While many animals are known to express their emotions through their voice, it remains to be understood how one animal perceives the emotions in the voice of another under different scenarios.
The new study, published on Wednesday in the journal Proceedings of the Royal Society B: Biological Sciences, used playback experiments to assess how Nile crocodiles reacted to cries of human and ape babies.
Despite the distant evolutionary relatedness between crocodiles and primates, researchers from CNRS in France said crocodiles may be capable of detecting the degree of distress in ape and human baby cries more reliably than humans themselves.
Scientists suspect this may be an innate ability in crocodiles to gauge the distress of potential prey.
In the study, researchers collected infant cries from a research database, with each expressing different levels of distress.
Each of these cries represented infants calling for their mothers’ attention with differing degrees of urgency.
Scientists then identified 18 different variables in the cries, including their pitch, as well as the number and duration of syllables, chaotic and harmonic sounds.
They then set up speakers playing these sounds at Morocco’s CrocoParc – an outdoor facility with numerous ponds housing about 300 Nile crocodiles.
Researchers found the crocodiles reacted swiftly to the playback and also use different sound criteria compared to humans to assess the distress encoded in the infant cries.
In particular, scientists said crocodiles are more attracted to infant sounds that seem more chaotic with “more intense energy in the high frequencies of the spectrum”.
They said this is in contrast to humans, who mainly use the pitch in an infant’s sound to judge the level of distress encoded in the cries.
“The acoustic features driving crocodile reaction are likely to be more reliable markers of distress than those used by humans,” researchers wrote in the study.
Experiments also suggested crocodiles may not be particularly sensitive to whether a call was from a bonobo, chimpanzee or human, but rather pay specific attention to features of the sound that indicate distress or chaos.
The new findings, according to researchers, is “interesting” for two reasons.
“First, it marks a difference with the way humans assess the level of distress in infant cries,” scientists said.
“Humans assign a distress level primarily from the pitch of the cry. The higher the pitch of a cry, the more humans judge the cry as expressing high distress,” they explained.
Since crocodiles do not pay attention to pitch, researchers said they seem particularly adapted at estimating the degree of distress regardless of the species.
The reptiles also have a poor perception of high frequencies, scientists said, adding that this may explain why human babies’ high arousal cries induce stronger and swift predatory responses from crocodiles.
However, the responses of some of the crocodiles in the study may have been a motherly reaction to the sounds of a baby in need.
The findings shed more light the different ways in which emotional communication via sound evolved across species.
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