One might say he died of a broken heart, because as humans, for whatever reason, it soothes our existential dread to anthropomorphize nature's simpler creatures. The Wildlife Conservation Society has the more scientific but still sad story: "Gus was euthanized yesterday while under anesthesia for a medical procedure conducted by WCS veterinarians. Gus had been exhibiting abnormal feeding behavior with low appetite and difficulty chewing and swallowing his food. During the procedure, veterinarians determined Gus had a large, inoperable tumor in his thyroid region."
He was 27, old for a zoo bear; still, Gus hasn't been the same since his companion of 24 years, Ida, died in 2011.
In the New York Times column "The Lonely Polar Bear" after Ida's passing, Diane Ackerman wrote:
My heart goes out to Gus, the famously neurotic polar bear in the Central Park Zoo, who used to swim endless laps around his pool. He’d dive to the bottom in a froth of bubbles, surge across and then surface like a bear obsessed. He’d backstroke to the other side, and with great paws splashing, dive down to the bottom and circle around again. Some wags called him the “bipolar bear,” but most zoo-goers sensed that he felt bored, pent-up, out of his element and depressed. [...]
But when Ida died recently from liver disease at the age of 25, Gus grew listless, slouching around his habitat and swimming little, obviously confused and greatly disturbed by her disappearance. [...] Gus probably misses Ida’s familiar scent, since polar bears are wizards of smell who will march over ice ridges for 40 miles to reach prey they’ve whiffed, can tell which way a human went 14 hours before, and follow a breeding female simply by sniffing her tracks.
Here he is in happier times, or so we'll tell ourselves to feel better about our own lives: