A zoo in Australia has euthanised a pair of African lions after one of them suffered a medical episode, sparking a backlash on social media.

The Adelaide Zoo said Amani and her male partner Mujambi were euthanised on Thursday. “Both big cats are now at rest,” the zoo said.

Mujambi, 19, suffered a medical episode over the weekend and, after the veterinary team failed to improve his condition, the zoo management decided to euthanise the animal to spare it further suffering.

Amani, 23, the oldest lioness in the country, was put to sleep shortly after.

The zoo said the decision was made as “negative impacts” on Amani without her partner “outweighed any benefits of keeping the lioness alive”.

Amani and Mujambi had been together for 16 years and were “very closely bonded”. The zoo described Mujambi as a handsome lion with a beautiful mane despite his old age. “She swiped at him, growled and bossed him around, but she also wanted to be no further away from him than his shadow,” said zoo curator Deb Barry.

The African lions had lived beyond their average life expectancy in the wild of around 15 years, the zoo said. “Under human care in a zoo or safari park setting, lions can live into their 20s. At 23 and 19 both lions were in the latter stages of their lives,” it said.

Zoo director Phil Ainsley said the decision was taken after carefully examining options, including moving Amani to another zoo that houses her species.

“Tigers, we know, potentially can live in isolation. That’s not how it works for African lions,” he said. “Normally you’ll either get a pride or, as they get older, a couple living together.”

Moving Amani and integrating her into another pride or environment “was considered too risky and stress provoking”, he added.

Amani turned 23 in May and her birthday was celebrated by her keepers. She got to play with special pinatas. The zoo, which has over 3,000 animals, shared a video of her rolling on the ground with different balls on her birthday.

“Choosing when it’s the right time to say goodbye to an animal is a hardship but also a blessing,” Mr Ainsley said.

Amani was brought to the Adelaide zoo from Auckland in 2002 and Mujambi arrived five years later from New South Wales.

But despite the zoo’s explanation, the decision to kill the healthy lioness sparked outrage online, with many people asking: “Was it the only option?”

“I just don’t understand that this was the only option,” Facebook user Debi Ratta wrote. “Seems so very very extreme.”

“Why couldn’t you have got another mate for her? Just because she was the oldest in captivity doesn’t mean you had to kill her? Also a healthy animal being killed is cruelty. I will not be supporting the zoo ever again,” said another user.

“I just don’t understand why taking Amani’s life is justified because of being the only one left,” said Verity Anne.

“How long after the medical episode did you euthanise him?” asked Natasha Maree. “How do you know he wouldn’t have come around naturally?”

Responding to the backlash on Facebook, the zoo said the decision was “not taken lightly”. It said Amani, given her age, was experiencing a “number of age-related health issues that were impacting her wellbeing”.

“As an accredited zoo with a highly regarded ethics and welfare policy, based on Amani’s age, her own health issues and her strong bond with Mujambi, euthanasia was deemed to be in the best welfare outcome interests of Amani because of her strong bond with Mujambi and not with her keepers,” it said.

The zoo created a website, “Remembering African Lions Mujambi and Amani”, to honour the legacy of the animals and asked people to post messages for the support staff who cared for them.