REMAIN STRONG DURING DISUSE
Hibernating bears maintain muscle strength and mass even though humans would lose 70% of their muscle strength during a similar period of inactivity. When human muscles are not exercised, slow twitch fibers that are important for posture (i.e., standing) and prolonged activity (long-distance running) convert to fast twitch fibers that have minimal endurance capability. Thus, bed-ridden patients need extensive rehabilitation in order to resume normal lives. However, bears are very capable of running and resuming their normal lives immediately after emerging from 5 months of hibernation. Our major findings thus far are that muscle types, sizes, and contractile properties of bears do not change during hibernation. It appears that one way bears may prevent these changes by having a series of whole body, isometric contractions that start at the neck and extend as a wave over the entire body several times per day. Thus, it seems that they are exercising while they are sleeping.
BEAR BONES STAY STRONG DURING INACTIVITY
When people are bed-ridden, bone loss and reduced bone strength occurs. To fully recover, most patients need therapy lasting 2-3 times longer than the length of time that they were inactive. However, bears from northern climates experience annual periods of disuse (hibernation) and activity that are approximately equal in length, yet they do not lose bone mass or strength even after many years of hibernation. We have found that bears maintain a balance between bone resorption and bone deposition such that bone strength is fully maintained throughout hibernation. These processes are controlled by hormones regulating calcium balance. These physiological processes could be used to develop therapies for people or pets with osteoporosis.