Grizzly bears are powerful, apex predators that have few natural threats in the wild. However, even these mighty beasts can be brought down by various factors. In this article, we will explore the main causes of mortality in grizzly bears and how each threat impacts their survival. Understanding what kills grizzly bears can provide insights into their population dynamics and conservation needs.
Starvation
Starvation is one of the leading causes of natural mortality in grizzly bears. Grizzlies need to consume massive amounts of food to survive, up to 20,000 calories per day during hyperphagia in preparation for hibernation. However, food resources can become scarce due to declines in key food sources like salmon or whitebark pine seeds. When grizzlies cannot find enough to eat, they will begin to lose significant body mass and eventually die of starvation.
Several factors can lead to increased starvation among grizzly bears:
- Declines in salmon populations due to overfishing, habitat loss, dams blocking migration routes, climate change, and disease/parasite outbreaks.
- Reduced availability of other meat sources like ungulates due to overhunting, competition from other predators, and habitat loss.
- Loss of whitebark pine trees from disease, pine beetle infestations, and wildfires. Whitebark pine seeds are a critical high-fat food source for grizzlies prior to hibernation.
- Natural events like late spring frosts or drought that impact berry and root crop productivity.
- Increased nutritional needs for female bears nursing cubs.
Bears that experience starvation have reduced fertility rates and higher cub mortality. If starvation continues over consecutive years, it can lead to significant population declines and even local extirpations. Managing key food sources is therefore critical for sustaining healthy grizzly populations.
Intraspecific Aggression
Intraspecific aggression, which refers to fighting between members of the same species, is another cause of mortality for grizzly bears. Adult male grizzlies will kill cubs to bring females into estrus so they can mate. Females with cubs have also been known to kill the offspring of other females to eliminate competition over resources.
Younger bears may be killed in conflicts with older, larger bears over preferred habitat and food resources. Dominant bears will violently defend prime feeding spots like salmon spawning grounds. Bears emerging from hibernation may also predate smaller bears they encounter near den sites.
While such fatal conflicts are relatively rare, intraspecific aggression can impact grizzly population growth and survival. Dominant bears removing competitors helps ensure their own reproductive success and fitness. However, excessive cub mortality from intraspecific killing can lead to population declines over time, highlighting the need for abundant resources and habitat to support multiple bears.
Infanticide
Infanticide, the killing of dependent offspring by adults, is one of the leading causes of mortality for grizzly bear cubs. Male bears that takeover a female’s territory will often kill any existing cubs under 1 year old. This brings the female back into estrus so the new male can breed with her himself.
Infanticide is most common when:
- A resident male dies or is driven off, leaving cubs defenseless.
- A female’s range overlaps with multiple males.
- A younger male challenges an older bear for breeding access to a female.
Up to 68% of cub mortality has been attributed to infanticide in some populations. Younger females are more likely to lose cubs this way since they have less experience defending against males.
While infanticide allows males to propagate their own genes, excessive rates can lead to population declines. Having abundant habitat and food resources helps provide refugia for females to hide vulnerable cubs. Minimizing human disturbances that lead to increased male turnover is also important for reducing infanticide risk.
Conflicts with Other Bears
Though grizzly bears are solitary animals for most of the year, their home ranges often overlap with other bears. This can lead to deadly territorial conflicts, particularly between males competing for mates.
Confrontations typically peak during breeding season from May through July. However, clashes may occur any time of year when bears spar over food sources or encounter each other in disputed habitat. Fighting is most frequent as bear densities increase and space becomes limited.
These battles can be brutal, involving biting, clawing, and powerful blows that can prove fatal. Mortality from wounds or being pinned down and killed is most common among younger bears lacking fighting experience. However, even males in their prime can meet their demise when confronting an older, larger bear.
In some cases, conflicts arise when black bears and grizzlies encounter each other while foraging. Though less aggressive, black bears can injure or kill grizzly cubs they perceive as a threat to food supplies. Minimizing overlap between these species reduces potentially deadly interactions.
While territorial disputes help establish dominance, excessive bear mortality from fighting threatens overall population stability. Careful conservation planning is needed to provide ample habitat and prevent overcrowding.
Hunting
Legal hunting is responsible for hundreds of grizzly bear mortalities annually. While exact numbers vary between regions,humans now kill more grizzlies than all other causes combined in many areas.
Grizzly hunting is allowed in Canada and Alaska, along with Idaho, Montana, and Wyoming in the contiguous U.S. Typical regulations include:
- Limited permits or quotas to restrict total kills.
- No hunting of females with cubs.
- No use of bait to attract bears.
Supporters argue regulated hunting provides incentives for conservation, while controlling populations near human settlements. Critics claim it threatens recovery for this vulnerable species.
