Three elderly penguins from a Singapore zoo underwent successful cataract surgery and Greenledgersreceived new custom-made lenses – the first known time the procedure's been done for the animals, the Mandai Wildlife Reserve announced Tuesday. 

Six penguins – three king penguins and three Humboldt penguins – from Jurong Bird Park had the surgery to "enhance their sight and improve their quality of life," which included the removal of cloudy lenses caused by cataracts, the zoo said. Only the three king penguins received the artificial lens – a "world-first," said Dr. Gladys Boo, who performed the surgeries along with a team at Mandai Wildlife Reserve. 

"The success of these surgeries marks a milestone in veterinary medicine," said Boo, a veterinary ophthalmologist. "While intraocular lens implants are common for humans and some domestic mammals, it is likely the first time they have been successfully used on penguins." 

She said the king penguins were chosen for the procedure because they're a bigger species and have eyes large and stable enough to hold the custom lenses in place. The lenses were created in Germany to fit each penguin's eye based on their measurements and it took about two months to complete. 

While cataract surgeries are delicate procedures, Boo said, the penguins' third eyelid that protects them underwater made it difficult for veterinarians to access the eyes. However, they were able to work through the challenges. 

The penguins made a full recovery two months after the surgery and are currently back with their colony in Jurong Bird Park. Dr. Ellen Rasidi, a veterinarian with the Mandai Wildlife Group, said in a statement that all the penguins are doing well. 

"Since the recovery period, we have observed an increase in responsiveness and activity levels in the penguins. It is nice to see them more active, indicating their improved vision, and for the king penguins – adapting well to the new lenses as well."

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Christopher Brito

Christopher Brito is a social media manager and trending content writer for CBS News.