Study sheds light on the evolution of emotion
Why we can tell if animals are upset by listening to their calls
CHARLES Darwin, when musing about the evolution of emotion, pointed out the similarities in how animals mouth off. “With many kinds of animals, man included,” he wrote in 1872, “the vocal organs are efficient in the highest degree as a means of expression.” Pigs grunt while eating, he noted. They scream while terrified. Darwin theorised that emotional sounds could be traced back to a common animal trait.
Now researchers are investigating the idea Darwin posed more than 100 years ago. They’ve demonstrated that humans can detect an animal’s arousal – which can range from low (asleep) to high (frenetic).
We can sense excitement in the sounds of species as different as frogs and pandas. The scientists also suggest these sounds are part of a very old “signalling system”.
“Our study shows that humans are naturally able to recognise emotional arousal across all classes of vocalising animals,” said Piera Filippi, who studies the evolution of cognition and communication at the Vrije University Brussels in Belgium. Filippi and her colleagues, psychologists and biologists from Germany, France, the Netherlands and Canada, have just published their study in the journal Proceedings of the Royal Society B.
“This outcome may find an important application in animal welfare, suggesting that humans may rely on their intuition to assess when animals are stressed.”
The scientists asked 75 college-age people to listen to sounds from nine species. To check for consistency across languages, the group included English, German and Mandarin-speakers. The researchers collected 180 recordings of animal vocalisations, reflecting high or low levels of excitement.
These were desperate or negative screams: the sounds of frogs in competition for mates, monkeys reacting to danger or ravens confronted by a dominant bird. Representing our species were actors speaking in Tamil, asked to read lines as if increasingly upset. The subjects had to identify which vocalisation, out of paired sounds from the same species, represented higher arousal.
“Humans performed better than expected by chance,” Filippi said. They accurately selected humans acting out emotional distress (95% correct) over humans talking regularly.
But the excited giant panda (94%) was close behind, followed by the aroused hourglass tree frog (90%), African bush elephant (88%), American alligator (87%), black-capped chickadee (85%), pig (68%), common raven (62%) and a monkey called a Barbary macaque (60%).
“These results suggest that fundamental mechanisms of vocal emotional expression are shared among vertebrates,” the authors wrote. Put broadly, every unhappy land-dwelling four-legged animal sounded unhappy in a similar way.
Harold Gouzoules, an animal behaviour expert at Emory University in Atlanta who was not involved with the research, called this study a “significant contribution” towards answering the questions that had puzzled Darwin. “It goes beyond previous studies of mammals and birds to include reptiles and amphibians,” Gouzoules said. “That also makes it challenging to understand.”
It struck Gouzoules as odd that humans were better at identifying arousal in tree frog sounds than for the monkeys. But Filippi said that it was possible the difference in emotional intensity of the low and high monkey calls was “less extreme” compared with the other species’ sounds. “However,” she said, “finding that humans perform better in evolutionarily more distant species corroborates the hypothesis that this is a biologically-rooted ability.”
What’s more, Gouzoules argued the study does not get to what Darwin would have considered emotion. “If emotions are equivalent to a carrot cake,” he said, arousal is like the “wheat before it becomes flour”.
Arousal is an important dimension of emotion, but “it doesn’t equate to assessing emotional processing,” he said. He offered the example of a chimp baring its teeth.
“A person will describe that as a smile,” he said. But “they’d get it dead wrong,” Gouzoules said. These chimp facial expressions are in fact “fear grins”.
The authors acknowledged that this research would benefit from physiological data. Having brain recordings or heart rate might paint a clearer picture of the animals’ true feelings.
Filippi also wants to repeat the experiment, but with a twist. “We are currently running the same study on black-capped chickadees. So we may soon know if these little birds can tell the difference between an excited or relaxed panda.” – Washington Post