In the last few years, though, the tribulations of age - not just the appearance of it - have begun catching up with Rollie. It wasn't immediately noticeable on the outside. But his keepers are reminded each time they get a look past the Emperor Tamarin's flowing whiskers, and into his jaws.
The tiny monkey, used to crunching away on raw sweet potato and celery, has surrendered all but 6 of his 32 teeth to the toll of time.
At 17, Rollie - a resident of Chicago's Lincoln Park Zoo - is a senior citizen of his species. In the wilds of the Amazon, his keepers say, he almost certainly would never have made it this long.
In captivity, he's got plenty of company.
The Golden Years have arrived at the nation's zoos and aquariums, and that is taking veterinarians and keepers, along with their animals, into a zone of unknowns.
Do female gorillas, now frequently living in to their 40s and 50s, experience menopause?
Can an aging lemur suffer from dementia?
How do you weigh the most difficult choice - between prolonging pain and ending life - when the patient is a venerable jaguar who's been around so long she's come to feel like a member of the family?
All of those questions hang on a larger one that, until recent years, has been left to educated guesswork based on limited evidence.
"How old is geriatric? How old do animals really live?" says Sharon Dewar, a spokeswoman for the Lincoln Park Zoo, whose keepers have adjusted to Rollie's toothlessness by serving him a diet of soft-cooked veggies. "That's the million-dollar question."
Zeroing in on the answer takes years of tracking births, deaths and the age of animal populations. But zoos, which have pooled information on animal births and genealogy since the 1970s, are drawing some early conclusions. For example, records show that the median age of Siberian tigers living in zoos in the two decades ending in 1990 were a little over 11 years old. Since then, however, the median age of those tigers has topped 15 years old.
The increase in animal longevity is no mystery. Just as with people, health care for animals has become much more sophisticated.
At the San Antonio Zoo, keepers noticed that George, a 37-year-old tapir, was slowing down. In the mornings, his legs seemed stiffer, and he had trouble getting up. The diagnosis was clear: arthritis.
At first they put him on dietary supplements. They moved on to Adequan, a prescription that helped ease the discomfort further. Still, wasn't there more they could do? The problem is there's no textbook for how to treat a geriatric tapir.
Reasoning that tapirs are not so different from horses, the zoo called in a specialist who performed acupuncture on George, inserting tiny needles at various medians in an effort to ease the pain.
Since then, George "acts like he's five years younger," says Rob Coke, the zoo's senior staff veterinarian.
Even as San Antonio and other zoos have improved on health care, they've also become much more careful and cooperative in managing animal populations, tracking their animals to make decisions about breeding. Keepers focus on more than just keeping animals healthy, creating habitats and social environments that will make them happy and less-stressed.
The result is more robust animals, with the potential to live longer. That potential is realized because life in a zoo or aquarium grants animals an exception to nature's laws of survival. In the wild, weaker animals fall victim to predators, parasites and poachers before they ever have a chance to grow too old.
"Life as a wild animal is tough," says Steve Feldman of the Association of Zoos and Aquariums.
Without predators, and treated for disease, animals are far outliving their wild counterparts.
At the Minnesota Zoo, a pair of bottlenose dolphins have reached 44 and 42 years old, and in Florida a couple have reached their 50s.
"We know from studying the teeth of animals (dolphins) that have washed up on beaches, in studies I've looked at, that there are no animals that old," says Kevin Willis, an expert on animal life expectancy at the Minneapolis zoo, in the Twin Cities suburb of Apple Valley.
But old age subjects animals to wear and tear and changes in physiology that they would never have known otherwise.
On a recent afternoon at the New York Aquarium, the uncertainties of animal aging are evident in the case of a California sea lion named Fonzie.
For years, he was one of the top performers for the crowds in the stands of the aquarium's amphitheater. But at 21, he's definitely slowing down. He started hobbling. The corneas on his eyes turned cloudy. He lost interest in his trainers. His weight dropped to 552 pounds. Under the X-ray, veterinarians noticed subtle changes in his bone structure.
"You know how it is when you have arthritis and in the winter time your bones creek because it's so damp and cold?" says Kate McClave, who runs the aquarium's onsite hospital. "Well, it's a similar thing for a marine mammal."
To help, vets moved Fonzie to an indoor pool where the water temperature is a closely controlled 55 degrees and he is protected from winter winds, and put him on anti-inflammatories. Nearly three months later, the eggplant-shaped mammal lumbers in to the checkup room with all the grace of a sandbag, his breath fragrant with fish. In exchange for a finned snack, he submits himself to the probe of a stethoscope, a few eye drops, an ultrasound and a look inside his mouth.
"This is one of our few patients that will actually say 'ahhhh'," says Paul Calle, senior veterinarian for the Wildlife Conversation Society, which runs the aquarium.
