Auckland Zoo has lost its last two lions.
Kura, 19, and her daughter Amira, 17, were euthanised on Wednesday morning in what staff have described as the "best call" for the animals.
In a statement, the zoo said that keepers had been monitoring Kura's health for the past few months and noticed she had been losing weight despite being fed more.
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She also began to display behaviours that are unusual for a pride matriarch, such as acting submissively toward her daughter Amira. This indicates she felt very vulnerable.
Senior Vet Manager Dr James Chatterton said it was obvious Kura's quality of life wasn't going to improve.
"Her decline was irreversible and would only worsen in the days ahead, making euthanasia the kindest, most humane and only option."
The decision to also euthanise Amira was difficult for the zoo, but they knew she wouldn't cope without her mother as the two were closely bonded.
"Experience has shown us that even the rare times Amira has been apart from Kura for a few hours, she has found it extremely difficult," said carnivore team leader Lauren Booth.
"She would become highly stressed and anxious, so much so that in March we gave her anti-anxiety medication."
Amira could not live alone as lions are pride animals who need to be around others of their species. The elderly lion wouldn't have been accepted by a new pride, either at Auckland Zoo or anywhere else, and Ms Booth said to attempt to reintegrate her would likely have been fatal for Amira.
"This is heart-breaking for us, we love both Amira and Kura and will miss them terribly, but we know that euthanasing Amira enabled her to die peacefully with Kura, and not endure any unnecessary suffering."
Auckland Zoo's carnivore team described Kura as "the most liony of lions" - a true matriarch and "tough old lady" who always looked out for her fellow female lions and would keep the men in line when needed.
"Personally, I'll miss the demeanour of strength and dignity she always exuded and be forever grateful for all that she taught me about lion dynamics and what it means to be a good leader," said Ms Booth.
Amira was "a real princess" and a playful "mummy's girl" who was also alert and astute, according to the team. She and her mother lived longer lives than their wild counterparts, whose life expectancy in the wild is only 15 if they're lucky.
The loss of Kura and Amira means Auckland Zoo currently has no lions, but the zoo is committed to housing more vulnerable and endangered African big cats in future.