Charles Darwin once noted that all terrestrial animal species seem to share a set of vocal emotional expressions to such a degree that many make similar sounds in similar situations. Now, researchers may have proven Darwin right, after finding that people can recognize differing emotional arousal in species as diverse as frogs and giant pandas.

The fact that animals can tell the emotional state of other animals, even if they are different species, has been shown before. One study found that when playing the noise of a crying infant, a mother deer will rush in protectively, even if the vocalizations are from baby seals, monkeys, or humans. But until now, this was only thought to apply to mammals recognizing other mammals' calls.

A new paper, published in Proceedings of the Royal Society B, has looked into whether or not humans are able to correctly tell the state of arousal in birds, reptiles, and amphibians too.

Now, before you jump to any conclusions, the definition of "arousal" in this situation is probably not what you’re thinking. The researchers define it as “a state of the brain or the body reflecting responsiveness to sensory stimulation, ranging from sleep to frenetic excitement.” On the lower end of arousal, there are emotions such as boredom, while the higher end can include anger.

For the study, the team got three groups of people who spoke either German, English, or Mandarin to listen to pre-recorded animal vocalizations and then to tell the researchers what emotional state the creatures were experiencing. The calls came from a wide variety of species representing over 350 million years of evolution, including a black-capped chickadee, American alligator, common raven, hourglass treefrog, African bush elephant, barbary macaque, domestic pig, giant panda, and finally humans.

Amazingly, they found that all of the participants, regardless of what language they spoke, were able to reliably identify when all of the species – spanning all classes of air-breathing tetrapods – were in high states of arousal. This suggests that the ability to recognize differing emotional states across all species is deeply rooted in humans.  

The researchers think that this might mean there is some common ancestral cause behind the ability to recognize emotional states from such a wide range of species. It might have helped our ancestors, for example, determine whether or not the noise they hear is an animal being threatened, which could prepare them for a threat that lies ahead.