They said the culling "may seem harsh, but in nature is necessary to ensure a strong pride of lions with the greatest chance of survival."
In February, the zoo faced protests and even death threats after it killed a 2-year-old giraffe, citing the need to prevent inbreeding.
This time the zoo wasn't planning any public dissection. Still, the deaths drew protests on social media, including an online petition with nearly 50,000 signatures Wednesday calling on the zoo to stop killing healthy animals.
Each year, thousands of animals are euthanized in European zoos for a variety of reasons. Zoo managers saying their job is to preserve species, not individual animals.