Maggie retired from the petting zoo at the Brevard Zoo shortly after her 16th birthday. A fallow deer with familiar white dapples on her back, Maggie had lived at the Florida zoo since 1994, where staff and guests alike knew her for her calm, sweet nature and attraction to men wearing strong cologne.

"Many times we would be sitting in the yard and she would put her head against our legs or in our laps if we were sitting down, and she just wanted us to be there and to pet her," said Brandi-Ann Pagano, a lead animal keeper who cared for Maggie. "A few people, either zoo guests or volunteers that Maggie came to recognize, she would actually walk up to those people to greet them if she remembered them."

At 16, she was edging past the life span expected for a fallow deer, and struggling with change.The zoo had just opened up a children's petting area, and it included a new herd of rowdy young goats that nibbled at her coat, which the deer apparently hated. [World's Cutest Baby Animals]

Because she didn't feel at ease with this new herd, she became more uncomfortable around people, according to Pagano. The keepers noticed Maggie pacing and saw that she was losing hair, and, about a year and a half ago, a decision was made to retire her to her own yard.

A familiar problem

Animals in zoos do not face the stresses and dangers of the wild: Food is guaranteed, there is no risk of being eaten, and ailments are treated. And many zoos offer enrichment, or activities intended to keep an animal occupied. As a result, zoo animals, like modern humans, can live into old age, and they, too, face the physical decline and illness of old age. Aging animals develop problems rare among wild populations, such as cancerous tumors, as well as more standard problems associated with aging, such as arthritis, according to Trevor Zachariah, director of veterinary services at Brevard Zoo.

As a result, zoos must treat geriatric animals and, sometimes, make difficult decisions.

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"The whole keeping of animals in zoos has just evolved dramatically over the years, and as that science evolves, I think more and more we are looking more specifically to the needs of the individual animals as opposed to the needs of the whole population of animals or the needs of the zoo," said Michael Loomis, chief veterinarian at the North Carolina Zoo. "For instance, if there was a genetically valuable animal in the past the zoo would try to keep the animal going at any cost in order to get another offspring, whereas in this day and age, this animal is looked at as an individual and the quality of life this animal is experiencing really figures highly into what is done to try to keep the animal alive."

Many zoos now practice a form of hospice — an approach to death that seeks to comfort the dying rather than extend life at all costs, includes loved ones and approaches death as part of a process. However, zoos have yet to formally acknowledge the practice, write two wildlife veterinarians in a recent article in the Journal of Zoo and Wildlife Medicine.

Both the public and zoo workers could benefit from formally extending the concept of hospice to animals, according to Cheryl Scott, one of the wildlife vets to make the proposal, who has practiced geriatric medicine for pets. [Growing Trend: Hospice for Pets]

"I think it used to be embarrassing to grieve over pets or animals when they die, and now when we just know the value of animals and we get attached to them, I think it is especially important to acknowledge that grief is part of the process," Scott said.

A sudden turn for the worst

When Maggie retired, she went to live in her own yard at the zoo, which at one point she shared with five African spurred tortoises. Her hair grew back, and she seemed more relaxed, Pagano said. Even in retirement, the deer kept some of her social calendar, receiving visitors, going for walks around the zoo and appearing at fundraisers or greeting wedding guests.

Maggie always had a sensitive stomach and was on a diet that restricted her intake of fruits and vegetables. After about a year and a half in retirement, the deer began regurgitating food immediately after eating and drooling.

"When we noticed it was becoming increasingly worse to the point where she wasn't holding any food down at all, we knew it was a chapter that would have to come to a close pretty soon," Pagano said. "I knew it was coming to this point, I knew she had a very long life, I knew she had touched many people's lives."

Zachariah examined her and found that her rumen — part of her stomach — was not functioning properly. Invasive diagnostic and treatment methods are less of an option for zoo animals than for pets simply because zoo animals are not as comfortable being handled and manipulated, Zachariah said. "Given what she would have to go through to diagnose and treat whatever was causing her problem, when you add her advanced age to that, the consensus feeling was that it wasn't fair to the animal to put her through all the stress of having to do all those things to her." [Zoo Animals Face Old Age and Illness: Gallery]

Saying goodbye

On Sept. 14, Maggie was euthanized, after goodbyes from people who cared about her. Pagano and another keeper were with her.

"It was a closure that I needed, I wanted to be there for her, I wanted her to have someone there with her so it wouldn't be as frightening to her," Pagano said. "It was the first and last time she walked into the hospital. She did great, I feel it was because she had people around her."

After Maggie had passed, Pagano made phone calls to volunteers and former staff members who were attached to Maggie, to let them know before the announcement went out.

"This is a biased opinion, but I believe she had one of the best lives we could have ever given any animal so it was a hard decision to make, but in the same thought it was also an easy decision to make because we truly cared about her, and we know that she had had a long life," Pagano said.

The story appeared in the local news, and a paragraph announcing her death went out to the world on Facebook, where it collected condolences and memories of her.

Public relations doesn't play a role in how the Brevard Zoo handles the deaths of most animals, only for those large charismatic animals like rhinos or giraffes and for animals with a community following, like Maggie. But, otherwise, the medical considerations are generally the same, according to Zachariah.

Quality of life assessments, too, are for all animals; it's possible for caretakers in zoos to get a read on any animal's quality of life, according to David Jessup, a wildlife veterinarian and co-author of the article about hospice practices in zoos.

