Auckland Zoo has made the "extremely tough" call to euthanise two lions amid concerns for their welfare.
Zulu and Malik, half-brothers both born in 2004, spent their lives together in Wellington and Auckland Zoos before being put down on Tuesday morning.
However the mobility of both lions has reduced over the past year, and the zoo's animal care team has since diagnosed them with a degenerative disease of the bones and joints.
"We first noticed Zulu experiencing stiffness and reduced mobility in his hind legs last November and began treating him with anti-inflammatory and pain relief medication," explained Dr James Chatterton, Auckland Zoo's Head of Veterinary Services.
"While Zulu's diagnostic results were not definitive, his deteriorating mobility indicates a painful and progressive disease process is present and this continued to worsen despite multiple medications over several months."
Recently, Malik started to display similar symptoms of his own.
"Malik is at an earlier stage of this degenerative disease, but having spent his entire life with half-brother Zulu, with whom he's so closely bonded, being left on his own would be unacceptably stressful and compromise his welfare," said Dr Chatterton.
"He also couldn't be sent to another zoo. As well as the risks and stress of moving an elderly animal like Zulu, due to the complex social dynamics of lions, attempting to integrate him with any other lions would also be extremely dangerous, and likely fatal."
Auckland Zoo says the decision to euthanise Zulu and Malik was "the kindest call".
"For their own survival, big cats are hard-wired to disguise any injury or pain, so once they start showing abnormal signs, we know there is significant disease present," Dr Chatterton said.
"As professionals who care deeply for our animal patients, being able to make the call to euthanase at the right time is very important, as it enables us to prevent any untreatable pain and ensure animals like Zulu and Malik can pass away peacefully and with dignity."
After fellow lions Kura and Amira were put down in 2018, Zulu and Malik became the last remaining lions at Auckland Zoo.
Their deaths on Tuesday mean there are now no lions left at the tourist attraction, but director Kevin Buley says they plan to get some more in the future.
"Wildlife conservation only ever happens because enough people care and are prepared to put animals and the environment first," he said. "You cannot underestimate the power of lions, giraffes, rhinoceros and all the other species at good zoos to inspire people about wildlife."
Zulu and Malik died aged 17, matching the average life expectancy of lions in captivity.
While the pair were born at Auckland Zoo, they spent the majority of their lives at Wellington Zoo before being transferred back in 2018.
Auckland Zoo Carnivore team leader Lauren Booth hailed their "majestic presence" and said she couldn't help but fall in love with them when they returned.
"Over their 17 years, many millions of people got to experience and connect with them, and in doing so contribute to our zoos' local and international efforts for wildlife and wild places," she said.
"My team and I loved these beautiful boys' natures and watching the bond they shared with each other, as we know so many of our visitors did too, and we will miss them greatly."
