Bears are well known omnivores. They are animals that eat both plant and animal matter including insects, plant matter, berries, tree cambium, fish and red meat.
At the North American scale, a high degree of available foods appear on the bear menu. Large salmon runs or even whale carcasses are available along the Alaskan Coast. Caribou range Northern Alberta. Huckleberries, whitebark pine, deer and elk litter the Rocky Mountains.
Composition of diet has been shown by to affect physical size, reproductive potential and survival. Specifically for bears, wildlife sciences have used ratios between types of carbon, nitrogen and sulfur stable isotopes in hair and tissue to examine the composition of plant, animal and marine material present in their diet. Examining food variation within ecosystems and across North America can help decrypt the “we are what we eat” trade-offs that exist between diet and biologic fitness.
Stable isotopes are nonradioactive atoms that have an extra neutron, or in other words they do not decay spontaneously. Differing types of stable isotopes are present in plants, animals and even human foods. These isotopes are transferred to the animals that eat them.
Using samples of tissue and laboratory equipment, concentrations of stable isotopes within tissue can be quantified. These concentrations can then be correlated to differing diets composed of meat, plants, marine life etc. Different samples from animals can infer different time periods for foods ingested.
Plasma for instance can provide information on what an animal has ingested in the last one-two weeks prior to collection and red blood cells provide information over the past two-three months. Hair can be segmented to provide information on diet over the time it has grown (summer to fall).
For instance, Carbon stable isotopes aid in determining the primary production (i.e. plants) energy source within an animal’s diet. Examining carbon signatures over time can be useful to identifying seasonal food usage by bears or selection of differing plant species within a particular ecosystem or habitat.
As organisms eat each other, stable nitrogen isotopes are transferred from one animal to another. Comparing carbon to nitrogen isotopic signature can be very informative about the composition of an animal’s diet.
Yet, large ecosystems that have a great spatial breadth and high diversity of vegetative foods, on-the-ground monitoring of availability and use of particular foods isn’t feasible. The use of stable isotopes is then a critical tool to inferring assimilated diets of bears to understand food use throughout and ecosystem. In other words, let the bears tell us what is important to eat.
Research has used stable isotopes techniques to better understand how grizzly bears use food resources within different habitats across large ecosystems. For instance, in Yellowstone, Sulfur isotopes helped set the nutritional importance of white bark pine nuts which was found to differ significantly within grizzly bear diet above all other plant and animal nutritional sources.
Within the Northern Continental Divide Ecosystem, early isotope work has indicated that bears on the eastern side of the Rocky Mountains are more carnivorous than grizzly bear on the western side of the Continental Divide. In research led by Justin Teisberg of the US Fish and Wildlife Service, blood and hair samples from grizzly bears across the ecosystem and compared. Male grizzly bears on the eastern Rocky Mountain Front averaged 81 percent animal matter in their diet and females averaged 61 percent. Comparatively, males in the Whitefish Range and North Fork of the Flathead consumed 18 percent animal matter during the summer months. Combined for sex, grizzly bears in the Swan Valley averaged just below 60 percent animal matter in their diets during the summer months.
Understanding the importance of food, especially to a bear, is not to be understated. Bears display delayed-implantation. Breeding occurs between mid-April and late July, the fertilized egg does not implant in the uterine wall until late autumn. About two months after this implantation the cubs are born in the den. However, a bear’s body fat, a measure of the animals physiologic requirements to successfully birth and nurse cubs in the den, needs to be greater than 20 percent before implantation will occur.
Research in the Northern Continental Divide Ecosystem (NCDE) has also examined body condition of captured grizzly bears and results thus far show that grizzly bear females across all portions of the NCDE enter the dens with fat levels above those needed for cub production.
Stable isotope and body condition monitoring will continue to occur across the ecosystem as state and federal agencies work on the delisting of the grizzly bear in the NCDE.
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