The slow shall inherit the earth

Why are sloths the darling of the internet? Simon Usborne studies the science of our obsession with furry animals

For animals so well known for their indolence, sloths have marched into the public consciousness with impressive speed. The tree-hugging mammals have wrapped their furry arms around the internet, revealing, via videos that gain millions of views, surprising truths (they're really quick at sex, for example) and an unstoppable human obsession with cute creatures that scientists say works like a drug addiction.

Kristen Bell appeared to be on something when she appeared on the Ellen Degeneres show in January. The Forgetting Sarah Marshall actor recalled finding out about a meeting with a sloth her boyfriend had organised for her birthday: "My entire life I had been waiting for this moment. And I started to have a fully fledged panic attack." A home video showed Bell curled up on her bed, apparently disabled by her love of sloths.

Bell's sloth meltdown has been viewed 10 million times on YouTube, ranking it alongside the videos that have helped fuel her obsession. For this we have a British zoologist and filmmaker to blame. Inspired by early clips on YouTube, Lucy Cooke travelled to the world's only sloth sanctuary, in Costa Rica. She filmed sloths falling off things, crawling over things, cuddling things, and looking generally cute. When the videos went viral, Cooke got a film deal. Meet The Sloths was aired in the US late last year, and will be shown on the Animal Planet channel on Sunday evening.

The star of Cooke's film is arguably Randy, a libidinous male who is eventually released from the sanctuary, which has cared for injured and orphaned sloths for more than 20 years, after recovering from a broken arm. To his dismay, a love rival has moved in on his patch. The rival is filmed mating, the first time the act has been caught on camera. It takes six seconds.

Sloths are full of surprises, it turns out. Some studies show they sleep for as few as 10 hours a day, while others insist they are highly evolved. Cooke, who studied zoology under Richard Dawkins at Oxford, says: "The secret of their success is their slothfulness. Their diet doesn't include many nutrients and is mildly poisonous so a high metabolism would be dangerous. Their slow movement also goes under the radar of their main predator, the harpy eagle."

There is hard science behind our love of cute creatures. Austrian zoologist Konrad Lorenz argues that a caring instinct means humans have evolved to react positively to animals that resemble babies. Other research has shown that images of cute creatures stimulate the same parts of the brain activated by drugs such as cocaine. Cooke calls her website: "The home of grade A sloth-based cute crack."

Our tendency towards anthropomorphism troubles some naturalists. But Cooke says she is primarily concerned with the welfare of a species whose habitat is in peril. If advertising cuteness and appealing to the baser instincts of people like Bell is a way to increase awareness, Cooke argues, it's worth it. "I'm hoping the publicity will mean more money is spent studying sloths and on conservation," she says.

In 2010, a South African study showed how even rational scientists are not immune to the lure of cute. Papers about "charismatic mega-fauna" such as pandas or meerkats far outnumbered those of less glamourous species. Nicky Jago, a senior keeper at London Zoo, says the zoo's sloths have never been more popular. "Mole rats don't get the same press," she says, adding: "They're less aesthetically pleasing but deserving and fascinating creatures."