A grown brown bear sits at the top of nearly every food chain it lives in, and almost no animal hunts a healthy adult. The animal that kills the most brown bears isn’t a predator at all: in many populations, most deaths trace back to people.1Journal for Nature Conservation. Characteristics of natural and anthropogenic mortality of an endangered brown bear population The animals that do eat brown bears mostly arrive after death, as scavengers and decomposers.2Wildlife Biology. Brown bear Ursus arctos scavenging patterns
The Few Animals That Hunt Brown Bears
The one predator that hunts bears regularly is the Amur tiger. In the Russian Far East, tigers kill both brown bears and Asiatic black bears.3Biology Bulletin. Interspecific Relationships between the Amur Tiger (Panthera tigris altaica) and Brown (Ursus arctos) and Asiatic Black (Ursus thibetanus) Bears Bears made up about 2 of every 100 tiger kills found on the ground, and bear remains turned up in about 8 of every 100 tiger droppings examined, which shows bears are a regular part of the diet there.4Biology Bulletin. Interspecific Relationships between the Amur Tiger (Panthera tigris altaica) and Brown (Ursus arctos) and Asiatic Black (Ursus thibetanus) Bears
A tiger can take a bear where a wolf rarely does for a few reasons. It ambushes alone from cover and kills with a bite to the neck. It is also close to a bear’s weight, which a single wolf is not. Tigers probably favor smaller, younger bears over big males, and even then a bear sometimes fights back or steals the tiger’s kill.
Elsewhere, nothing routinely hunts a grown brown bear. Its size, claws and strength usually make it too costly a target, because brown bears often stand and fight rather than run.5Natural Habitat Adventures. Brown Bear Facts | Alaska Wildlife Guide A kill only pays if the meal outweighs the risk of injury, and a wound that stops a predator from hunting can mean slow starvation. Even a pack has little to gain by testing a healthy adult, which is why predators near bears usually go after elk, deer or moose, far less dangerous animals.
Wolves and bears mostly meet at carcasses rather than hunting each other. A pack may harass a bear, or a sow with cubs, but a lone bear frequently drives wolves off a kill. Deaths happen on both sides, though they are uncommon. Still, the idea that nothing ever kills a brown bear is a myth: cubs are vulnerable, and old, injured or starving bears lose their advantage.
Dangers to Cubs, Including Other Bears
Cub survival can be higher than many people expect. In one hunted grizzly population, about 93 to 95 of every 100 cubs survived the spring mating season, and about 85 of every 100 survived the full year.6BioOne (Ursus). Sexually selected infanticide in grizzly bears: the effects of hunting on cub survival Still, around 15 of every 100 litters followed for a year were lost entirely.7BioOne (Ursus). Sexually selected infanticide in grizzly bears: the effects of hunting on cub survival
Across that first year, the dangers come from several directions. Adult males are a threat, especially in spring when bears gather and mate. Wolves or big cats can catch a cub that strays from its mother. Starvation takes cubs when the mother is in poor condition, and accidents such as falls or drowning at rivers claim others.
Male bears are widely suspected of killing cubs so the mother will mate again, but in that population the evidence did not show this clearly.8BioOne (Ursus). Sexually selected infanticide in grizzly bears: the effects of hunting on cub survival Proving it requires knowing which male killed which cubs, and that is very hard to observe in wild bears that roam over wide areas. A lost litter can also come from starvation or accidents, so researchers cannot assume every missing cub was killed by a male.
Mothers are the cubs’ main defense. A mother brown bear defends her cubs with force rather than fleeing.9Natural Habitat Adventures. Brown Bear Facts | Alaska Wildlife Guide She sends them up trees, which adult males climb poorly, and she avoids areas used by big males. If she has to, she will charge intruders much larger than herself.
Yearlings remain at risk after they leave her side. They must find food and space alone, and they often wander toward roads and towns, where people become the danger.
Humans Are the Biggest Threat
In one endangered brown bear population, 84 of every 100 deaths were linked to people.10Journal for Nature Conservation. Characteristics of natural and anthropogenic mortality of an endangered brown bear population Being shot was the most common single cause, at about 59 of every 100 deaths, and collisions with vehicles came next, at about 19 of every 100.11Journal for Nature Conservation. Characteristics of natural and anthropogenic mortality of an endangered brown bear population In Japan’s Shiretoko Peninsula, bears are killed in conflicts even near protected land, and unreported killing may be substantial.12Ursus. Modeling the effects of human-caused mortality on the brown bear population on the Shiretoko Peninsula, Hokkaido, Japan
“Linked to people” includes legal hunting, poaching, killings in defense of life or property, and vehicle or train strikes. The categories overlap with other causes, such as a bear hit by a car while crossing a road near a town where it was seeking food. Because these figures come from specific studied groups, they show how heavy human impact can be, not a number that applies to every brown bear.
Roads, railways and garbage drive deaths in some valleys. Among collared grizzlies in British Columbia’s Elk Valley, 12 of 14 deaths came from conflict with people or road and rail collisions, so only one confirmed death in that group was natural.13Conservation Science and Practice. Unsecured attractants, collisions, and high mortality strain coexistence between grizzly bears and people in the Elk Valley, southeast British Columbia Death rates there ran about ten times higher than elsewhere in the province.14Conservation Science and Practice. Unsecured attractants, collisions, and high mortality strain coexistence between grizzly bears and people in the Elk Valley, southeast British Columbia
Spilled grain on tracks draws bears, and bears also walk rail lines as easy travel routes. Trains cannot stop quickly, and the deaths happen far from witnesses.
