The culling of 57 snow monkeys, also known as Japanese macaques, living in Japan's Takagoyama Nature Zoo, located in Chiba's Futtsu city, took place after the monkeys were found to carry genes of an "invasive alien species", officials stated on February 21, Tuesday.
A DNA test was conducted on the monkeys and they were identified as having been crossbred with another species of old world monkey, known as the Rhesus macaque.
Japanese law bans the possession and transportation of this species of monkey, as well as its use in crossbreeding.
"Because they get mixed in with indigenous animals and threaten the natural environment and ecosystem" Junkichi Mima, a spokesman for conservation group WWF Japan told AFP news agency.
A memorial service for the "souls of the monkeys" was carried out by the zoo operator in a Buddhist temple located nearby.
"The culling was unavoidable because there were fears they might escape and reproduce in the wild," an official from the Office for Alien Species Management, part of the country's environment ministry told the local media.
"They had to be killed to protect the native environment," a local official was quoted as saying by Reuters.
After Japan's environment law was revised in 2013, the Snow monkey species, also known as the Rhesus macaque crossbreed, was listed for culling.
Another instance of culling due to a genetic anomaly took place in Denmark when a healthy young giraffe, Marius, was killed at the Copenhagen Zoo.
The healthy giraffe was considered genetically unsuitable for future breeding. Though there were various petitions signed to save Marius and have him adopted, the zoo euthanised him on 9 February 2014.