Maggie, the second-oldest known female orangutan in captivity, was euthanized on May 20 after declining in health over the past several weeks at the Brookfield Zoo. She was 55 years old.

Described as “curious, intelligent and playful” by the primate staff at Brookfield Zoo, Maggie arrived at the zoo’s Tropic World exhibit in 1995. She was born at the San Diego Zoo in 1961 and gave birth to four offspring there.

In Brookfield, Maggie was a surrogate mother to two infants. When Kecil, a male, arrived from the Toledo Zoo as a 10-month old in June 2014, he was introduced to Maggie, who raised him. Kecil’s own mother, who had raised other offspring previously, showed no interest in raising her newborn after an apparently difficult delivery.

Kecil is now 2½ years old and is one of five orangutans remaining in the Brookfield Zoo collection. The others are Ben, 37 (arrived at Brookfield Zoo in 1992); Sophia, 35 (arrived at Brookfield Zoo in 1992); Brunei, male, 25 (born at Brookfield Zoo); and Kekasih, female 7, (born at Brookfield Zoo).

Maggie reportedly always stayed close to Kecil in the Tropic World exhibit and was protective of him.

Zoo staff have arranged “play dates” with Kekasih and over the weekend was introduced to her 7-year-old’s mother, Sophia, in order to try to incorporate him into the family.

A little more than a decade ago, Maggie made national headlines when she was treated by a University of Chicago endocrinologist for a hypothyroid condition that had caused her to gain weight and become lethargic.

According to a Chicago Tribune article from 2004, Maggie had been obese most of her life and suffered from severe headaches and a host of other ailments.

After being diagnosed with the hypothyroid condition, Maggie, then in her 40s, was placed on medication that stabilized her body chemistry and helped change her life. She lost about 100 pounds, became more active and began mating with a male in the collection, though the two never produced offspring.

According to a press release issued by the Chicago Zoological Society, which operates Brookfield Zoo, on Sunday, Maggie’s health had been declining and “degenerative changes associated with her advanced age began to severely impact her quality of life.”

She was suffering from a variety of ailments, including arthritis, heart disease, round cell sarcoma and, of course, the hypothyroidism that had plagued her all her life.

Maggie was the second-oldest confirmed orangutan in captivity, according to Sondra Katzen, media relations manager for Brookfield Zoo. The oldest, an orangutan named Gypsy who is estimated to be 62 years old, is in the collection of the Tama Zoological Park in Tokyo.