The Asian elephant was put to sleep Tuesday at the zoo where she lived for almost a half-century. Zoo officials said some age-related conditions accelerated in recent weeks, and she showed signs of a falloff in kidney or liver function. A necropsy revealed arthritis and large, inoperable uterine tumors that are impossible to diagnose in elephants through exams or imaging, the zoo said.

“She was a wonderful elephant,” interim zoo director Craig Piper said in an interview Wednesday, as heavy-hearted staffers absorbed the loss of an animal some had tended for over 30 years. “She served as a tremendous ambassador for elephants and for elephant conservation.”

Since Happy’s death, the zoo’s 57-year-old Patty is the last elephant on exhibit in the United States’ largest city. The zoo’s parent institution, the Wildlife Conservation Society, decided 20 years ago to stop acquiring pachyderms.

Born in the wild in Asia, Happy was brought to the U.S. as a 1-year-old. She was named for a character from “Snow White and the Seven Dwarfs” before arriving at the zoo in 1977.

Happy keenly engaged with her keepers and was easy to motivate with favorite treats, such as watermelon or strawberries, said Keith Lovett, the zoo’s director of animal programs. Piper said she sometimes stashed treats in her ear to save for later.

In 2005, she showed researchers that elephants can recognize themselves in a mirror — a sign of self-awareness seen in only a few other species. During the experiment, Happy faced her reflection and repeatedly used her trunk to touch an “X” painted above her eye, a mark she could see only in the mirror.

She was paired with other elephants until her last partner died in 2006. Happy then lived separately from Patty and a third elephant out of concern that they wouldn’t get along, though Lovett said the animals could see, smell and touch each other over a divider. The third elephant, called Maxine, died in 2018.

Zoo officials said the median life expectancy for Asian elephants in U.S. zoos is about 45 years. Their life expectancy in the wild is more difficult to pinpoint.

During Happy’s lifetime, zoo elephant exhibits came under increased scrutiny. Some experts said urban animal parks were too small for creatures that roam extensive distances in the wild. Animal rights activists argued that zoo enclosures were no place for big-brained, social pachyderms.

Some zoos phased out their exhibits and sent elephants to sanctuaries, though some other zoos remain committed to keeping and breeding the creatures, arguing that they help keep people interested in saving wildlife.

One activist group, the Nonhuman Rights Project, sued the Bronx Zoo in 2018, seeking to have Happy declared a “person” for legal purposes and moved to a large animal sanctuary. It was the first such case about an elephant, according to the group.

Citing a principle that’s used to challenge the legality of a person’s imprisonment, the activist group said Happy was “an extraordinarily cognitively complex and autonomous nonhuman being” who was unlawfully deprived of her liberty and suffered from being pent up in a exhibit without other elephants.

Zoo officials said Happy was assiduously cared for and had space for swimming, foraging and other natural behavior. Uprooting her from her longtime home could harm her, the zoo said.

New York’s top court ultimately rejected the activists’ claim, by a 5-2 majority. Colorado’s highest court later issued a similar ruling about five elephants in a zoo there.

Still, two of the New York high court judges wrote pointed dissents. One called Happy’s captivity “inherently unjust and inhumane” and “an affront to a civilized society.”

The Nonhuman Rights Project has continued pursuing cases about elephants in various other states.

The group’s executive director, Christopher Berry, said in an statement Wednesday night that Happy “will always be remembered as the elephant who opened the courtroom doors to consideration of nonhuman animals’ legal rights.”

Happy spent her final weeks, by her choice, in an off-exhibit barn and yard within her enclosure, Piper said. In a zoo version of hospice care, staffers provided hydration, nutrition and pain management, he said.

Meanwhile, Patty is doing well, the zoo said.

The Wildlife Conservation Society said in 2006 that once there was only one elephant, the animal might be moved to another zoo if circumstances were right. Piper said the zoo will be “really thoughtful and careful” in contemplating whether to move Patty from her home of 53 years.