According to an article by Brian Owens in New Scientist, a German scientist working in the Congo has discovered a fun new fact about gorillas, that they hum and even sing during mealtimes. Food-related calls had been documented in chimpanzees and bonobos, but never in gorillas.

But far from just vocalizing, gorillas appear to generate two different types of sound while eating. One of them was “a steady low-frequency tone” that sounds rather like a sigh of contentment, or a hum:
 

 
The other was “a series of short, differently pitched notes” which resembles “a random melody”:
 

 
And it’s not like they “sing the same song over and over,” commented Luef. “It seems like they are composing their little food songs.”

According to Ali Vella-Irving of the Toronto Zoo, “Each gorilla has its own voice: you can really tell who’s singing. And if it’s their favorite food, they sing louder.”

The behaviors, however, differ according to whether the primates are in captivity or not. In zoos every individual sings during meals, but Luef found that in the wild “it was generally only dominant silverback males that sang and hummed while eating.”

She speculated that vocalizing might be the silverback’s method of informing the group that the meal is not yet concluded and that the time to move on has not yet arrived. “He’s the one making the collective decisions for the group,” Luef says. “We think he uses this vocalization to inform the others ‘OK, now we’re eating.’”

Because there is so much variation in calls both between individuals and species, food calls provide a good way to study the origin of language, says Zanna Clay, a psychologist at the University of Birmingham: “It gives a good insight into the origin of meaning in animal signals, and also the social pressures that might drive the flexibility we see in language.”