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The gorilla who learnt sign language to communicate with humans has adopted two kittens after being unable to conceive her own children.

To celebrate her 44th birthday keepers at San Francisco Zoo introduced Koko to a litter of cats .

The moving moment was captured on video showing the colossal ape gently handling the kittens given to her by trainer Francine Patterson.

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At one point Koko is seen tickling one of the cats on its stomach with her index finger.

According to the video uploaded on to YouTube through the gorilla’s kokoflix account, the three are now part of one happy family.

(Image: YouTube)

The western lowland gorilla is known for having learnt a large number of hand signs from a modified version of American Sign Language.

Patterson says Koko is able to understand more than 1,000 signs of what she calls "Gorilla Sign Language".

(Image: YouTube)

In contrast to other experiments attempting to teach sign language to non-human primates, Patterson exposed Koko to spoken English from an early age.

It is reported the gorilla, who was born at the zoo, understands more than 2,000 words of spoken English, in addition to sign language.