Nile Crocodiles Recognize and React to the Sound of Crying Babies | Science

As one Nile crocodile rests, another perks up near a river in Tanzania.
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In the animal kingdom, including humans, parents have a remarkable ability to detect distress in their offspring. The cries of human babies, as well as those of chimpanzees and bonobos, are so persuasive that even creatures unrelated to them, such as Nile crocodiles, recognize and respond to them. However, to a crocodile, a crying human baby may be more reminiscent of a meal rather than a call for help.

A recent study reveals that crocodiles are immediately drawn to the cries of a human baby because these distress signals trigger a predatory response in the hungry reptiles. Surprisingly, certain female crocodiles may also respond as their maternal instincts are oddly appealed to by the cries. Various species, from humans to birds to crocodiles themselves, employ distress vocalizations to express danger to their own kind. The report, published in the Proceedings of the Royal Society B: Biological Sciences on Tuesday, adds to the intriguing notion that there must be something universally understood in these calls that transcends closely related species.

Scientists conducted experiments using audio recordings of cries from human, chimpanzee, and bonobo babies. The researchers discovered that Nile crocodiles not only paid attention to these cries, but also exhibited swift reactions when they heard the sounds of distressed infants. While some of these reactions may have been predatory, others could be attributed to mothers reacting to familiar sounds resembling their own offspring in need. Élodie F. Briefer, a behavioral ecologist from the University of Copenhagen, who was not involved in the study, suggests that the crocodiles’ response could be an innate reaction to either a distressed prey or the cries bearing similarities to their own young.

The researchers, led by Julie Thévenet from Claude Bernard University Lyon, collected cries of various levels of distress from bonobo infants recorded in European zoos and chimp calls from a wild population in Uganda’s Kibale National Park. Human cries were also sourced from different situations, ranging from everyday activities like bathing to more stressful events like receiving shots at the doctor’s office. The team analyzed the calls, identifying 18 different acoustic variables such as pitch, syllable count, duration, and presence of chaotic or harmonic sounds.

The researchers then played the recorded cries through speakers placed at CrocoParc in Agadir, Morocco, an outdoor facility housing around 300 Nile crocodiles. These reptiles are known to have excellent hearing. Many of the crocodiles responded promptly to the cries. Some investigated the speakers from the surface, coming within inches and staring intently at the devices. Others approached the speakers underwater, displaying what appeared to be predatory behavior, with some even attempting to bite the speakers. However, certain responses did not appear overtly predatory.

“It cannot be entirely ruled out that some individuals, particularly females, responded in a parental care context,” state the authors of the study. Female crocodiles, and occasionally some males, do respond to the distress cries of their own young, which share acoustic features with the calls of primate infants. The fact that Nile crocodiles are so attuned to the cries of distressed human babies may indicate that these reptiles have been attentive to such calls for a significant period of time, posing a threat to our ancestors throughout evolution.

The team employed software analysis to examine the acoustic elements of the recordings and compared the crocodiles’ reactions with those of a group of human subjects exposed to the same cries. Surprisingly, when it came to bonobo cries, crocodiles displayed more accurate analysis of the infant’s distress levels compared to humans, despite the evolutionary distance between the two species. Humans tended to rely heavily on pitch, overestimating the level of distress in bonobo babies since they typically produce higher-pitched cries. In contrast, crocodile reactions were triggered by other aspects of the cries that were more indicative of distress in primates, such as chaos. Élodie F. Briefer remarks, “Crocodiles, with their broader focus on acoustic features, demonstrated more effective cross-species comprehension, unlike humans who tend to focus more on pitch.”

Charles Darwin himself hypothesized that distress calls across different species may have ancient evolutionary roots, dating back to the earliest terrestrial vertebrates. Natural selection could have favored those vocalizations that were effective across diverse species. Élodie F. Briefer explains that vertebrates often exhibit consistent reactions to stress, which can be observed through changes in their vocal apparatus. This indicates that even distantly related species can understand and communicate with each other.

Scholars have discovered other intriguing connections that support this idea. Brain imaging studies have revealed that dogs can recognize human emotions by listening to their voices. This may not be entirely surprising given the extensive co-evolution between humans and dogs. However, a 2019 study conducted by Piera Filippi, a cognitive scientist at the University of Zurich who specializes in vocal and emotional communication across animal species, found that chickadees, birds that learn vocalizations from their parents, could identify distress calls from widely divergent species, including humans and giant pandas, which they had never encountered before.

While the aforementioned study provides fascinating insights, Filippi, who was not involved in the Proceedings B report, acknowledges that our understanding of the behavioral and cognitive responses of many species, including crocodiles, to different vocalizations remains limited. Filippi states, “Testing a wider range of species, especially those far removed from primates on the phylogenetic tree, will provide a more comprehensive understanding of the evolution of vocal communication, particularly emotional communication.”

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