"Zookeepers and zoo medicine has a great deal of insight into animals that are not so much like us," Jessup said. "Birds are physiologically nothing like humans yet we have a great deal of insight into their behavior, how active they are, what they eat."

A question of quality

Officials at the four zoos interviewed, which include Reid Park Zoo in Arizona, said they kept tabs not just on an aging or ill animal's health but also its quality of life; the North Carolina Zoo employs the most formal means of evaluating quality of life. A form, with queries about an animal's life expectancy, signs of pain, presence of normal behavior, veterinary prognosis and other considerations, is filled out periodically for animals with chronic illnesses, and then used in any discussion about euthanasia, according to Loomis.

While many animals thrive in the safe haven offered by zoos or other facilities, living longer lives and producing more offspring, there is evidence that captivity can cause stress and shorten life spans in some species. For instance, a study published in 2008 in the journal Science found that elephants living in European zoos, particularly Asian elephants, had dramatically shorter life spans than those in protected populations in their native lands.

Great apes, particularly males, frequently succumb to heart disease, though the reasons aren't understood. Doc, a 27-year-old orangutan and fixture at the Houston Zoo, was euthanized Aug. 30 when it became clear that he had lost his battle with a heart condition.

"He had a certain sweet, gentle quality to him even as a baby that was quite endearing," said Lynn Killam, the Houston Zoo's assistant curator of primates. "When he got older, when he turned into a fully mature male, I used the word bluster for lack of a better one. He made lots of displays, he was magnificently male, he was stunning, he was imposing and yet he had this gentle quality."

Doc, the almost 300 pounds (136 kilograms) of him, would play gently with his young son, Solaris, something that would never happen in the wild, Killam said. He was also an accomplished artist. As part of an enrichment program, Doc would dip vegetation in paint and brush it onto a canvas held by a keeper, some of this work was used to raise funds for the Kinabatangan Orang-Utan Conservation Project. He also developed a following among visitors.

"They would put their hands up to window, and he would put his hand up to the window and they would feel like they had looked into the eyes of a sentient being when they looked into Doc's eyes. And they had," Killam said.

During a routine examination about a year and a half ago, the zoo staff discovered that part of his heart was no longer functioning properly, a sign of degeneration of the heart muscle that has stricken many great apes, she said.

Doc was not an old orangutan. In captivity, these apes can live into their 50s. The wild populations, which live in the forests of Borneo and Sumatra, are endangered species threatened by the loss of habitat, and it's not yet certain how long a wild orangutan can live, Killam said.

Doc went on medication, and things seemed fine for about a year. Then he lost his appetite. Another exam revealed his condition had worsened considerably. A cardiologist then updated Doc's medication, a combination of oral and injectables.

Many animals at the Houston Zoo, including big cats and primates, are trained to accept injections in return for a treat, according to Joe Flanagan, director of veterinary services at the Houston Zoo. Doc was no exception.

"Things that in most orangs would have been stressful and impossible, he took readily," Flanagan said. "He took injections better than most adult men."

Getting him to take his oral medication was a challenge, because Doc had no appetite and the keepers had to get creative to find foods that he was willing to eat, eventually resorting to ice cream donated by the zoo's restaurant, Killam said.

"There were times the keepers told us they thought Doc was taking the medications for them. He didn’t want it but he knew we wanted him to have those medications," she said. "It was very poignant."

He seemed to be doing better, but then about a week before he died, something changed and his gentle eyes became listless, almost as if he was giving up, Killam said.

"This type of disease just makes you feel really tired, it's not like a heart attack that hurts," she said. "There was something about his demeanor that suggested to us he was just done. … It was terrible, because everybody had had such high hopes and everybody wanted him to go back outside and play with Solaris again. There was such a terrible gap between what had happened and what we hoped before that there was a lot of grief.”

On Aug. 30, 2011, Killam and others who worked closely with him held on to him as he was euthanized.

"We held his hands, we held his feet, we held his check pads (the large flaps on the sides of his face, characteristic of male orangutans). He had to have that last shot given to him by the people he loved," she said. "We all held on to him and were there with him when he went, and this is usually what happens with our loved ones, we will not abandon them even though it is so hard."

While zoo officials routinely conduct a necropsy after an animals' death, Doc received a particularly thorough one with the help from an outside pathologist.

"Essentially, a team of eight people spent seven hours looking through tissues and that was before they went to the microscopic stage," Flanagan said.

There are so few orangutans in the United States and, thankfully, so few in Doc's condition that a case like his could add volumes to the amount of information available about the heart problems plaguing great apes, Flanagan said.

Information gleaned from the necropsy went to the Great Ape Heart Project, which as its name indicates is trying to understand heart disease and improve cardiac health for great apes (excluding humans, who have their own cardiovascular projects). Doc's skeleton went to a university and his brain went to the Great Ape Aging Project.

Doc's contributions to science and possibly to other great apes with bad hearts make his loss easier intellectually, but not emotionally, she said.

"When we have the joys of the births and the excitement of new animals when you have all those endorphin-laden moments that you get as a zoo worker, you have to accept that part of what you are going to feel is the downside of that, the death and illness," Killam said.

You can follow LiveScience writer Wynne Parry on Twitter @Wynne_Parry. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.