Gus, the beloved and famously neurotic polar bear at the Central Park zoo in New York, has died aged 27, the Wildlife Conservation Society said Wednesday. Some 20 million people are thought to have visited Gus during his time at the zoo, which saw him become famous for his obsessive swimming routine, and even become the star of a book based on his life. In 2006 TV-chef Rachel Ray prepared a special meal -- tilapia on a block of ice -- for Gus and his partner Ida, who died in 2011. According to the Wildlife Conservation Society Gus was euthanized on Tuesday while under anesthesia for a medical procedure after a loss of appetite and difficulty eating. \"During the procedure, veterinarians determined Gus had a large, inoperable tumor in his thyroid region,\" read a statement from the conservation group. It said the average life expectancy for a male polar bear in a zoo is 20.7 years. Gus, who was born in Toledo in 1985 and came to the Big Apple in 1988, became the face of the zoo, appearing in many newspapers and advertisements. \"Gus was an icon at the Central Park Zoo and a great source of joy for our visitors and staff,\" said Jim Breheny, WCS Executive Vice President of Zoos and Aquarium. \"He was an important ambassador for his species bringing attention to the problems these bears face in the wild due to a changing environment. \"Polar bears are apex predators - the kings of their domain, but vulnerable in a world affected by climate change brought on by human activity.\" Gus gained fame in 1994 when his obsessive swimming in endless figure eights raised concerns. A special \"enrichment program\" was designed for Gus, providing moveable items for him to manipulate, implementing positive reinforcement training sessions, and having him forage for food to keep his mind and body active and healthy. In 1995 a book called \"What\'s Worrying Gus?\" was published. According to its description on Amazon, the book \"recounts the timeless tale of the quintessential outsider coping with the harsh reality of New York in the \'90s, starring Gus, the lovable and neurotic polar bear of the Central Park Zoo.\"