Behavior

A Universal ‘Language’ of Arousal Connects Humans and the Animal Kingdom

New research suggests that the ability to intuit emotional states from vocalization is hardwired in humans and land animals.

New research not only supports Darwin’s views, but also identifies a universal “language” of arousal emitted and understood by amphibians, reptiles, birds, and mammals. The findings, published in the journal Proceedings of the Royal Society B, suggest that we are at least somewhat like the famous fictional character Doctor Dolittle, who could decipher animal communications with ease.

“Our study shows that humans are naturally able to recognize emotional arousal across all classes of vocalizing animals,” said lead author Piera Filippi, a postdoctoral researcher at the University of Aix-Marseille and the Max Planck Institute for Psycholinguistics. “This outcome may find an important application in animal welfare, suggesting that humans may rely on their intuition to assess when animals are stressed.”

Prior research additionally suggests that animals understand human emotional vocal expressions. Pet owners are often more attuned to this, given the reactions that dogs, cats, horses, birds, rodents, and more have to a range of owner outbursts, from angry scolding to happy praise.

For the latest study, Filippi, senior author Onur Gunturkun, and their team went beyond investigating such a familiar collection of animal pets. They instead gathered 180 recordings of vocalizations from nine different and very diverse species: hourglass treefrog, black-capped chickadee, common raven, American alligator, African bush elephant, giant panda, domestic pig, and Barbary macaque. People who spoke English, Mandarin, and German were then recruited to evaluate the levels of arousal communicated in each animal recording.

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The human listeners, no matter their native language, aced the tests. This indicates that human ability to assess vocal expressions of arousal — whether emitted due to sexual bliss, infant distress, or terror over a predator — is biologically rooted and somehow cemented in our DNA.

The scientists further conducted an acoustic analysis of the recordings. Pairing this data with the findings concerning the human listeners, the researchers discovered that people use multiple acoustic parameters to infer levels of arousal in vocalizations. Mainly, however, humans rely on fundamental frequency cues and the forcefulness of the sounds.   

The primary explanation for the ability, seemingly shared across much of the animal kingdom, has to do with the body-sound connection.

“Across vocalizing animals, higher levels of emotional intensity may induce the contraction of muscles that are required for vocal production,” Filippi explained. “This modification alters the quality of the sounds produced, often resulting in changes related to the perceived frequency of the sound.”

Whether the vocalizer is a pissed off pig or a playful elephant, the resulting sounds will then be directly affected by muscle contractions that, in turn, are impacted by the animal’s emotions at the time.

The researchers believe their study’s findings could extend to marine species with audible forms of communication. We therefore should be able to identify a dolphin’s levels of arousal, for example, just by hearing its vocalizations, and vice versa.

It may be that skills for vocal expression in marine and terrestrial species evolved from a common ancestor. This underlying connection, as well as the shared basic universal “language,” are tied to critical life-and-death concerns.

“The ability to recognize emotional content across diverse species may have favored the perception of heightened levels of threat or danger in the surrounding environment,” Filippi said. “This may have increased survival opportunities.”

Infant distress calls appear to be the most easily understood, from baby alligators calling for their mothers to young giant pandas squeaking, growling, barking, and huffing. The researchers suspect that this primal ability to decipher baby speak is “particularly salient to caregivers.”

Some animals can even manipulate humans via their infant-resembling cries. Earlier research, for example, found that cats can purr in the same frequency range of a crying baby when soliciting food from their owners.

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It remains unclear if insects share the ability of other animals in deciphering the arousal levels of various species’ communications. Filippi explained that insects produce sounds that differ in the level of emotional intensity through stridulation, which refers to vibration resulting from rubbing two body structures against each other. Since the sound-production mechanism is different from that of most other animals, insects may not be included in the universal “language” that connects other creatures.

Another identified disconnect exists between how humans mentally process speech versus nonverbal emotional communications like screams of pain, pleasure, and fear. “Spoken language can be highly emotional,” Filippi said. “However, the fine-tuned articulatory movements involved in speech production may constrain the perception of emotional intensity encoded in the signal.”

Future research could help to determine if intensely emotional forms of speech, such as certain spoken poems or songs, help to blur those probable constraints. Future studies might also lead to improved technologies for emotion recognition and the production of synthesized speech.

Currently, Filippi and her colleagues are collaborating with another research team from the University of Alberta to determine how accurately black-capped chickadees assess human vocal expressions of emotion. It could be that common garden songbirds, along with other animals, are paying more attention to us and our expressed moods than we might tend to think.

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