Koko the gorilla can understand 2,000 words of spoken English

Nancy Alperin and Kendra Keller claim they were told to show their nipples to the gorilla, Koko, as a way of bonding.

Sign language requests from Koko, the "talking" gorilla, were allegedly relayed to the women by the Gorilla Foundation's head, Francine Patterson.

The Gorilla Foundation, based in San Francisco, strongly denies the claims.

Expose breasts

Ms Alperin and Ms Keller are seeking more than $1m (£528,000) in damages for alleged sexual discrimination, wrongful dismissal after reporting health and safety violations and outstanding overtime pay.

The pair were among 16 employees at the foundation, best known for the 33-year-old female Koko, who has been the subject of several books and television shows.

The gorilla is said to use a sign language of 1,000 gestures to communicate with humans and can understand 2,000 words of spoken English.

The pair allege Ms Patterson pressured them on several occasions to expose their breasts to the gorilla, at least twice outside where other employees could have seen them.

Allegations denied

Their lawsuit said: "Through sign language, as interpreted by Patterson, Koko 'demanded' plaintiffs remove their clothing and show Koko their breasts."

It went on: "On one such occasion, Patterson said, 'Koko, you see my nipples all the time. You are probably bored with my nipples. You need to see new nipples. I will turn my back so Kendra can show you her nipples'."

The women, sacked in August last year, say they never followed the requests to undress for Koko.

The foundation's lawyer, Todd Roberts, said in a statement: "We unequivocally deny these allegations and are confident that this case lacks merit."

He said the lawsuit was an attempt by the women to "manipulate a purported employment issue and miscast it purely for publicity purposes".