Koko the gorilla, famed for the ability to use sign language to communicate, has become the adoptive mother of two kittens.

For her 44th birthday, the gorilla chose a grey kitten and a black-striped kitten — Ms Grey and Ms Black — to join her family, signing the words "cat" and "baby".

In the video, Koko is presented with a litter of kittens in a box and "immediately falls in love" with Ms Grey, picking it up and stroking its stomach.

Ms Black is seen attempting to climb out of the bag in which she is placed to reach Koko.

She later signs "put on head", indicating for Ms Grey to be placed on her head.

The gorilla's love of kittens is well known with the story of Koko and her first kitten, All Ball, documented in the award-winning book Koko's Kitten.

Trainer Francine Patterson, who has worked with Koko since 1971, says Koko is able to understand more than 1,000 signs.

Koko was also exposed to spoken English from an early age and can reportedly understand about 2,000 words of the spoken language.