The Takagoyama Nature Zoo in Futtsu, Japan killed 57 snow monkeys after discovering they possessed "alien genes" earlier this year. It sounds more sci-fi than it really is: In this situation, alien genes refers to the monkeys being crossbred with rhesus macaque—another species of monkey.
According to BBC, the rhesus macaque is banned under Japanese law since they are labeled as an invasive species and can threaten Japan's natural environment. The fear is that if the invasive monkeys were to escape, they would reproduce in the wild and create an uncontrolled community. "They get mixed in with indigenous animals and threaten the natural environment and ecosystem," says Junkichi Mima, a spokesman for WWF Japan.
It was previously believed that all 164 snow monkeys at the museum were pure, meaning they were not crossbred in any way. The 57 monkeys were put down over a one-month period, which ended in early February. A memorial service was held for the monkeys at a nearby Buddhist temple.