Gorillas in the wild will sing and hum when they eat to show they're happy with their meal or to communicate to others that it's time for dinner.

That's what researchers found when they observed two groups of wild western lowland gorillas in the Republic of the Congo. Their findings were published in the journal PLOS ONE.

They discovered that the gorillas made two types of food-associated noises: humming and singing.

Humming is a prolonged, tonal low-frequency sound, which you can hear in the clip below — and turn up the volume on these clips or you'll miss it!

Singing is characterized by multiple short and differently pitched sounds following in close succession. Here's the clip of a gorilla singing:

The researchers, led by Eva Luef, a primatologist at the Max Planck Institute for Ornithology in Germany, point out that many species of birds and mammals produce vocalizations when they eat or when they encounter certain foods. The food-associated noise-making has been studied in chimps and bonobos, but only anecdotal evidence had previously existed for gorillas, mostly from zookeepers.

“They don’t sing the same song over and over,” Luef told New Scientist. “It seems like they are composing their little food songs.”

The study's authors tracked the two groups of gorillas, recording the different humming and singing that gorillas of different sexes and ages produced in response to different types of food. They found that dominant silverback male gorillas were the most vocal when it came to their meals.

“He’s the one making the collective decisions for the group,” Luef said. “We think he uses this vocalisation to inform the others ‘OK, now we’re eating’.”

The information was collected between July and September 2012 at the Mondika Research Centre in the Republic of Congo.

The authors only noticed humming and singing in relation to feeding with the gorillas, especially when they were eating seeds, flowers and aquatic vegetation. The researchers suggest that the sounds the gorillas make may demonstrate a general sense of well-being during the meal or contentment over individual food choices. The vocalizations may also help with group dynamics, signaling when it's time to eat or finish eating.

"Food-associated calling could represent a form of collective decision-making in the feeding context and allow group members to coordinate their feeding activities," the researchers write in the study.

First-hand experience

"My view has always been that the humming is a contentment issue," says Michael P. Hoff, Ph.D., who studied gorillas for years at Zoo Atlanta and the Yerkes Primate Center. He is also co-author of the book "Gorilla Behavior."

"It's 'Oh boy, here comes food!' and they really like it. It's very similar to what my child would do when she was little and we would bring her favorite food to her," Hoff said. "She would bounce up and down and make these humming vocalizations."

Hoff, who is chair of the department of social sciences at Georgia's Dalton State College, says the gorillas he has observed got much more vocal about foods they really liked. Hoff also says that gorillas have unique voices.

"It's highly individualistic. I can tell who is humming even when I'm not watching them. I can tell their humming and their singing."

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.