Media playback is unsupported on your device Media caption Digit with her adoptive father

The Thivillons have been caring for Digit the gorilla for more than a decade, but having a primate at home throws up some unlikely problems.

In a village near Lyon in south-east France, a couple have become local celebrities by virtue of their adopted "child".

Digit, as she is known, has been living in the same room as Pierre and Eliane Thivillon for nearly 13 years.

Their relationship began in 1999, when the young primate came into the care of the couple, who manage the zoo at Saint Martin la Plaine where she was born.

Within three days of her birth, it was clear that Digit's mother Pamela was refusing to feed her. The Thivillons took her into their care, bottle-feeding her during the day and returning her to her enclosure at night.

By 18 months old, the baby gorilla had begun to show a deep attachment to her foster parents. After an illness left her requiring 24-hour care, Pierre and Eliane took her into their bedroom at night, where she has slept ever since.

For the first 10 years, she snuggled up between the couple, but now at 130kg (287lbs) she has to have a bed of her own.

Although she has free reign of the couple's small apartment, during the day she rambles around her enclosure which is next door.

Image caption A younger Digit on the bed she then shared with Eliane and Pierre

"I don't go in there as it's Digit's home," says Eliane, acknowledging the young teenager's right to privacy. "Only Pierre is allowed in there."

Having a gorilla in the family is not without its problems.

"This morning I brought four raisin buns for our breakfast," says Pierre. "When I had my back turned she managed to finish four of them before I could save one. Then she turned her attention to Eliane's coffee."

"She loves chocolate," says Eliane, and sometimes she will drink an entire cup of coffee in search of the little sweet Eliane dunks in there.

The couple, who have no children, are often asked if they consider Digit as their daughter.

"I wouldn't say we see her as our child as such, but she is someone who is very dear to me," explains Eliane.

Image caption Digit plays with other animals at the Saint Martin la Plaine zoo

"She is part of our family and I do treat her as a mother would. Sometimes I will say: 'No, no more sweets Digit', and then she will give me three or four kisses and I give in."

The trio have learned to transcend the barriers of language to communicate with each other.

"We recognise the noises that she makes. This morning she made a little grunt to show me that she was happy," says Eliane. "She understands what we tell her too."

"The other day she was sitting on the sofa and I said, 'Come on Digit, give me some space,' and she moved over.

"In the morning, when she wants her toys or her Lego or her books, she just points to the cupboard where they are kept."

Encouraging a teenager to read is a challenge for any parent, but what kind of books is a 13-year-old gorilla interested in?

"She has animal books and catalogues," Eliane explains. "Sometimes she looks through them and she'll stop on a page and I'll explain to her, this is a cat or this is another animal."

If she is thirsty she will find my hand and give it a little tug Pierre Thivillon

When playing with her Lego she is limited to the big chunks as the little ones slip through her enormous fingers.

Every evening when Pierre has finished his rounds at the zoo, he and his wife have a quick meal before they retire to their bedroom.

"I stay and play with Digit until she makes signs that she wants to go to sleep," says Pierre. "Then we prepare a pillow for her, we cover her up and she makes happy little grunting sounds. Then we just watch her until she falls asleep."

Her brother Ginko joined them too for a while but when he grew too big he moved out into one of the enclosures. Digit remains, never more than a metre away from the couple.

"In the night, if she is thirsty she will find my hand and give it a little tug, sometimes she will tug at my feet," says Pierre.

"We try to be attentive to her needs - not that she is spoilt rotten or anything - but we just try to make sure that she has what she needs and she is happy."

Image caption Digit was taken into their care after her mother rejected her

But looking after a teenage gorilla means forsaking more than the average parent. Finding an appropriate babysitter is difficult and Pierre and Eliane have been with Digit every night since they took her into their care.

Once, when Pierre had to travel to a zoo in Kent in the UK, he arranged the trip so that he could go and return within the day. And the same applied when he had to go into hospital.

Both Pierre and Eliane are in their sixties and with no clear successor to take on the running of the zoo their biggest worry is who will take care of Digit.

"It is not good getting old," says Pierre. "I think that parents who have children have their worries but when these children get to a certain age they can take care of themselves. But when you have a gorilla like Digit you are completely responsible for her, it is not like having a dog that you can leave with a neighbour.

"She is a very special creature and that's why it has been such a joy to have her with us. But we know that her only chance will be not to live with other humans, but with other gorillas."

For the past few years, the Thivillions have been trying to reintegrate Digit with her gorilla family and she is showing some progress, getting along well with her brother.

In the meantime Pierre is busy helping his charge to get on the property ladder, building her a huge 3,000 cubic metre enclosure which she will share with her brother and two half-sisters.

"We are hoping that eventually she will have a baby of her own and she will still have this bond with us," he says. "We hope when she does she will still come to us for advice."

The Thivillions told their story to Vanessa Mock for Outlook on the BBC World Service. Listen to the programme here.