Polar Bear Ursus maritimus is the largest land predator on earth. It is a huge carnivorous bear that averages larger than Brown Bear. Adult males weigh 350-75 kg (770-1500 lbs) while an adult female is about half that size. The largest recorded male was shot in northwest Alaska in 1960, weighed 1002 kg (2209 lb) and stood 3.4m (11 ft) tall when mounted on its hindlegs (Wood 1973).

Despite the claim of “largest land predator,” Polar Bears can be called "marine mammals." Much of their life is spent on the sea-ice and they swim readily and easily. They routinely cross large bodies of ocean and have been seen in the Arctic Ocean 300km (200 mi) from land. They concentrate at the edge of the polar ice-pack, where their prey [primarily Ringed Seals Pusa hispida and Bearded Seals Erignathus barbatus] is most abundant.

For the most part, humans are few within the Arctic range of Polar Bear, and deadly confrontations are few. Humans kill far more bears than bears kill people. Yet, Polar Bear is a very dangerous animal. Bears are fearless towards people; they are very curious and are unpredictable in temperament; and in the right circumstances one will stalk, kill, and eat a human as prey. Many attacks by Brown Bears are the result of surprising the bear, but this is not the case with Polar Bears. These alpha predators are stealth hunters and a victim is often unaware of the bear’s presence until the attack is underway. Whereas Brown Bears often maul a person and then leave, a Polar Bear attack is more likely to be predatory and is almost always fatal; Stirling (1988). Yet, man is not a preferred prey and attacks may be by hungry bears in unusual circumstances.

While Polar Bears have attacked large mammals like Svalbard Reindeer or even Musk-ox, their biology requires a large amount of fat from marine mammals; there are insufficient calories in terrestrial food; Ramsey & Hobson (1991), Ovsyanikov 1996. Humans are at a huge disadvantage in the barren and frozen habitat of Polar Bears. A bear can smell a seal a mile away and in a den buried under 3 feet of snow. It can smell a human a long way off. It has good hearing and excellent vision. It knows how to blend into the habitat through cunning and stealth. A bear can outrun you. Man’s only significant defense is a gun. In places like Svalbard, north of Norway, people are warned at the edge of each town that this is Polar Bear country – enter at your own risk. Even wildlife tours to Arctic Svalbard routinely have leaders carrying guns at all times – and posted as look-outs – any time tourists are permitted off ships and come ashore.

Brown Bear Ursus arctos is the alpha predator across the forested Holarctic. There are several subspecies but their limits are subject to debate. Genetically, it appears that in the Old World there are two clades within the nominate subspecies, and others in Central Asia. In North America, there are 3 or 4 clades (including a California clade that is extinct); Garshelis 2009). Within the subspecies currently called Ursus arctos horribilis, two groups are generally recognized—the coastal Brown Bear and the inland Grizzly Bear -- these two types broadly define the range of sizes of all brown bear groupings. An adult Grizzly living inland in Yukon may weigh as little as 80 kg (180 lb) while an adult coastal Brown Bear living on a steady diet of spawning salmon may weigh as much as 680 kg (1500 lb). Obviously it is these huge bears that appear to be the most scary. Yet, the abundance of salmon, especially during the summer spawn, focuses the attention of these bears and their interactions with people at such places are mostly without incident. One of those locales is Brooks Falls, Katmai NP, Alaska, where these photos were taken.

It seems in the incidence of bear attacks is primarily inland, perhaps due to limited food and hungry bears, or perhaps because it is easier to come unexpectedly upon a bear inland. Brown Bears do not take well to surprise. Fatal attacks by Brown Bears in North America average about two per year (Herrero 2002). The Alaska Science Center (Smith & Herrero 2008) ranks the following as the most likely reasons for bear attacks: surprise, curiosity, protection of young, predatory intent, wounded hunted bear, carcass defense, and provoked charge. Unlike smaller Black Bears Ursus americanus that can climb trees, adult Brown Bears respond to danger by standing their ground and warding off their attackers. Mothers defending cubs are the most prone to attack, being responsible for 70% of Brown Bear-caused human fatalities in North America (Herrero 2002).

A wounded bear is a dangerous bear. In 1915, a gigantic male known as "Kesagake" visited the village of Sankebetsu, Hokkaido, Japan, to feed on harvested corn. The bear was shot by two villagers and fled to the mountains, injured. On 9 Dec 1915, Kesagake showed up again, killing 6 people in two days, eating some and stashing others away. A famed bear hunter finally shot "Kesagake" on 14 Dec. The bear was almost 3m (nearly 10 ft.) tall and weighed 380 kg (837 lb). Human remains were found in his stomach. The region was abandoned by villagers for fear of more bear attacks, and it became a ghost town. Even today, the Sankebetsu incident remains the worst animal attack in the history of Japan. Kimura Moritake (1994) published a book on the incident, and there are Japanese movies and plays based upon the story.

Given all these concerns, one does not wish to come upon a Brown Bear unexpectedly, or get between a mother and cub. However, like many land predators, humans are more a threat to bears than they are to us. Quammen's Monster of God chapter on bears focused on Brown Bears in Romania; the purpose of bear reserves there [i.e., as hunting locales for the rich and famous, and most infamously, dictator Nocolae Ceausescu; the wardens relationship with the bears and the dictator; and what happened to all of them.... great reading].