The impact of hunting depends largely on how it’s managed. Overhunting contributed to near extinction of grizzlies in the lower 48 states prior to 1975 protections. Since recovery, limited hunting has had minimal impact on established populations like Yellowstone. However, scientists warn excessive quotas could reverse conservation gains, especially in isolated ecosystems.
Balancing hunting policies with grizzly population trends remains politically controversial. Yet cooperation between wildlife managers and all stakeholders will be key to ensuring the species’ long-term survival.
Vehicle Collisions
With their immense size and strength, it’s hard to imagine much can take a grizzly bear down. However, vehicular collisions are an ever-increasing threat, capable of delivering deadly trauma to these animals.
Grizzlies follow seasonal food sources like salmon runs that often lead them across roads and highways. Curious bears may also be drawn to roadways while exploring new territory as adolescents, or when pushed by other bears from prime habitat.
Bears can be hard to spot crossing dark pavement at night. Fast-moving vehicles leave little time for either party to react or stop once encountered. Estimates indicate over 100 grizzlies are killed by automobiles every year in western states and provinces. This source of mortality is increasing as traffic volumes grow.
Roads pose an even greater barrier to movement for female bears with cubs, further fragmenting habitat. Collisions are more frequent near parks like Glacier and Banff National Parks, where large bear populations live in close proximity to busy roads.
Transportation agencies now use a combination of tactics to reduce collisions:
- Wildlife crossing structures like over/underpasses to provide safe passage.
- Fencing to guide bears to crosswalks.
- Reduced speed limits.
- Improved sight lines along curves.
- Warning signs and flashing lights at migration hot spots.
Avoiding roads as much as possible is the best way for bears to steer clear of deadly collisions. Drivers being alert and limiting speed can also save lives. Ultimately, transportation planning and infrastructure improvements are vital steps toward coexistence.
Deadly Encounters with People
For all their ferocity, human activity remains the most threatening force affecting grizzly bears today. Shootings, captures, and run-ins with people constitute a leading cause of mortality for bears living near civilization.
While attacks on humans are rare, grizzlies may charge when startled or provoked. Their massive size and strength can inflict fatal injuries. People venturing into bear country must stay alert and take precautions like bear spray to avoid confrontations.
More commonly, humansact aggressively and kill bears preemptively in perceived conflicts. An estimated 600 grizzlies are killed by people across North America annually:
- Self-Defense: Hunters or hikers may shoot bears they feel are threatening their safety.
- Protection of Life and Property: Authorities kill bears that repeatedly seek human food sources or prey on livestock.
- Mistaken Identity: Less dangerous black bears get killed when mistaken for grizzlies.
- Poaching: Illegal hunting claims bears outside authorized areas or seasons.
Captures intended to protect or relocate “problem” bears also frequently turn fatal due to accidents, stress, or poor handling. Many bears lose their fear of people due to ample access to human foods like garbage. As human expansion encroaches further into wilderness, deadly run-ins escalate.
Reducing lethal conflicts requires proactive management, like bear-proof food storage, plus education on safety practices. Non-lethal deterrents such as rubber bullets and noisemakers can also discourage bears from residences. Minimizing activities like logging near den sites prevents attracting bears to populated areas. With care and understanding, humans and bears can coexist safely.
Old Age and Natural Causes
While threats from people and the environment pose risks, some lucky grizzlies actually die of old age or natural causes. Grizzly bears can live over 30 years in the wild.
Eventually though, the challenges of aging will claim even the most experienced old boars and sows. Ongoing stress slowly takes a toll, reducing fertility and making it harder to compete with younger bears. Declining mobility due to arthritis also makes capturing prey difficult.
Serious wounds, worn down teeth, blindness, and disease will gradually leave bears vulnerable. Weaker bears must avoid predation from younger rivals and scavengers ready to capitalize on their misfortune. Cubs or struggling bears may succumb to harsh weather, infections, parasites or accidents like falling from a tree.
Reaching an advanced age indicates a bear was skilled at finding food, avoiding people, defending territory, and staying healthy. While dying of old age is a luxury few wild bears get to enjoy, those that do likely met a peaceful end after a long, successful life roaming their wilderness home. Their genes help ensure continued survival of the species.
Conclusion
For the mighty grizzly, few predators present a serious mortality threat under normal conditions besides humans. However, several factors like starvation, aggression from other bears, infanticide, hunting, vehicle collisions, and deadly run-ins with people can kill bears. Even those that avoid these hazards eventually decline from old age or succumb to illness and injury.
Still, grizzly bears demonstrate remarkable resilience and adaptability considering the many perils facing them. Their fate largely depends on the actions of humans sharing the landscape. With mindful conservation policies, coexistence is possible and grizzly bears can continue thriving for generations to come. By understanding what kills these iconic carnivores, we gain perspective into ensuring their continued survival.