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Gorillas react more strongly to the voices of unfamiliar humans and those they like less, researchers have found.
A study of zoo animals found that they act more warily around these individuals, offering hope that those studying the apes in the wild aren't putting the animals at greater risk of poaching.
Gorillas are able to tell humans apart, a new study suggests, becoming four times more stressed when they hear disliked or unfamiliar voices.
Apes at Atlanta Zoo were much quicker to react to the voices of visitors and vets over those of humans they liked, such as their keepers, and would stare at the source of the sound for longer.
Researchers believe that this knowledge will ensure that wild gorillas react with caution to poachers, even if they have good relationships with other humans in the wild.
'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.
'If they are actually able to distinguish between people, there is hope.'
Dr Natalie Cooper, a mammologist at the Museum who was not involved with the study, welcomed the study but said that more research was needed.
'My issue with a lot of these kinds of studies is that they're using very few individuals, all from one zoo, and they're not testing them with that many people,' she says. 'It makes sense as they don't know that many people, but I'd be interested to see if this applied to gorillas at other zoos and if this applied to just this particular group.'
The research, led by scientists at the University of Georgia, was published in Animal Cognition.
While communication between members of a species is ubiquitous across the natural world, the ability to interpret what other species are saying is less common.
Though animals may not know exactly what another species is saying, some are able to understand the general meaning of a call. For instance, two species of American rodent, the yellow-bellied marmot and golden-mantled ground squirrel, are able to recognise and react to alarm calls made by each other.
Some species can also understand specific contexts within the wider message. White-browed scrubwrens and superb fairy-wrens, both found in Australia, react more strongly if a more urgent alarm call is made by the other bird species.
However, eavesdropping on others can have its consequences, with the fork-tailed drongo occasionally mimicking the alarm calls of other species amid genuine alarm calls. By crying wolf every so often, the bird is able to steal food from meerkats by making them flee to their burrow when the mammals hear its fake alarm call.
Broadly speaking, however, understanding the voice of another species has its advantages. Humanity has used this to its advantage by training domesticated animals to carry out specific actions on command, such as teaching a dog to sit.
In the wild, however, getting animals used to a human voices can be an issue. When researchers are studying wild apes, such as gorillas, their aim is to get the animals used to human presence to enable the scientists to watch and record their behaviour. This is vital to better understand the apes and ultimately allows for better protection and management of their populations.
But there are also worries that this familiarity with humans leaves gorillas open to being taken advantage of. As a result, researchers wanted to investigate whether gorillas could tell the difference between a familiar scientist researching them and a poacher wanting to hunt them.
They did this by using a group of gorillas living in captivity at Atlanta Zoo, and playing them a recording of one of three types of people saying a phrase similar to that used by their keepers: 'Good morning. Hello.'
This phrase was either said by a zookeeper they were positive towards, another zoo worker such as a vet or maintenance worker they were negative towards, or a voice they hadn't heard before. Each gorilla in the group heard each type of voice once in a random order.
After the experiment was complete, researchers found that the gorillas were more likely to look towards the speaker when an unfamiliar or negative voice was being played. Negative voices elicited around twice the looks a positive voice did.
They also looked at the speaker for longer on average when unfamiliar or negative voices were played, with unknown voices receiving the most attention. They also looked towards the speaker much more rapidly under these conditions.
'If there's any sound that seems threatening or unfamiliar, they stopped what they were doing and focused their attention on it,' Roberta says. 'It's something humans do too. If it's not a threatening sound, I keep doing my own business. If I hear that there is someone in my house, I immediately stop what I'm doing to hear what's going on.'
The unfamiliar and negative voices caused around four times as many behavioural changes as a positive one, which was interpreted by the researchers as a sign of increased stress.
Natalie, however, says that the interpretation 'feels a little bit anthropomorphised.'
'If a human hears the voice of someone they know, they might move towards it, but I'm dubious that animals in the field are focusing on voice cues,' she says.
'Gorillas are going to be hearing anything moving through the bush, not just voice, while scent is also really important for them. I think habituated gorilla groups are not as focused on the voices, just the presence of people close to them, so I don't think it will necessarily be the most helpful thing in the world.'
The researchers, however, contend that their findings mean the animals' wild relatives are able to identify friendly humans from foes.
This is vital to the survival of the two species of gorilla, both of which are critically endangered. Their number one threat is poaching, with the apes hunted for bushmeat and sometimes to capture baby gorillas after the parents are killed.
Though this study will not protect gorillas from the guns of poachers, it does offer some reassurance that those fighting to protect them will not put their charges in any additional danger.