To better understand the sounds, Luef and her colleagues tracked two wild western lowland gorilla populations in the Republic of Congo. The scientists recorded and analyzed the singing and humming calls that gorillas of different ages and sexes produce in response to various foods.
They found that adult males make the sounds much more frequently than females. The type of food mattered as well.
According to the authors, "Aquatic vegetation, flowers, and seeds led to a high likelihood of call production. Insects, in particular, showed the lowest probability and rate of calling."
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As for why males sing and hum more while eating, the researchers note that males in general tend to vocalize more frequently. They point out that "females and immatures are generally at a higher risk of predation and thus behave more quietly."
Other factors could be at work too. The calls are too quiet to be "food advertisement signals," which are essentially dinner bells in the world of wild great apes. The singing and humming might, however, "represent a form of collective decision-making in the feeding context and allow group members to coordinate their feeding activities," according to the authors.
This could also help to explain why people often share with each other how they feel about food in melodic ways. This usually happens right at the table, too. Few say a rapturous "mmmm" about food out of the direct context of actually eating.
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Just as humans also sing and hum to themselves when content, gorillas appear to do this as well. Such reports are mostly anecdotal, so more research is needed to determine the full communication abilities of gorillas, their meaning, and how they connect to the evolution of vocalizations among all higher primates.