Japanese Zoo Culls 57 Snow Monkeys Due To Mixed Genes From "Invasive Alien Species"

Japanese monkey

Some of the monkeys have formed a hot tub culture, using local hot springs to keep warm in the depths of winter. divedog/Shutterstock

A zoo in Japan has had to cull at least a third of its snow monkeys after they discovered that the primates were carrying the genes of an “invasive alien species”. It turned out that what the park thought were genetically pure Japanese macaques, native to the islands, were actually hybrids with the invasive rhesus macaques.

Since 2013, after a revision of Japan’s environment laws, it has been illegal to keep or transport invasive species, including their hybrids. This meant that the 57 monkeys found to be Japanese-rhesus macaque hybrids had to be culled using lethal injections. “They have to be killed to protect the indigenous environment,” an official from the Chiba Prefectural Government told AFP.


Following the cull of the monkeys, it is reported that the zoo held a memorial service to honor the monkeys at the local Buddhist temple to “appease their souls”.

Japanese macaques (Macaca fuscata) are endemic to Japan, and the most northern living non-human primate. In their mountain habitat they frequently encounter snow, which gives rise to their other name, the snow monkey. Some populations have even developed a “hot tub culture” as in the depths of winter the primates keep themselves toasty warm in hot springs.

The rhesus macaque (Macaca mulatta), however, is widely distributed across much of Southeast Asia, and has been introduced to Japan. In a bid to protect their native wildlife, the Japanese government has tried to eradicate the primates, and prevent them from interbreeding with the native snow monkeys.

It was feared that the hybrids kept at the zoo might escape, releasing their “alien” genes into the wider population of local wild Japanese macaques. The step was therefore taken to kill the genetically mixed monkeys, though it is not yet clear if other steps could have been taken, such as sterilization.


Hybridization can be a very real threat to native animals if they frequently encounter invasive species. Wildcats in Scotland, for example, are teetering on the brink of extinction as it was found that they commonly interbreed with domestic cats. The only remaining native cat to the British Isles, there are now thought to be only a few hundred genetically pure Scottish wildcats surviving in the most remote parts of Scotland, and efforts are underway to try and protect them further.


  • tag
  • genes,

  • genetics,

  • interbreeding,

  • hybrid,

  • cull,

  • invasive species,

  • zoo,

  • rhesus macaque,

  • Japanese macaque,

  • snow monkey