(CNN) -- What's white and black, and has people fawning all over? Meet Siku, a polar bear cub born in captivity in Denmark.





The cute cub is being reared by handlers at the Scandinavian Wildlife Park because his mother isn't producing the milk to feed him. Since his birth last month, Siku has become a Web star. He has his own page on the park's website and he's even on Facebook.





Park director Frank Vigh-Larsen says he's stunned by the cub's rapid transformation into an Internet sensation.





Video footage posted on YouTube of Siku bottle-feeding, rolling around and snoozing has been viewed more than 2 million times in six days, he told CNN, and the cub gained thousands of friends on Facebook within the space of a few hours.





As of Wednesday, at 36 days old, Siku has just opened his eyes and weighs in at 4.2 kilograms (9.2 pounds), more than five times his initial weight, his handler said.





"He's just a little solid cannonball," Vigh-Larsen said. "He's doing well."





Such growth is a testament to the dedicated care Siku is receiving. For the first three weeks, Vigh-Larsen fed the cub every two hours -- and still feeds him every three hours, meaning the keeper gets little sleep.





In the new year, two other wildlife keepers will start to care for him, too, in shifts, making sure the cub is never alone during the first 12 months of his life.





Siku's popularity comes at a time when concern over the plight of polar bears is at an peak. Corporate giant Coca-Cola recently launched a campaign in conjunction with the World Wildlife Fund to raise awareness about the threat of climate change to polar bears' Arctic habitat.





And Vigh-Larsen is determined Siku -- whose name means "sea ice" in Greenlandic, an Eskimo language spoken in Greenland -- will thrive so he can play a role in highlighting the risk. Polar bears rely on sea ice in the Arctic to hunt.





"Siku is going to be an ambassador for polar bears, for global warming," he said.





Vigh-Larsen draws a contrast with the last polar bear cub to become an Internet star, Germany's Knut. That bear died suddenly at age 4 last March at Zoo Berlin, to the sorrow of many worldwide who had followed the cub's progress after he was abandoned by his mother and hand-raised by a zookeeper.





The case of Knut was a disaster, Vigh-Larsen said, "because he was only ever used for selling tickets and teddy bears."





Siku will have a different mission, he says: using his cuteness to persuade people to listen to warnings about climate change and take steps to protect their planet.





It could prove a powerful weapon. The cub has already appeared on front pages of newspapers around the world and graced international TV news bulletins.





New pictures will be posted on the Wildlife Park's website Thursday, Vigh-Larsen said, with the pictures likely to win him even more fans.





Vigh-Larsen also hopes that the facilities the huge park can offer will make it possible for Siku to join its four other polar bears -- including Siku's mother -- as a normal adult male in three or four years' time, despite being bottle-reared.





This is doubly important because of a shortage of polar bears for breeding programs in captivity -- set to become ever more important as the numbers in the wild drop.





Scientists believe there could be as few as 25,000 polar bears left in the world, according to Polar Bears International, a conservation group. The animals are only found in five nations: the United States (Alaska), Russia, Norway, Canada and Greenland, the group said.





In 2008, the U.S. Fish and Wildlife Service listed the polar bear as threatened under the Endangered Species Act.





Without action, Vigh-Larsen says, research suggests that there could be no polar bears at all in the wild within 40 years. He hopes
What bears rely on sea ice What do polar bears rely on to hunt? Who is the park director? What handler says the cub is growing What does Vigh-Larsen want Siku to highlight? What do polar bears rely on sea ice for? Who is being bottle-reared?
Polar sea ice Frank Vigh-Larsen Frank Vigh-Larsen persuade people to listen to warnings about climate change to hunt. Siku
question: What bears rely on sea ice, answer: Polar | question: What do polar bears rely on to hunt?, answer: sea ice | question: Who is the park director?, answer: Frank Vigh-Larsen | question: What handler says the cub is growing, answer: Frank Vigh-Larsen | question: What does Vigh-Larsen want Siku to highlight?, answer: persuade people to listen to warnings about climate change | question: What do polar bears rely on sea ice for?, answer: to hunt. | question: Who is being bottle-reared?, answer: Siku