Poor old Ivan, pushing 50, takes a long time to climb into bed at night, and a longer time to get started in the morning.

His knees are stiff, his eyesight is failing, his teeth are falling out and his digestion hasn't been so great.

Still, life is good for this senior citizen at Zoo Atlanta, especially compared to his previous incarnation.

Ivan was captured in the wild and taken from his mother as an infant. He wouldn't see another gorilla for more than 30 years. He was raised as a pet by the E. I. Irwin family in Tacoma, Wash., where he ate hambugers, smoked cigars, and rode in Irwin's hotrod. When he grew too large and strong to control, Irwin built a glass-and-steel cage at his circus-themed Tacoma shopping center, B&I Circus, and put Ivan on display, along with an elephant and a pair of chimpanzees.

Ivan stayed in that solitary box for the next 28 years, a terrible hardship for a social animal. By 1994, public pressure to provide Ivan with a better habitat, plus money troubles, convinced B&I to turn Ivan over to the Woodland Park Zoo in Seattle, which immediately shipped him to Atlanta, where there was room for him to roam.

Today Ivan is among the oldest gorillas in captivity. Four of these gray-hairs live in Zoo Atlanta, including Ozzie, 51, the oldest male gorilla in human care. This quartet of Methuselahs represents the challenges that will face zoos around the country, as their gorillas grow older.

As owners of older pets know, older animals are more labor intensive, said Kristen Lukas of Cleveland's Metroparks Zoo. Lukas is chair of the Species Survival Program for gorillas, a coordinated effort by zoos around the world to improve the odds for this critically endangered species.

Like their human counterparts, older animals often need more medical care and more intervention. Their appetite drops and their nutrition is threatened. (Ivan's keepers regularly boil and soak his food, to make it easier to chew, and supplement his diet with Centrum vitamins.)

Like the keepers in Atlanta, staff at zoos around the country will have their hands full as the gorilla demographic ages. Ten years ago there were 18 gorillas over age 40 in North American zoos, or 5 percent of the captive gorilla population, said Lukas. Today that percentage has doubled: Among the 338 gorillas in human care, 34 are over age 40.

These boomers will tax the resources of zoos, but they can also serve as a kind of eminence grise for the younger set, offering grandparently guidance and a calming hand. "They can be the most steady personality in the group," said Lukas.

"Ivan is my favorite gorilla," said Dr. Hayley Murphy, director of veterinary services at Zoo Atlanta, who keeps a large framed photo of Ivan on her wall. "He's a quirky guy."

When Ivan arrived in Atlanta he followed in the footsteps of the zoo's most famous resident, Willie B., who, like Ivan, had been in solitary all his adult life. After the construction of the Ford African Rain Forest exhibit in 1988, Willie B. had his first chance to walk outside, to feel the sun on his face and the grass under his feet.

Astonishingly, he made the transition from loner to successful silverback, mating with several females and siring a soccer-team-sized tribe of offspring, before passing away in 2000. When former Zoo Atlanta director Terry Maple talks about Willie B.'s funeral service, which drew 8,000 visitors, his voice still catches. "There wasn't a dry eye in the house."

Ivan was another story. He enjoyed moving to his outdoor habitat (though he hated getting his feet wet) and successfully integrated into different social groups, but he never hit it off with the women. "Unfortunately I think he'd been [in solitary] too long before we moved him," said David Towne, retired director of the Woodland Park Zoo, who, along with Maple, was instrumental in getting Ivan out of his shopping center cage. (Ivan remains the property of Woodland Park, but is cared for in Atlanta, by mutual agreement.)

"He never really accepted female gorillas," said Towne. "The first couple they put him with, they tended to pick on him and beat him up, because he didn't know how to deal with them."

Until last October Ivan was living happily with a younger female, Kinyani, though Lori Perkins, vice president of collections, said the two were more like roommates than a couple. "They were living parallel lives."

Ivan recently dropped from 353 to 314 pounds and developed a cough, worrying his keepers. They started him on antibiotics and tried to boost his calorie intake with Clif bars and peanut butter shakes. They plan a coordinated diagnostic session late this month to check for cardiac disease, which is a significant problem for gorillas.

Using general anesthesia to immobilize the ape, a team of 10 or 15 medical personnel, including a cardiologist, an ophthalmologist and a dentist, will check his major systems.

The anesthesia is dangerous for an older animal, and the team will move quickly, finishing within 45 minutes to an hour.

Ivan's colleague Joe, an older gorilla living in the Dewar Wildlife Trust in North Georgia (formerly known as Gorilla Haven), suffered from severe cardiac disease, and passed away earlier this summer, after a similar examination.

Today, ever since Kinyani moved out, Ivan has lived alone, though he can see other social groups around him. He shuffles out of the gorilla building in the mornings, placing burlap sacks down where he walks, to keep his feet dry. ("It's a Sir Walter Raleigh sort of thing," said Perkins.) Then he'll sit in the sun, and enjoy a lunch of prunes, celery, cabbage, peppers, and gorilla chow — and maybe the occasional dose of Metamucil — before spreading a bale or two of hay around to make himself an open air sleeping couch for an afternoon nap.

"He's having peaceful, enjoyable twilight years," said Perkins.

Zoo Atlanta. 9:30 a.m.-5:30 p.m. Mon.-Fri.; 9:30 a.m.-6:30 p.m. Sat. and Sun, $15.99-$20.99; 800 Cherokee Ave. 404-624-9453; www.zooatlanta.org