Born in captivity at the Berlin Zoo in 2006, Knut became an international sensation after he was hand-raised by zookeeper Thomas Dörflein

NEED TO KNOW

• Born in captivity at the Berlin Zoo in 2006 and abandoned by his mother, Knut the polar bear became an international sensation after he was hand-raised by zookeeper Thomas Dörflein
• The bear cub became a symbol of climate change awareness and earned titles like “the world’s cutest polar bear”
• Researchers later identified a rare autoimmune brain disease that led to his death at age 4

When a German zoo welcomed a polar bear cub in 2006, zookeepers never expected him to draw international media attention—or that his death several years later would become a mystery that briefly baffled the scientific community.

Knut the polar bear was born on December 5, 2006, at the Berlin Zoological Garden to mother Tosca, who rejected him (and his brother cub, who lived only four days) at birth. Zookeeper Thomas Dörflein quickly began hand-raising the abandoned cub, keeping him in a warm incubator for 44 days and bottle-feeding him formula, the BBC reported in 2011.

One children’s book published in 2007 about Knut’s young life, Knut: How One Little Polar Bear Captivated the World, described Dörflein as giving the cub round-the-clock care, feeding him every two hours, bathing him, rubbing him with baby oil, and even playing him songs on the guitar.

Dörflein’s extreme dedication to Knut’s (pronounced K’-noot) life quickly drew media attention in Germany, helped by the fact that the cub was the first polar bear to have been born at the Berlin Zoo and survive for over 30 years.

By March 2007, the zookeepers decided it was time to debut Knut to the public, and the cub quickly became a fixture at the Berlin Zoo.

The park’s officials said that after Knut’s introduction, average attendance had doubled, and more than 800 Knut stuffed animals were sold every day, Al Jazeera reported at the time. In 2008, a movie, Knut and His Friends, opened in theaters across Germany, and he quickly earned titles like “the world’s cutest polar bear” from media publications and fans alike.

Knut eventually began to play a political role for environmentalists and zoo officials, too. The German government chose to use Knut as the face of its campaign against climate change, as environmentalists argued that anyone who got to see a polar bear like Knut up close could be moved to advocate for the planet and its wildlife. In May 2007, Knut even appeared on the cover of Vanity Fair next to Leonardo DiCaprio for the magazine’s Green Issue.

“In a world where polar bear habitats are rapidly shrinking, Knut reminds us just what we are at risk of losing,” Craig Hatkoff wrote in How One Little Polar Bear Captivated the World. “He makes it clear that we all breathe the same air at one point or another and encourages us to respect all habitats.”

The bear’s life in captivity wasn’t without controversy. PETA’s German branch repeatedly raised concerns about him being hand-reared and having such a media-driven life.

“We have been after this zoo ever since they made Knut a tourist attraction to his detriment,” PETA President Ingrid Newkirk said in 2011, per the Associated Press. “When he was small, cuddly and fuzzy, he was cute, and everybody loved him. But then he grew and became boring and everyone lost attention. He was not hand-fed anymore or babied and loved. He was put in an enclosure about 1 millionth of the space he would be in nature.”

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On March 19, 2011, Knut died from a seizure caused by encephalitis, or a swelling of the brain. Smithsonian Magazine reported that he shocked guests when he began behaving erratically, spinning in circles, and losing control of one of his shaking back legs. Seconds later, he collapsed, tipping backward into his enclosure’s pool before finally drowning as zoo-goers — and thousands of others who saw the moment after it was captured in a viral video — watched in horror.

Criticisms of the 4-year-old bear’s life in captivity intensified after his death, particularly considering that polar bears can live for up to 20 years in the wild and sometimes longer in captivity.

Speaking with the BBC in 2011, the director of the Oxford Center for Animal Ethics, Andrew Linzey, argued that it would have been better for Knut to have been put down rather than hand-reared by Dörflein (who died of a heart attack in September 2008 at age 44).

“Frankly, it would have been better for Knut not to have existed at all than live such a miserable life,” Linzey told the outlet, echoing critics who said that he suffered problems as a result of his bottle-feeding and the crowds he encountered at the Berlin Zoo.

“Zoos impose unnatural lives on most of their captives,” he argued at the time. “People just see a cuddly bear, and they want to gawk at him, but what they should see is an animal deprived of its natural life, exhibited for entertainment and profit.”

Outside of ethical debates, Knut’s death at such a young age also stirred scientific conversations, as his caretakers, zoo officials, and researchers struggled to identify what caused his encephalitis.

“Knut’s extensive postmortem report indicated that the bear’s encephalitis was so severe that it would have killed him even if he had not fallen into the pool. But critically, the report failed to identify what caused that deadly condition in the first place,” Smithsonian reported previously.

By 2015, the mystery had been solved. German researchers published a report in Scientific Reports announcing that Knut had suffered from anti-NMDA receptor encephalitis. This disease causes the immune system’s antibodies to attack nerve cells in the brain, per Smithsonian.

Although the disease is the most common type of non-infectious encephalitis that affects humans, the researchers’ findings stated that Knut’s death marked the first time the condition had been found in an animal.

“We are relieved to have finally solved the mystery of Knut’s disease, especially as these insights could have practical application,” the research paper’s co-author Alex Greenwood, an expert in wildlife diseases at the Leibniz Institute for Zoo and Wildlife Research, told the magazine at the time. “If the current therapy for human patients is also suitable for wild animals, many cases of fatal encephalitis in zoos may be prevented in the future.”

As of 2026, the Berlin Zoo is home to several bears, none of which are polar bears. The U.K.-based non-profit organization Bear Conservation reports that at least 124 zoos, aquariums and parks (including one circus) are known or believed to be keeping polar bears.

Speaking to the BBC after Knut’s cause of death was found, Greenwood said he believes that the research developments made by examining the bear’s brain mark a pretty fitting end to his story.

“Pretty much every aspect of Knut’s life was played out in the public sphere,” the study’s co-author said. “And reflecting on it now, we’re very happy to reach the point where we can end the story by saying why he died.”

“There’s some closure. Closure for him, but it opens up possibilities for other animals,” Greenwood added. “He will be the trigger for research that may help not just other polar bears but other wild and captured animals as well.”

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