Toi, a 21-year-old female red-shanked douc langur monkey — the last of her species in North America — was euthanized Wednesday at the Philadelphia Zoo because of failing health, the zoo said Friday.

"Toi was a favorite of many. Her gentle demeanor and unique behavior of covering her eyes with her hand endeared her to staff and visitors alike," Andy Baker, the zoo's chief operating officer, said in a statement. "We are truly saddened by this loss, Toi's long and healthy life was a testament to the excellent care she received here from the keeper team and veterinary staff."

Toi was born in 1997 at the San Diego Zoo and came to the Philadelphia Zoo in 2009. Douc langurs are found in the rain forests of Vietnam and Laos, and are considered an endangered species, the zoo said.

"Their striking features" — golden faces with white ruff and powder-blue eyelids — "and distinct personalities made them excellent ambassadors for all of the highly endangered primates of Southeast Asia," the zoo said Friday.

Toi, one of the oldest ever of her species in the United States, had recently experienced a loss in mobility and appetite, and her behavior had started to worsen, the zoo said. Veterinary staff tried to treat her but determined she had a low chance of recovery. They concluded that euthanasia "was the most humane option," the zoo said.