Gorillas bare their teeth in a playful "grin" to reassure one another during play, scientists have discovered.

This "flash of teeth" seems to let their playmate know that they do not intend to harm them.

The researchers, from the University of Portsmouth, study the facial expressions of primates to uncover the evolutionary origins of human smiling and laughter.

They published their findings in the American Journal of Primatology.

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, is a gorilla and has been living in the same room as Pierre and Elaine 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 Elaine took her into their bedroom at night, where she has slept ever since.