Gorillas can recognize familiar human voices based on the relationship with the speaker, according to a new study based on playback experiments. The animals responded negatively when hearing the voices of people they don’t know or with whom they had a bad interaction, suggesting they can tell human voices apart.

Many animals can recognize and differentiate the vocalizations of members of their own species. Some can even do so with other species, such as when a monkey recognizes a distress call from a human and knows something is wrong. But the capability of undomesticated animals to recognize and distinguish between individual humans is not clear yet.

Previous studies showed that crows, pigeons, and wild elephants can distinguish between voices that they were and weren’t familiar with, suggesting that being able to do this could be important for animals much exposed to humans. Researchers at the University of Georgia wanted to further investigate this, now focusing on gorillas.

“I worked mostly with wild gorillas, and one downside of working with wild primates is that through the habituation process we could make them much more susceptible to hunters because they become used to seeing and hearing people,” said Roberta Salmi, lead author of the study, said in a statement. “If they are actually able to distinguish between people, there is hope.”

Investigating gorillas 

In her usual work with gorillas, Salmi and her team realized that the animals reacted negatively when specific humans entered into their indoor enclosure, specifically some veterinarians and a maintenance worker. But it wasn’t really clear whether they were reacting to seeing these people or also to their voices. Here’s where the new study comes in. 

From October 2015 to April 2016, the researchers worked with a group of twelve zoo-housed gorillas, four females and eight males, living at Atlanta’s Zoo. All had been part of the zoo gorilla population for all or most of their lives. Nine were born there, the oldest male and female were each taken from the wild, and one female was transferred from another zoo.

The team played three types of audio recordings to the gorillas. People who were unfamiliar to the animals, people that the gorillas knew but had negative interactions in the past, including the previously mentioned veterinarians, and long-term keepers that worked with the gorillas for more than four years and had a good bond with them.

All the participants of the study recorded the same phrase that was then played to the gorillas: “Good morning. Hello,” which is how keepers usually greet the animals. While they didn’t react much to the voices of the keepers, the gorillas showed signs of distress such as vocalizations when hearing the voices of other people they didn’t know or had bad experiences with. 

The gorillas stopped doing whatever they were doing, such as eating their treats, and looked toward the sound to identify whether the voices were a threat. This is what usually happens in the wild, Salmi said, and something humans do too. If we hear that somebody is in our house that we weren’t expecting, we stop anything we were doing to see what’s going on. 

“Some primates are able to distinguish and have different reactions to humans, according to whether they are hunters or researchers,” Salmi said. “If wild gorillas are able to distinguish between people who behave differently, not only by sight but also by voice, it would be extremely helpful. It would help me sleep better to know that researchers aren’t making the gorillas more vulnerable to hunters.”

The study was published in the journal Animal Cognition.