A panda at Berlin's zoo in July. (Axel Schmidt/Reuters)

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 theorized 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 researchers say. The scientists also suggest that these sounds are part of a very old “signaling system.”

“Our study shows that humans are naturally able to recognize emotional arousal across all classes of vocalizing 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, published their study Tuesday in the journal Proceedings of the Royal Society B: Biological Sciences. “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 vocalizations, reflecting high or low levels of excitement.

This was not excitement like the kind you'd find at Six Flags. Instead, 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 vocalization, out of paired sounds from the same species, represented higher arousal.

“Humans performed better than expected by chance,” Filippi said. As you might suspect, they accurately selected humans acting out emotional distress (95 percent correct) over humans talking regularly.

But the excited giant panda (94 percent) was close behind, followed by the hourglass tree frog (90 percent), African bush elephant (88 percent), American alligator (87 percent), black-capped chickadee (85 percent), pig (68 percent), common raven (62 percent) and a monkey called a Barbary macaque (60 percent).

“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, a bioacoustician and animal behavior expert at Emory University in Atlanta who was not involved with this research, called this study a “significant contribution” toward 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 that this study does not get to what Darwin would have considered emotion. “If emotions are equivalent to a carrot cake,” he said, then 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. They would read it as emotional — they'd get that, easy,” 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, in order to assess their ability to identify emotional intensity,” she said. So we may soon know whether these little birds can tell the difference between an excited or relaxed panda.

Listen to the animal calls — can you tell which sound is more excited?

A domestic pig:

A black-capped chickadee:

An hourglass tree frog:

(For the pig, the first track is more excited. For the chickadee, the first, too. For the frog croak it's the second file.)

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