A gorilla’s world can be very loud and dramatic when they need to make themselves seen and heard through the dense vegetation they live in.

Screams, hoots, roars and growls are all part of their repertoire but so too are smaller, subtler sounds and gestures that are just as important for being understood in their social groups.

These vocalisations, or verbal communications, can be associated with different behaviours including play, feeding, anger and alarm, and can also be used alongside specific gestures and expressions.

The vocalisation that people are perhaps most aware of is the screaming charge which is the big, dramatic 'I’m angry and frightened and you’re a danger to me, so I’m going to scare you away' display

Ian Redmond is a tropical field biologist and conservationist and has been associated with mountain gorillas (Gorilla beringei beringei) for more than 30 years. He is best known for his time as a research assistant with the famous primatologist Dr Dian Fossey, who identified 17 gorilla vocalisations.

Mr Redmond had to learn those sounds and the body language which often accompanied them to be able to carry out his research in close proximity to the gorillas. He also helped train the actress Sigourney Weaver ahead of winning the part of Dr Fossey in the 1988 film 'Gorillas in the Mist'.

Here is his guide to understanding what gorillas have to say.

Belch vocalisation (BV)

Known as a contact call, the sound got its name because it was first thought to be the gorillas belching. As Mr Redmond explains in the clip above, recorded for BBC Radio 4’s Natural Histories programme, this is the call he used most frequently in his encounters with gorillas, as part of what he calls “gorilla etiquette”.

“The principles of that are to use body language to show non-aggression,” he told BBC Earth.

“So you keep low, you don’t stand bipedally so you’re towering over them, you fold your arms and look away from them and glance out of the corner of your eye to show that you’re not threatening them or being aggressive, and you announce your presence, you tell them that you’re coming so that you don’t take them by surprise and you use gorilla vocalisations to do that.”

Scream

“The vocalisation that people are perhaps most aware of is the screaming charge which is the big, dramatic 'I’m angry and frightened and you’re a danger to me, so I’m going to scare you away' display," says Mr Redmond.

"That’s what got them their reputation as being the monsters of the forest because when you’ve got a 200kg (440lb) animal with very large teeth going ‘waaaah’ very loudly at you while hurtling towards you, it is very intimidating – that’s what it’s designed to be.”

Dian Fossey’s work with the mountain gorillas in Rwanda changed this image. Gaining the animals’ trust, she was able to get close to them and discover that their intimate communication was much more complex than these alarm bell cries had led other observers to believe.

"Back-off"

This warning call was an important one for Mr Redmond’s student Sigourney Weaver to learn ahead of her starring role in 'Gorillas in the Mist'. Along with the contact calls and other reassuring sounds he taught her to do, he says she also needed to know how to react to a gorilla vocalisation.

“If a gorilla is about to eat a plant, they’re sitting in one place eating all the plants within arm’s reach and they usually eye up the next place they’re going to move to, and if somebody else goes to eat that plant then you’ll hear ‘uh,uh,uh’," Mr Redmond explains.

"It means ‘back off, I’ve got my eye on that’, in the context it means ‘stop that’ or ‘move away’. It’s sometimes called a pig grunt because when they get going it almost sounds like pigs grunting or a cough grunt. It’s very clear, you don’t mistake it."

Play chuckle

All gorillas play and express playfulness, although this characteristic chuckling or laughing sound is usually made by youngsters.

“It's a very hoarse 'ha, hahaha, hahahaha, ha, ha' and that’s both an expression of fun and I guess joy, but it can also be used as an invitation to play," Mr Redmond says.

"So a gesture to come and play accompanied by a play chuckle will elicit a response, and that’s a clear communication – both verbal and using gestures.”

Contentment/singing

The contentment sound, which is often termed singing, can extend from the belch vocalisation call when gorillas are at ease and can take many different forms. Mr Redmond thinks gorillas can recognise each other’s voices so they can tell who is making the reassuring noises from the tone of their voice.

“When gorillas are relaxed and the sun’s shining and there’s lots of food they’ll start singing, it’s going ‘mwaaah, mwahwah, hwah, hwah, hwah, hwah’, they’re usually eating when they’re singing, because they sing when they’re happy and they’re happy when they’re eating," he says.

“It’s high-pitched, almost like a dog whining sound and sort of like 'mwah, mwahmwah', low rumbling sounds.”

Silence

If a gorilla hears something that it suspects might be dangerous it will not immediately give alarm calls, instead it will stop what it is doing and look towards the source of the sound and listen.

“Very, very quickly members of the family realise that the lack of sound and the focused attention on a particular spot in the forest might mean danger,” says Mr Redmond.

“So silence is actually a means of communication when it’s accompanied by a direct gaze and an expression of concern and if that is then reinforced with more worrying signals, more movement or evidence of a poacher or a threatening animal, then there might be an alarm bark and if they’re really frightened then they scream and run away.”

If the danger remains then the silverback will again sit quietly behind a bush or a tree and observe the threat, waiting for the perfect moment to jump out and give a frightening roar.

“A lot is inferred from position and expression with little or no vocalisation, they’re just not that vocal," Mr Redmond says.

"If an argument breaks out and they scream at each other it’s loud but most of the time there is little sound and the sounds are very soft, but they do gesture and signal by their body,” Mr Redmond says.

Chest beat

The gorilla’s most famous gesture is the chest beat, standing on two legs and hitting the chest alternately with their open hands, rather than their clenched fists – as has been portrayed in films.

“For a silverback the stylised display to show other gorillas what a big fine fellow he is, is to start off hooting, sometimes biting off a leaf, then the hoots speed up until they slur together and he stands up bipedally and does a rapid chest beat and then finishes off the display by hitting the ground or tearing off a sapling, or if someone’s sitting there maybe thumping that someone,” Dr Redmond explains.

This display can be directed towards other males as well as females, with nuances depending on whether the recipient is at a distance or close by.

Touch

In the video clip above, two females who are both nursing offspring use touch to share affection, while one uses it to offer an invitation to the other.

"It’s a very clear conversation, one goes over to the other and says 'hi' and then starts to move away and the other one says 'no don’t go' and the first one says 'well I’m going but why don’t you come with me', all of those words are unnecessary because of the gestures and the expressions of desire," Mr Redmond says.

"I don’t think it’s anthropomorphism to read that intent, what they’re saying to each other is perfectly clear from the gestures; it’s non-verbal communication. What we don’t know is if any of it is accompanied by specific vocalisations."

Mr Redmond admits there is still a lot to learn about gorilla communication. Studies of gorilla vocalisation have been forced to take a back seat to research dedicated to their conservation, but with their populations now shown to be more stable he says the time is ripe for more work to be done. 

They clearly have the capacity to understand language

“If you define language as having nouns and verbs there isn’t evidence of that in the wild but in captive language studies apes, including gorillas, very quickly seem to understand syntax and learn the gestures and symbols for particular things and learn to associate human words, spoken words with things,” he says.

“So they clearly have the capacity to understand language and it would be surprising if that was there and there was no natural use of it.”

You can listen to the first episode of Natural Histories, 'Monkeys and Apes', on Tuesday 2nd June at 11:00 BST on BBC Radio 4.

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