There's a Komodo dragon with an arthritic knee, a red wolf that's had several root canals and a white tiger with weakening hind limbs. An old sea lion is battling a chronic infection, an elderly gorilla suffers from a bad back and an aging stork has been losing weight.

Parts of the National Zoo these days look like a geriatric ward. Its most celebrated elderly patient, of course, is Hsing-Hsing, the 28-year-old giant panda with irreversible kidney disease. But the infirmities of advanced age are an increasingly common theme at this and other zoos.

Captive animals are living longer, thanks to improved care and nutrition. New medical techniques are extending their lives and making for difficult decisions about when to euthanize. Zoo officials now face the results of their success.

"We're writing the new chapters in geriatric veterinary medicine," said Robert Hoage, a spokesman for the zoo. "We have a number of animals that are getting up there. This presents real challenges."

Elderly animals suffer from many of the same degenerative diseases that humans do--failing organs, arthritis, cancer, cataracts. They can't jump as high as they used to. Many begin to shed weight. Sometimes, whether from illness or age-related orneriness, they are less friendly and cooperative.

The aging of their animals has forced a change in focus for zoos, more accustomed to dealing with the young. National Zoo keepers have added Ensure and fatty cottage cheese to their charges' diets and have bumped up their vitamin dosages. The Los Angeles Zoo is replacing the hard concrete floor of its elephant enclosure with softer material that is easier on old bones. Instead of putting sick animals in out-of-sight enclosures, the Hogle Zoo in Salt Lake City has added signs explaining that certain animals look bad because they are ill.

The availability of high-tech medical care for animals--organ transplants, artificial hips, corneal implants--also gives zoos new options. But for some creatures, the "cure" is as painful as the disease.

The zoo's 45-year-old African elephant, Nancy, has a bone infection in her toe. So far, the infection doesn't hurt, but if it spreads to the tendon, it will. Ultimately, it could cripple or kill her.

Other zoos have tried surgery with similarly afflicted animals, but veterinarian Lucy Spelman rejected it as too invasive. She was concerned that it would cause pain and potentially worsen the infection.

"We're not going to put this elephant through something that has a small chance of working but will make her life painful and horrible," she said.

So Spelman is trying another technique, shooting antibiotics into the animal's bone marrow to fight the infection. The most recent treatment was on Friday.

As keepers positioned themselves around the elephant, Spelman gently slid the needle into Nancy's giant, craggy leg. The leg jerked, and Nancy repeatedly swung her trunk toward her keeper seeking a reassuring biscuit snack.

Nancy is a well-loved animal and has been at the zoo for more than four decades. She is also a three-ton bundle of ill health--an abscess on her sole, fluid buildup in her abdomen and possible kidney and intestinal inflammation. She has lost weight. She is on steroids, two types of antibiotics and a drug to restore her immune system.

Spelman is optimistic that the intravenous antibiotic, begun in July, will work. But to give everyone plenty of time to think about it, she also had a talk with Nancy's keepers about the eventual possibility of euthanasia. Any decision about putting an animal down is made by a group that includes keepers and Spelman. (In Hsing-Hsing's case, it also would involve highest-level zoo officials.)

"It isn't easy to make decisions to euthanize," Spelman said. "But I always feel good about it in a small way to be able to put an animal out of its misery."

In a pool out of public view, keeper Linda Moore is hand-feeding tasty fish tails--with vitamin pills stuffed inside--to a pair of 22-year-old sea lions. Maureen and Esther have outlived their life span in the wild and eat about half the amount of fish they did several years ago. The nutritional supplements make up for that.

Many other elderly animals have to be tricked into taking their vitamins. Moore stuffs a cartilage-protection supplement into a dead chick that she feeds Cherokee, an ailing red wolf. A geriatric deer happily scarfed down medicated apples coated with peanut butter until it had to be euthanized this year. Hsing-Hsing happily downs his daily arthritis pill, but only because it is buried inside his favorite blueberry muffin treat.

One reason for hand-feeding is that it lets the keeper know whether an animal has lost its appetite, often a sign of ill health. Animals frequently hide illness because in the wild it could cause a predator--or a member of their own species--to attack them.

The sea lions snap up the fish that Moore offers. But Esther is reluctant to come out of the pool to be weighed. It takes Moore several attempts to lure her out with a fish snack.

"I'd always heard from people who train sea lions that, as they get older, they give you a hard time," Moore said. "I thought, noooo. But--yes."

Elderly sea lions also "certainly slow down with age," she said. "We used to have sea lions who would go up on the huge rocks and do flips. We try to adjust our training to them getting older."

When the sea lions see Spelman coming, they flee. Both have extensive medical histories and don't like needles.

Esther has acute muscle inflammation and is being treated with steroids. Maureen has a chronic infection that began as a bite wound. Spelman has put her under anesthesia a dozen times to drain the wound, using a technique she developed herself.

Treatment has not been cheap. Maureen's drugs alone cost $1,000 a month for several months.

"We had a big discussion about her value and how long she has been here," Spelman said. "The director's office helped" with the cost.

A year ago, Maureen could barely eat. Now, she fights Esther for food again.

"It's an incredible change," Moore said. "She's back to being her normal self."

At the ape enclosure, Mesou sits on the straw in her cage, leaning against the wall for support. At 44, the gorilla equivalent of a human's 90 years, she has severe arthritis in her vertebrae and can barely bend her back--a "common theme" in older mammals, Spelman said.

"When you see Mesou's back, it looks a lot like my father's back," she said. "It's a disaster."

Spelman tried over-the-counter painkillers: ibuprofen, acetaminophen, aspirin. They did some good, but not enough. And Mesou kept throwing up, which Spelman countered by giving the animal ginger bitters, although she usually shies away from such forms of alternative medicine.

A month ago, Spelman put the gorilla on a new anti-inflammatory medication for humans called Celebrex. Bingo. Mesou also is on anti-ulcer medication and regular antibiotic treatment for gum disease. Her drug bill totals several hundred dollars a month.

After decades of raising obese apes, zoos now serve more health-conscious diets. But because Mesou is losing weight, her keepers indulge her with Ensure bars and spoon-feed her fatty cottage cheese.

Like most zoos, the National Zoo gives annual physical examinations to big mammals. With most other creatures, they wait until a problem crops up.

But Spelman is considering expanding the annual exam list. Last week, she stopped by the bird area to check on a relatively elderly stork that had come to the National Zoo in 1972. The bird was not eating well, so Spelman wanted a blood sample.

Two keepers and a summer intern restrained the stork, one grasping the wings, another the head, a third the legs. Spelman wiped the bird's leg with a cloth dipped in alcohol, then stuck the needle in to retrieve blood.

"Some of the cranes we have on the annual exam list," Spelman said. "We probably should add these guys."

"If they insist on living this long," keeper Debi Talbott told her, "we might as well do them."

A growing segment of the National Zoo's animal population is elderly. Maureen, a 22-year-old sea lion, above, and Nancy, a 45-year-old African elephant, are among the animals that have age-related health problems. The oldest include:

Animal Birth year

Ambika, Asian elephant 1948

Arusha, hippopotamus 1952

Nancy, African elephant 1953

Mesou, gorilla 1955

Aldabra tortoises (2) 1956

Cuban crocodile 1958

Flamingo 1964

Sarus crane 1964

African pancake tortoise 1965

Stanley crane 1966

SOURCE: National Zoo; all birth dates are estimates