Deaths away from witnesses are part of why official counts fall short. Researchers can only count deaths they find, and a bear struck by a car may wander off and die out of sight. Collision deaths are often never reported,15Conservation Science and Practice. Unsecured attractants, collisions, and high mortality strain coexistence between grizzly bears and people in the Elk Valley, southeast British Columbia and poached bears are rarely reported either. Collared bears help, because a transmitter signals when an animal stops moving, which lets researchers find carcasses that would otherwise go unnoticed. The true toll is probably higher than the tallies, so reported numbers work best as a minimum.
Many conflict kills follow the same path. A bear finds easy food like trash or livestock feed, returns, and loses its fear of people. Eventually it damages property or threatens someone, and wildlife officers or residents kill it. Bears drawn to food near people are the ones most often shot in conflicts.16Conservation Science and Practice. Unsecured attractants, collisions, and high mortality strain coexistence between grizzly bears and people in the Elk Valley, southeast British Columbia
People nearby can break that chain. Securing garbage, pet food, bird feeders and fruit keeps bears from learning to visit homes. Driving slowly at dawn and dusk in bear country lowers the chance of a collision.
Brown Bears Versus Polar Bears and Other Rivals
On an Alaskan beach where whalers leave bowhead whale remains, grizzlies and polar bears feed side by side in autumn.17Journal of Mammalogy. Polar bear–grizzly bear interactions during the autumn open-water period in Alaska Across 137 encounters, polar bears usually acted submissive, though grizzlies seldom acted aggressive.18Eye on the Arctic. Small grizzlies dominate bigger polar bears: study Even so, grizzlies fully pushed polar bears off the bone pile in about half of encounters.19Eye on the Arctic. Small grizzlies dominate bigger polar bears: study They held the upper hand even though they are smaller and fewer there.20Journal of Mammalogy. Polar bear–grizzly bear interactions during the autumn open-water period in Alaska Polar bears may avoid fights to save energy and avoid injury, and confidence and behavior can matter more than raw size. This reflects competition over food, not a fight to the death, and it doesn’t mean grizzlies would beat polar bears everywhere, since the data come from one site.
Disease, Parasites and Hunger
Parasites are common in bears but usually mild. In Alaskan brown bears, hookworm eggs showed up in about 10 of every 100 droppings, and roundworm and tapeworm eggs appeared less often.21U.S. National Park Service. Parasites in Alaska Brown Bears (U.S. National Park Service) About 10 of every 100 bears tested had been exposed to the mite that causes mange, but blood parasites and certain bacteria were not found in any bear tested.22U.S. National Park Service. Parasites in Alaska Brown Bears (U.S. National Park Service)
Finding hookworm eggs shows a bear carries the parasite, but it doesn’t say how sick that bear is. Mange exposure shows contact with the mite, not an active, serious case. Mites burrow into skin and cause hair loss, and badly affected bears lose insulation and condition, but most exposed bears show no serious illness.
Food supply shapes survival more. Salmon made up over 60% of Kodiak brown bears’ diet in good years, but in a year with 28% fewer salmon, salmon’s share dropped to 44%.23Elsevier / Global Ecology and Conservation. Phenological overlap of terrestrial and marine food resources did not reduce salmon consumption by Kodiak brown bears Future salmon declines from climate change could hurt coastal brown bears.24Elsevier / Global Ecology and Conservation. Phenological overlap of terrestrial and marine food resources did not reduce salmon consumption by Kodiak brown bears
Hunger turns deadly in several ways. A bear must store enough fat to last the winter in the den, and a thin bear may not survive hibernation. Hungry bears roam farther and run into people more, and mothers in poor condition may lose cubs.
How a Warming Climate Adds Risk
Warmer weather can delay when bears enter dens, so hibernation gets shorter.25PubMed Central. Drivers of hibernation in the brown bear Warm dens also raise a bear’s metabolism, using up stored fat faster.26PubMed Central. The seasonal sensitivity of brown bear denning phenology in response to climatic variability Bears that wake early may find deep snow and little food, and cubs may leave the den in worse condition.27PubMed Central. The seasonal sensitivity of brown bear denning phenology in response to climatic variability A mother who emerges thin has less milk and less strength to protect her cubs.
The period before den entry is when bears injure people most.28PubMed Central. Drivers of hibernation in the brown bear A longer pre-den season and warm springs can raise conflicts, which often end with bears killed.29PubMed Central. The seasonal sensitivity of brown bear denning phenology in response to climatic variability
What Eats a Brown Bear After It Dies
Ravens and golden eagles are the main birds that skin and clean bear carcasses.30Wildlife Biology. Brown bear Ursus arctos scavenging patterns Foxes, wolves and other bears may feed on one too, and big bears will eat dead bears they find. Bones can lie for years as smaller animals gnaw them.
Fungi, bacteria and insects then break down what remains, so the bear’s nutrients end up in the soil.31Montana Fish, Wildlife & Parks. Grizzlies Bears of Montana Beetles and fly larvae work through the flesh and hide first. Fungi and bacteria handle the tougher material, releasing nutrients into the ground beneath. Plants nearby take them up through their roots, which is how a dead bear feeds the living forest.
Bears feed the land even while alive. Where bears eat salmon by streams, soil ammonium triples, though the effect fades within a year once bears leave.32Springer / Ecological Research. Large predators and biogeochemical hotspots: brown bear (Ursus arctos) predation on salmon alters nitrogen cycling in riparian soils Salmon remains dragged ashore fertilize streamside plants and create small nutrient-rich patches that support insects and plants for months.