In this video segment excerpted from the “City of Bears” episode of the PBS series NATURE, guide and host Chris Morgan observes the annual late summer ritual of the salmon run, when normally solitary Alaskan brown bears come together to feed on the massive concentrations of salmon swimming upstream to spawn. Particular focus is given to the challenges faced by mother bears, who must protect their cubs even as they must teach them to survive in this dangerously competitive environment.
- How closely do brown bear cubs follow their mothers? Why do they do so?
- Why do brown bears prefer eating female salmon to male salmon?
- Do adult brown bears prefer to hunt together or individually? How does this affect their interaction during the salmon run?
- Why is it so essential for brown bears that they consume as many fish as possible during the salmon run?
- What are the challenges and dangers faced by mother bears during the salmon run?
The brown bear is the most widely distributed species of bear in the world, found in the northern regions of North America, Europe, and Asia. It is also the most broadly defined of bear species, with subspecies ranging from the smaller 350 pound “grizzlies” found in mountainous inland regions to the massive 1700 pound “Kodiaks” native to coastal lowlands regions like southern Alaska.
Mating season for brown bears lasts from early May through late July, although a process of “delayed implantation” allows a pregnant female to actively hunt, fish, and forage for several months before embryo gestation begins. For brown bears on the coastal plains of Alaska, the annual culmination of this feeding season is the “salmon run,” during which salmon swim upstream to spawn and die in the rivers where they were hatched. For this, the entire brown bear population descends in concentrations of up to several dozen to fish in the teeming streams.
Brown bears are omnivores, meaning they will eat whatever is available, from plants to animals to insects. While their smaller grizzly cousins in Wyoming’s Yellowstone Park consume half their annual calorie intake by eating up to 40,000 moths a day, Alaskan brown bears rely heavily on the fatty calories of salmon and salmon roe (eggs), consuming up to 40 pounds a day to see them through the long and lean months of winter hibernation, during which time they will lose up to half their body weight.
During hibernation—or “dormancy”—pregnant female brown bears give birth to litters of between one to four cubs, which generally weigh less than 1 pound each at birth. The cubs feed on their mother’s milk through the spring and into the summer, until they reach a weight of between 15 to 20 pounds and are able to learn fishing, hunting, and foraging skills from their mother. These skills are acquired by following and observing their mother over the course of two to four years, during which time the mother avoids male bears, which are known to kill and eat cubs. Male brown bears take no part in raising their own cubs.
Being at the top of the food chain with no need for safety in numbers, brown bears are generally solitary animals. Their concentration along rivers during the salmon run provides wildlife researchers with rare opportunities to observe their social interactions, which include physical competition among females for males, females teaching their cubs survival by demonstration, and female protection of their cubs against larger and aggressive males.
Life Science, Content Standard C
As a result of their activities in grades 9-12, all students should develop understanding of
- Interdependence of Organisms
- Organisms both cooperate and compete in ecosystems. The interrelationships and interdependencies of these organisms may generate ecosystems that are stable for hundreds or thousands of years.
- Living organisms have the capacity to produce populations of infinite size, but environments and resources are finite. This fundamental tension has profound effects on the interactions between organisms.
- Matter, Energy, and Organization in Living Systems
- The distribution and abundance of organisms and populations in ecosystems are limited by the availability of matter and energy and the ability of the ecosystem to recycle materials.
- Behavior of Organisms
- Organisms have behavioral responses to internal changes and to external stimuli. Responses to external stimuli can result from interactions with the organism’s own species and others, as well as environmental changes; these responses either can be innate or learned. The broad patterns of behavior exhibited by animals have evolved to ensure reproductive success. Animals often live in unpredictable environments, and so their behavior must be flexible enough to deal with uncertainty and change. Plants also respond to stimuli.
- Like other aspects of an organism’s biology, behaviors have evolved through natural selection. Behaviors often have an adaptive logic when viewed in terms of evolutionary principles.