Do black, brown/grizzly or polar bears ever prey on humans?

Question

Do bears, as opposed to defending cubs, being startled by humans, or trying to get into food, ever decide to prey on humans?

As in stalk, kill and then eat them?

Answer 1

TLDR: While rare, it definitely happens.

In fact the majority of people killed by black bears in North America were killed by aggressive bears not by say surprised bears or mothers defending their cubs.

The study found that 63 people were killed in 59 incidents in Canada, Alaska and the lower 48 states. The researchers determined that the majority (88%) of fatal attacks involved a bear exhibiting predatory behaviour, and 92% of the predatory bears were males. The authors suggest male black bears have evolved some different behaviours than females.

Source

We judged that the bear involved acted as a predator in 88% (49 of 56) of fatal incidents. Adult (n = 23) or subadult (n = 10) male bears were involved in 92% (33 of 36) of fatal predatory incidents, reflecting biological and behavioral differences between male and female bears. That most fatal black bear attacks were predatory and were carried out by 1 bear shows that females with young are not the most dangerous black bears.

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Ken Marsh, a spokesman for the Alaska Department of Fish and Game, said the animal responsible for the attack was an adult male black bear. The carcass of the animal, which mine officials said was shot at the scene, was retrieved.

A preliminary examination of the case, Marsh said, appeared to rule out any sign that the mauling was the result of a surprise encounter or a defensive attack by the bear.

"Initial indications, the information we have, are consistent with a predatory attack," Marsh said.

...

Murphy, the ABR president, said employees told him the fatal mauling involved an "extremely aggressive, predatory black bear."

"The bear appears to have stalked them," Murphy said the employees told him. "It came up from behind them and attacked."

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WEST MILFORD, N.J. — Authorities say the first known fatal bear attack in New Jersey was unprovoked and the bear was acting in a "predatory" manner.

Officials attribute the death of a 22-year-old Rutgers University student from Edison to "mauling."

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Rancher Sevend (Sven) Satre might have been able to provide some answers to these questions if he were still alive. On June 14, 1996, the 53-year-old man was riding his horse, checking on his cattle in the timbered country near the central British Columbia community of Tatlayoko Lake. He never returned. Satre's partially eaten body was located the next morning and an aggressive black bear was dispatched at the gruesome site by the search party. A necropsy performed on the bear proved that indeed it was the man-eater. Interestingly, it was in perfect health.

By all indications, the bear had stalked Satre and his horse for more than half a mile before rushing in for the kill. The horse and rider had only just begun to react to the attack when Satre's saddle rolled after a tight left turn and he was thrown to the ground. Trackers concluded that Satre fought off the predatory black bear for less than 10 seconds before he was overpowered and killed. His axe was found at the site where the one-sided fight took place.

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Grizzly bears will also prey on people,

The man, who isn't being named by conservation officers, was moose calling with a hunting partner along the Smart River in northern B.C. on Wednesday, when a female grizzly attacked him from behind.

Conservation officers say the attack was predatory, meaning the bear wanted to eat the man.

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Predatory attacks by bears are very rare, but do occur. Any bear that continues to approach, follow, disappear and reappear or displays other stalking behaviors is possibly considering you as prey. Bears that attack you in your tent or confront you aggressively in your campsite or cooking area should also be considered a predatory threat.

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Polar bears will also prey on humans

Atwood was a member of a team that combed through nearly 150 years of records of bear attacks in Canada, Greenland, Russia, the US and Norway. They drew their data from government agencies, news reports and, in the older cases, from ships’ logs.

Between 1870 and 2014, they found 73 cases of polar bears attacking a group of people or an individual, with 63 people injured and 20 people dead. Bears were acting in a predatory manner in most attacks, and it was male bears that were more often involved.

Where details were available, the researchers assigned the attacking bear a score reflecting its body condition. It turned out that 61 per cent of these bears were in “below average” condition – a situation Atwood says is down to them not finding as much food because of dwindling time on sea ice, their habitual seal hunting ground.

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The last thing to note is that while people say that polar bears are the only animals that hunt humans, this is not true, since black bears and brown bears will do it as well.