Koko: The Gorilla Who Talks To People is a documentary exploring the story of the only "talking" gorilla in the world and her relationship with animal psychologist Penny Patterson.
Project Koko began as a PhD project to teach sign language to a baby gorilla, but Koko and Penny have now been together for more than 40 years.
Koko's extraordinary life with Penny prompted a good deal of reaction on Twitter, with viewers sharing their thoughts on this remarkable programme.
Viewers were mostly amazing by the incredible intelligence of Koko.
#kokothegorilla will always be amazed by the intelligence of gorillas ???
-- nicole (@nicolextovey) June 15, 2016
#kokothegorilla on BBC1 is mind-blowing to me. She knows how to sign over 200 words!
-- hannah ? (@hannahrai92) June 15, 2016
-- Bryony Symes (@BryonySymes) June 15, 2016
Animals are so intelligent, such a beautiful, heart warming story. #kokothegorilla ?
-- emma (@emma_cowen) June 15, 2016
I'm loving the #kokothegorilla documentary, it's so interesting. It's amazing to see just how intelligent she really is
-- ohbitemehobbit (@chloecoates666) June 15, 2016
But equally, they felt uneasy about a gorilla being kept in captivity. The programme highlighted how Koko not having children had deeply affected her.
koko is just amazing, i wish she had a family
She looks so sad