Crocodiles Are Drawn to the Cries of Distressed Human Babies

Crocodiles react to the distress of a crying infant, even from very distant species.

Nile crocodiles were found to be able to tell the level of distress of a crying baby from several species, including bonobos, chimpanzees and humans, according to research published in the journal Proceedings of the Royal Society B: Biological Sciences.

As crocodiles will often respond to distress signals of their prey species and make a bee-line towards their potential easy meal, this indicates that the crocodiles can also determine distress in species not traditionally their prey.

Researchers from the University of Saint-Etienne and the University Lyon in France tested these sounds on crocodiles at the CrocoParc in Agadir, Morocco, playing sounds of crying infants of varying degrees of distress. They found that the majority of the crocs responded to the sounds, and these responses were more urgent with the more intensely distressed crying.

Most of the crocodiles reacted immediately, even more so when the cries were more distressed.

"Here we found through playback experiments that Nile crocodiles are attracted to infant hominid cries (bonobo, chimpanzee and human), and that the intensity of crocodile response depends critically on a set of specific acoustic features (mainly deterministic chaos, harmonicity and spectral prominences)," the authors wrote in the paper. "Our results suggest that crocodiles are sensitive to the degree of distress encoded in the vocalizations of phylogenetically very distant vertebrates."

Nile crocodiles are large reptiles native to freshwaters across 26 African countries. They usually reach between 11 feet and 16 feet long, but larger specimens over 20 feet have been recorded.

They usually eat fish and larger mammals feeding at the water's edge, including, occasionally, humans. It is estimated that all species of crocodiles kill 1,000 people each year worldwide, according to CrocBite, a worldwide crocodilian attack database, but the Nile crocodile is responsible for more attacks on humans than any other crocodilian species, Justin R. Dalaba and Frank J. Mazzotti wrote for the University of Florida's CrocDocs.

Between 1949 and 2016, 214 people were attacked by crocodiles in South Africa and Eswatini, a 2020 study found, and around half of all attacks by Nile crocodiles are fatal, compared to 25 percent of saltwater crocodile attacks.

The easiest member of any species to take as a snack is the infant, as they are the most defenseless. This study indicates that crocodiles have evolved to tune into the sound of a distressed infant—and to approach a more distressed infant faster before the mother reaches it.

The researchers did find one strange thing in their study: one croc suddenly turned around and faced others who had responded after initially responding herself. The researchers suggest that this was a similar move to one that a mother would perform when protecting her young.

"A comparison of these results with those obtained with human subjects confronted with the same stimuli further indicates that crocodiles and humans use different acoustic criteria to assess the distress encoded in infant cries," the authors wrote. "Interestingly, the acoustic features driving crocodile reaction are likely to be more reliable markers of distress than those used by humans. These results highlight that the acoustic features encoding information in vertebrate sound signals are not necessarily identical across species."

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