Crocodiles were played the sound of human babies crying. The higher the infant's distress level, the more the carnivores would respond.

• Nile crocodiles react to the cries of infants from species such as bonobos, chimpanzees, and humans.
• New research shows that crocodiles also seem to distinguish levels of distress in cries.

New research suggests that crocodiles may be drawn to the sounds of crying babies.

Nile crocodiles were found to react to the cries of baby bonobos, chimpanzees, and humans — and they appear to be able to detect degrees of distress, research published in Proceedings of the Royal Society B, the Royal Society's main biological-research journal, found.

Researchers played the crocodiles audio recordings of infants crying and discovered they were drawn to those that seemed the most distressed.

The reactions could be interpreted as predatory by the large semiaquatic reptiles, but the researchers said it may also suggest a maternal instinct in female crocodiles.

Researchers used loudspeakers to play the cries to about 300 Nile crocodiles at Crocoparc in Agadir, Morocco, The Independent reported, with many of the crocodiles responding quickly.

The higher the infant's distress level, the more the crocodiles would respond.

"Our experiments obviously do not mean that crocodiles cannot be attracted by other signals than distress calls — they are opportunistic hunters," the authors wrote in the study. "But they suggest that the readiness of these animals to react increases with the presence of acoustic features marking a level of distress."

"It cannot be entirely ruled out that some individuals responded in a parental-care context," the researchers added.

The researchers compared the findings to another study in which researchers played the same cries for a group of humans. The study found that humans and crocodiles use different criteria to judge distress in other species and that humans' judgment tends to be less accurate.

While humans primarily responded to the pitch of the cries, crocodiles responded based on levels of "deterministic chaos, harmonicity, and spectral prominences."

The authors noted that crocodiles could recognize the distress levels of species very distantly related to them.

Nile crocodiles can grow to about 20 feet long and can weigh up to 1,650 pounds, National Geographic said.

The creatures "live throughout sub-Saharan Africa, the Nile Basin, and Madagascar in rivers, freshwater marshes, and mangrove swamps," it added.

According to National Geographic, Nile crocodiles generally live close to humans, meaning encounters happen relatively often — the publication says that "firm numbers are sketchy, but estimates are that up to 200 people may die each year in the jaws of a Nile croc."