At age 16, Molniy is the resident senior citizen among the Minnesota Zoo's four tigers. The geriatric cat with arthritic hips is the age equivalent of an 80-year-old man.

When keepers try to enrich his daily routine with an exercise ball and treats, he prefers to nap in the sun. Like many seniors, he has his good days and his bad.

"We've seen him go from a very active young rambunctious male to what he is now," Diane Weinhardt, the zoo's Northern Trail supervisor, said.

The Minnesota Zoo has nearly 5,000 animals. Most of them will die there.

End-of-life planning is part of the routine at the 40-year-old zoo in Apple Valley. In the past year, 177 animals died, from old age and other causes. Zoo officials are constantly looking at when and how they must replace animals in their exhibits.

"We have a lot of end-of-life conversations because every animal is going to die," Kevin Willis, the Minnesota Zoo's vice president for biological programs, said.

Victims of old age this year included a 24-year-old and a 25-year-old bison, a 22-year-old camel, a 16-year-old Holstein bull and a 31-year-old African penguin. Willis said all of the animals lived past their average life expectancy for their species.

Many animals live longer in captivity than they normally do in the wild, said Rob Vernon, spokesman for the Association of Zoos and Aquariums.

"With that comes a whole new set of care standards for a senior population or a geriatric population of animals," Vernon said.

The unexpected death this month of a 3-year-old tiger, Nadya, offered a look behind the glass into how the zoo cares for animals in their last leg of life — and how it handles their deaths. On Friday, necropsy results for Nadya revealed an infection or inflammation of her liver, kidneys, stomach and intestines. Her results were inconclusive, according to the zoo.

Handling end of life

Older animals at the zoo are not removed from exhibits and sent to a sanctuary to live out the rest of their lives. When to euthanize them is a case-by-case decision. Veterinarians meet with the specific species department, review the animal's symptoms and make the final call.

"At the end of the day, it is the vet's call if it is time to die," Willis said.

Some dead animals are sent to the University of Minnesota for necropsy and then chemically cremated, while others are necropsied and incinerated on site in the zoo's incinerator, methods commonly used in zoos across the nation. Parts of the animals are often saved for research and educational purposes.

In chemical cremation, "basically you dissolve the body," Willis said. "We don't bury them."

The zoo's incinerator, Willis said, is typically used for smaller animals when the cause of death is apparent, such as a stillborn piglet or a bird that flew into a window in the zoo's tropics building.

Animal rights groups advocate for a somewhat different final chapter.

"As animals have been at the zoo for a long time, it would be great if they would relinquish them to a qualified sanctuary," said Chelsea Youngquist, program director for the Minneapolis-based Animal Rights Coalition. "We are not opposed to euthanasia when it is the best thing for the animal."

From May 2015 to this May, of the zoo's 177 deaths, 96 were birds, 73 were mammals and eight were reptiles. The numbers do not include smaller animals managed in groups, such as hissing cockroaches and most fish.

Thirty of the mammals were Daurian pika, relatives of the rabbit that have a short life span and a high rate of stillbirths. "In years where we are doing a lot of breeding, and we have been lately," Willis said, "we are going to get more deaths."

In 2014, the zoo had about 120 deaths out of its 4,700 animals.

When they die, they leave behind a void that has to be filled.

The zoo's two pumas have reached the age where time is ticking, and the zoo is looking to fill their exhibit.

"They are both old, and I need to start looking for replacement animals," Willis said.

The pumas may be considered elderly, but they are not record-holders. The record-holder at the zoo is Nikko. At 32 years old, Nikko is the oldest living snow monkey in North America, Willis said.

In some cases, the zoo relies on its breeding program to replace animals. After Nadya's death, Willis said the zoo will not bring in another tiger to replace her. Instead, it is relying on two of its tigers, Patrice and Putin, to breed.

"We have a really valuable pair of tigers, and we really want them to get along and to eventually breed," Willis said.

Common conditions

The zoo has several animals being treated for age-related conditions, including arthritis.

X-rays of Molniy's hips show the arthritis that has set in — bumpy edges have overtaken the once smooth curve of the tiger's joints. It's common among the zoo's older animals.

In the morning, Molniy can sometimes be quite the grumpy cat, his keepers say.

They first put the cat on pain medication for his arthritis in 2013. He now takes three medications, including a steroid for inflammation. His handlers keep the 325-pound beast on the lean side to avoid putting too much pressure on his arthritic hips. "Older animals, regardless if it's a tiger, the hip issue is big," said Diana Weinhardt, the zoo's Northern Trail supervisor.

Aging affects all animals, not just the ones on dry land.

The zoo inherited a group of five rehabilitated Hawaiian monk seals, all in their early 20s, a year ago. Four of the five elderly seals have limited or no sight. One, Nani, developed an issue with hind flippers due to age, said Melanie Oerter, Minnesota Zoo zoologist.

"She can swim around slowly using her front flippers," Oerter said. "We keep the pool levels really high. So if she wants to come out of the water, she is able to pull herself out of the water using those front flippers."

To help Jake, the zoo's 36-year-old red-tailed hawk, with his arthritis, zookeepers created a new perch for him by filling a tube sock with rice. Kevin Wier, supervisor of outreach education, said zookeepers make a point of slowing themselves down to Jake's pace when they work with him.

"He probably has got some special genes as part of why he's lived so long," Wier said. "But he also gets incredible care from the staff and veterinarians here at the zoo."

Wier has spent 21 years watching over the hawk.

Jake has traveled around the state with the zoo's Zoomobile program, educating children. Because of his age, the zoo has limited his travels and his program involvement. He's semiretired, Wier said.

"One of the things I find interesting as the animals age," he said, is "there are a lot of similarities between them and between us."

Beatrice Dupuy • 612-673-1707