The ultra-rare footage was filmed by a robotic spy designed to look like a young gorilla.

The singing apes featured in the recently aired PBS series "Nature: Spy in the Wild 2".

Human camera operators are supposed to keep a safe distance from wild gorillas.

However, the lifelike animatronic spy robot was able to infiltrate the group and film the gorilla serenade.

A submissive gaze and an ability to beat its chest on demand was said to help the robot gain the trust of the group.

The footage it captured shows the great apes relaxing in foliage in a sanctuary in Uganda.

They were caught enjoying a feast of leaves and stems so much that they began to hum.

The PBS series narrator described this as a "chorus of appreciation".

Lots of leaves also meant that the mealtime was musical in more ways than one.

The apes were filmed farting for almost all of their meal.

The narrator explained that the vast vegetarian diet of the apes means that "they live in a semi-permanent state of flatulence".