Auckland Zoo has announced it euthanised its two elderly male lions Zulu and Malik this morning due to chronic health issues.
The zoo's head of veterinary services Dr James Chatterton said both lions had been experiencing stiffness and reduced mobility and were being closely monitored.
"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."
Chatterton said as professionals who care deeply for animal patients, being able to make the call to euthanise at the right time was very important.
"It enables us to prevent any untreatable pain and ensure animals like Zulu and Malik can pass away peacefully and with dignity."
The two lions, who were half-brothers, were described by zookeepers as "beautiful boys" with a strong majestic presence you can't help but fall in love with.
Born at Auckland Zoo in 2004 just weeks apart, the pair spent their entire lives together - primarily at Wellington and Auckland zoos.
The life expectancy for lions in zoos is 17 years, and over the past year, both lions had been intensively monitored by animal care and veterinary teams.
Chatterton said after not long after Zulu was discovered to have degenerative arthritis, Malik also started to show similar signs though his was at an earlier stage.
He said leaving Malik on his own would be "unacceptably stressful and compromise his welfare."
"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 carnivore team leader Lauren Booth and her team, who manage and care for these big cats, said Zulu and Malik were a privilege to look after in their twilight years.
"As many people will be aware, our Wellington Zoo colleagues have also just had to make a similar call for their elderly lionesses Djane and Zahra.
"It's really tough when any animal in our care dies, but like Djane and Zahra, Zulu and Malik have had long enriching lives due to the exceptional levels of care zoos like ours are dedicated to and able to provide – we're giving them lives as good, if not better than they'd experience in the wild," she said.
Auckland Zoo director Kevin Buley said while Auckland Zoo was now currently without lions, it did plan to work with them again in the future as part of the collaborative Australasian breeding programme for the species.
"Wildlife conservation only ever happens because enough people care and are prepared to put animals and the environment first.
"You cannot underestimate the power of lions, giraffe, rhinoceros and all the other species at good zoos to inspire people about wildlife. If we all make small changes in our own lives as a result, then the whole planet will be better off," Buley said.