A zoo in Japan has culled 57 snow monkeys because they carry "invasive alien" genes, the Telegraph reports.
DNA testing of 164 snow monkeys at the Takagoyama Nature Zoo in Futtsu city revealed that about a third of the monkeys had been crossbred with the rhesus macaque. Japan considers the rhesus macaque an invasive species and Japanese law prohibits the possession of invasive species and crossbreds, according to the Japan Times.
"They have to be killed to protect the indigenous environment," an official said of the hybrid monkeys, at the Telegraph.
Another official from part of Japan's environment ministry added that "there were fears they might escape and reproduce in the wild," according to the International Business Times.
The Telegraph notes that the zoo operator reportedly held a service for the culled monkeys at a local Buddhist temple.
Snow monkeys, it adds, are a popular attraction in Japan as they live in mountainous snowy regions in winter months and congregate around hot springs. Numbers of rhesus macaque, which are native to China and Southeast Asia, began to rise in the area in the 1990s.