Careful treatment appears to have eased Fonzie's discomfort and he's ready to rejoin the other sea lions. But his days as a performer are probably over. At the aquarium, his seniority is far from unusual. Immediately after his exam, keepers moved on to take a blood sample from Spook, a 43-year-old gray seal believed to be the oldest on record. Earlier in the week, the aquarium lost a sand tiger shark named Bertha who, at 65, also held an age record.
That longevity confronts zoo managers with mysteries and doubts they've never really had to deal with before.
"The simple question was: 'Does a 41-year-old gorilla need to be on birth control?' And nobody really knew," says Sue Margulis, curator of primates at Lincoln Park.
Years ago, there wouldn't have been much need to consider such a question. Even today, a gorilla that reaches 30 is getting up there. Now, though, the question applies to far more than the one gorilla at nearby Brookfield Zoo that provoked it. When Margulis and a fellow researcher set out to study the possibility of menopause in gorillas, they looked at 30 gorillas in 17 zoos around the country. Of those, 22 are considered geriatric, including one who's now 55.
They found that about a quarter were no longer going through monthly menstrual cycles, while others were in transition. But while gorillas in menopause spent much less time with the male silverbacks, most were quite healthy. In the wild, female gorillas typically leave the group in which they're born. In zoos, older female gorillas stick around, sometimes playing a grandmother role in childcare that is likely unique to captivity.
At the St. Louis Zoo, the uncertainties of aging have keepers wondering about the well-being of Ruffles, a black-and-white ruffed lemur. At 31, he's a sage.
Some of Ruffle's problems are easily identifiable and treatable. He gets an anti-inflammatory pill twice a day - he likes it tucked inside a grape - to combat the pain of spinal arthritis. When blood tests showed he had liver problems, he was put on medication for that, as well.
But there's no easy diagnosis for another symptom. At times, Ruffles seems to be staring off into nowhere.
"Dementia is one of those things that's very difficult to pin down just because we can't use the same sort of testing as we do with humans," says Joe Knobbe, St. Louis' zoological manager of primates.
Ruffles has good days and others that could be better. The best keepers can do is make him comfortable, including installing a tiny hanging platform where the lemur, who no longer climbs like a young primate, enjoys resting with a blanket.
Many zoos have been making similar changes to animal habitat to ease geriatric residents into retirement. At the Arizona-Sonora Desert Museum, a black bear named Spike and his sister Missoula are no longer youngsters. The 22-year-old siblings both have arthritis and Missoula has a problem with inner ear infections that makes it difficult for her to keep her balance. They struggled to climb to their den on the third tier of an exhibit featuring steep, rugged artificial cliffs.
"You start seeing these changes and you realize that if you just let it go, eventually it's going to be a problem where they can't get up there," said Craig Ivanyi of the museum, which is just outside Tucscon. "You realize it's just a matter of time."
So in December, keepers moved to the pair into retirement in a new, specially designed enclosure, with gently graded ramps and a large, sloping pool. Spike and Missoula will spend their lives there, off-exhibit, while the zoo renovates the old enclosure so that when new bears arrive, they will be able to age in place.
Ivanyi says that, even with the bears now too old to be exhibited, the zoo is obligated to take care of them and make them comfortable as long as their quality of life can be assured. The challenge for his institution and others is deciding what to do when quality of life begins to ebb away.
Even in old animals that appear healthy, examination after death often finds they "suffer from a range of health problems that may not have been apparent when they were alive," a group of mostly Swiss veterinarians wrote in an article published last year in the journal Animal Welfare.
"Zoos often unwittingly condemn their animals to long painful lives," wrote the authors, calling on zoos to use a scoring system to evaluate geriatric animals' quality of life in order to make more informed decisions about euthanasia.
Animals don't make diagnosis easy. Their instincts remain rooted in the wild, where survival requires covering up weaknesses and infirmities. But keepers who spend years watching these animals sense when something's wrong.
At the El Paso Zoo, keepers noticed six years ago that Sheba, their regal black jaguar, was faltering. Worsening arthritis made it difficult for her to climb. Her kidneys were failing. Cataracts limited her ability to see.
Keepers fashioned a hammock from old firehose, and hung it low so she could climb in more easily, but even that became difficult. At day's end, Sheba would retire from the exhibit space to be near her keepers as they cleaned up, quietly absorbing the sound of their voices.
But by last fall, as Sheba neared her 27th birthday, it became clear that pain and weakness were winning out. That left the zoo's veterinary staff, managers and keepers with a very difficult choice.
"It's a lot easier to second-guess yourself when you say, well, she probably would've lived four more days, slipping slowly down the slope," said Victoria Milne, the zoo's veterinarian.
They decided not to wait. On Nov. 8, vets anesthetized Sheba, then administered a solution by intravenous drip that, in a few seconds, shut down the big cat's body for good.
Then, as she lay there, keepers, vets and other zoo workers gathered around the cat they'd cared for for 17 years. Some whispered a few words, others reached out to lay a hand on her glossy black coat as they wept.
Like many of the zoo's other geriatric animals, their girl had lived a long, full life. But that didn't make it any easier to say goodbye.