A few weeks ago, Philadelphia Zoo keeper Mandy Fischer heard a small cough.

It was Spot, 25, a grandmother and matriarch of her group of spectacled langurs.

X-rays and ultrasound showed that the elderly Asian primate is not just losing her teeth. She has advanced cardiac disease.

Small wonder: The zoo says she's the second-oldest spectacled langur in North America.

And she represents a significant change in zoos across the country.

With improved nutrition, veterinary care and other advances, the animals are living longer.

Animal welfare advocates say this is good. It shows zoos are doing their job.

But it also means zoos must deal with increasingly difficult geriatric problems. Animals that would have succumbed in the wild are developing ailments ranging from renal failure to liver disease.

BooBoo the Andean bear, now 30, is slowing down and gaining weight.

Puzzles the giraffe was euthanized a week ago. Her companion Twigga is a year older - the fifth-oldest female giraffe in North America. At 28, she has swollen joints. Keeper Betsy Karkowski puts her arthritis pill - big as a nickel - inside a banana.

"It parallels what we see with humans," said Andy Baker, the zoo's vice president of animal programs. "It's not that there weren't ever geriatric animals. What we're seeing is a growing proportion."

No one is sure how many. It is difficult to define geriatric across hundreds of species. And there are little survival data, although researcher Lisa Faust, at Chicago's Lincoln Park Zoo, is crunching age data from zoos in North America to determine median life expectancies.

Estimates do exist. They suggest that more than 20 of Philadelphia's 250 larger mammals have survived at least five years longer than what is thought to be typical.

The aging of zoo populations prompts a cascade of concerns.

Are social animals that have lost companions lonely? Are elderly specimens taking too much space? Zoos are arks for species such as the Micronesian kingfisher, and young breeders are crucial for species survival.

Research is proliferating.

Veterinarian Robin Barbiers, Lincoln Park's vice president of collections, plans to study brain function in older animals of various species. "We know nothing about cognition as animals age," she said.

A colleague, the primate curator, is already studying aging female gorillas to see if they go through menopause. Few animal species do.

Behavioral training is helping zoos both diagnose problems and monitor them without resorting to anesthesia, often a risky procedure.

When Philadelphia's Mrs. Emu, an aged 24, developed a chronic limp, keeper Wendy Lenhart spent months training the large Australian bird to walk up a ramp and stand still. This allowed the vet to use a portable machine to get X-rays of her leg.

It was arthritis.

Now, Lenhart adds anti-inflammatory medication to Mrs. Emu's food pellets and apple treats, then feeds her by hand.

"She eats more of it when I stand here," she said.

Lenhart also has trained George, an Andean condor more than a half-century old, to step on a scale so she can monitor his weight daily.

Philadelphia has adapted living spaces for the elderly.

A tree kangaroo that could no longer climb her pole got handrails and ramps.

A snow leopard with failing vision got brighter lights.

"We all get old the same way," said zoo veterinarian Donna Ialeggio. "The idea is to make sure it is a good aging process."

For oldsters who are losing teeth, zoo nutritionist Barbara Toddes orders food pellets that dissolve easily in saliva. (Keepers used to soften yams in the microwave for a kangaroo, now gone.)

An elderly zebra, Esther, gets Purina's Equine Senior. It's easier to digest. Overall, her diet is twice as expensive as those of younger mares.

If it's getting close to the end for an animal, Toddes will relax the regime and allow more treats: "It's a lot like taking care of a very old relative. If Grandpa loves his candy and he's 92 . . . "

It would, of course, be nice if animals lived to the maximum age and died naturally.

Gorillas are susceptible to cardiac problems; a stroke killed the zoo's venerable Massa in 1984. He was 54, the oldest gorilla in captivity, and researchers who examined his remains said it was like studying Methuselah. (The Dallas Zoo's Jenny is now 55, eclipsing him.)

Decisions on euthanasia fall to a 14-member animal management group, and it can be complicated.

"Just as you face it in the human community, you face tougher medical choices when you have more ailments and diseases" in an animal, said Wayne Pacelle, president of the Humane Society of the United States.

Likewise, the emotions are complex when staffers - and visitors - have grown to love an animal over decades.

But a European study of 70 geriatric animals, published last year in the journal Animal Welfare, concluded that many had pain and "a significantly reduced quality of life." The researchers proposed a scoring system for making objective end-of-life decisions.

These days, Spot's keeper watches carefully, so as not to miss any clues such as a change in her routine or in the group's dynamics. If she weakens, the other langurs might challenge her.

Fischer slips the little primate's medication - digoxin, lasix, enalapril and baby aspirin - inside grapes, which Spot gobbles instantly.

Her cough is gone. She's strong. She's eating.

"She's a happy girl," Fischer said. "This is more grapes than she's ever had in her life."

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com.