Crocodiles are so attuned to the cries of baby humans and the young of other great apes that they might recognise their distress better than people do.
Relying on changes in frequency rather than pitch, crocodiles move swiftly towards the sounds of distressed human, chimpanzee and bonobo infants. While most of the reptiles seem to want to gobble up the source of the crying, some might seek to protect them, says Nicolas Grimault at the University of Lyon in France.
“It’s fascinating that crocodiles are able to gather information from the cries of primate babies – and in fact they’re better at it than humans are, which was really unexpected,” he says.
To test the responses of crocodiles, Grimault and his colleagues used the recorded cries of 24 hominid infants – including 12 humans (Homo sapiens), six captive bonobos (Pan paniscus) in European zoos and six wild chimpanzees (Pan troglodytes) living in Uganda.
The bonobo and chimpanzee infants were crying at varying levels of distress in natural circumstances, such as during conflicts with other apes or when distant from their mother. The human infants were crying either during low-distress bath time at home with their parents, or during vaccination at a health centre.
The researchers ran analyses of 18 acoustic variables on each sound file to identify which patterns were associated with different levels of distress.
They then placed two large speakers around each of four ponds at the Crocoparc zoo in Morocco. Each pond hosted a group of up to 25 adult male and female Nile crocodiles (Crocodylus niloticus). The researchers played 30-second recordings of infant cries to each group, at least 10 minutes apart, starting an hour after the park closed.
The crocodiles responded to the cries from all three species by turning their heads, swimming towards the sound and sometimes even biting the speakers.
“Crocodiles are usually rather immobile animals,” says Grimault. “So, when you play a recording of a baby crying and you get five or seven crocodiles suddenly up and moving, that’s a pretty strong reaction.”
“If the baby primate is in great distress – injured and/or isolated – it will be slower trying to escape,” he says.
The crocodiles’ reactions were stronger when the recordings had more non-linear acoustic characteristics – sound irregularities such as vocal roughness, frequency jumps or subharmonics sometimes caused by going beyond the normal range of an animal’s vocal chords – and more intense energy in higher frequencies. These traits are associated with higher emotional arousal, says Grimault.
By contrast, humans assess distress levels based on the pitch of the cry, a characteristic that crocodiles seem to ignore, he says. As such, whereas humans can mistake the distress levels of infants of species with naturally higher-pitched cries, such as bonobos, crocodiles won’t be as easily confused, says Grimault.
The findings suggest that crocodiles – which were abundant in the African cradle where human lines first evolved – could have been a significant danger in the earliest human settlements, say the researchers.
Although most of the crocodiles reacted in a predatory way to the cries, one individual seemed to show protective behaviour. “We think it was a female, and she just really positioned herself in front of the speaker as if to defend it from the other crocodiles,” says Grimault. Female crocodiles often defend their young from cannibalistic males, he says.
The attractiveness of babies’ cries to crocodiles may not have been scientifically studied until now, says Grimault, but the general effect has long been known about. European hunters in Sri Lanka apparently used crying human babies to lure crocodiles into shooting range during the 19th century, he says.