A zoo in Japan has culled 57 of its snow monkeys by lethal injection after discovering they carried the genes of an 'invasive alien species'.
Takagoyama Nature Zoo in Chiba, northern Japan, said DNA testing showed the monkeys had been crossbred with the rhesus macaque, which is non-indigenous and banned under Japanese law.
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A local official said the animals had to be killed to protect the native environment. If they escaped, they could reproduce in the wild and threaten the native species.
According to the BBC, Junkichi Mima, a spokesman for conservation group WWF Japan told AFP news agency that invasive species, including crossbreeds, cause problems "because they get mixed in with indigenous animals and threaten the natural environment and ecosystem".
According to the Telegraph, the zoo operator held a memorial service for the monkeys at a nearby Buddhist temple.
Japanese macaques, known as snow monkeys, are a popular tourist attraction, and can often be seen bathing in hot springs in the wild.
Individuals have brown-grey fur, red faces, and short tails.