Some things need no translation. No matter what language you speak, you can probably recognize a fellow human who is cheering in triumph or swearing in anger. If you are a crocodile, you may recognize the sound of a young animal crying in distress, even if that animal is a totally different species — like, say, a human baby. That sound means you are close to a meal.
In a study published Wednesday in Proceedings of the Royal Society B, researchers put speakers near crocodiles and played recordings of human, bonobo and chimpanzee infants. The crocodiles were attracted to the cries, especially shrieks that sounded more distressed.
“That means that distress is something that is shared by species that are really, really distant,” said Nicolas Grimault, a bioacoustic research director at the French National Centre for Scientific Research and one of the paper’s authors. “You have some kind of emotional communication between crocodiles and humans.”
These infant wails most likely drew crocodiles because they signaled an easy meal nearby, the authors say. But in some cases, the opposite may have been true: The crocs were trying to help.
The animals in the study were Nile crocodiles, African predators that can reach up to 18 feet long. Understandably, the researchers kept their distance. They visited the reptiles at a Moroccan zoo and placed remote-controlled loudspeakers on the banks of outdoor ponds.
The researchers played recordings of cries from those speakers while groups of up to 25 crocodiles were nearby. Some cries came from infant chimpanzees or bonobos calling to their mothers. Others were human babies, recorded either at bath time or in the doctor’s office during a vaccination.
Nearly all of the recordings prompted some crocodiles to look or to move toward the speaker. When they heard the sounds of human babies getting shots, for example, almost half the crocodiles in a group responded.
Dr. Grimault said the reptiles seemed most tempted by cries with a harsh quality that other studies have linked to distress in mammals.
“The more distress there is, the more easy the animal will be as a prey,” Dr. Grimault speculated. A helpless young mammal calling for its parent sounds like lunch.
Piera Filippi, a cognitive scientist at the University of Zurich who was not involved in the research, said the findings “speak to what could drive survival.” If crocodiles have evolved to listen for scared baby animals, that skill could help them stay fed and alive.
In her own work, Dr. Filippi and her collaborators have studied a similar question from the opposite direction: Can humans detect intense feelings in other species? From frogs to birds to pandas, people proved capable of distinguishing between calls from more distressed or excited creatures, versus calm ones.
Dr. Filippi noted that although she was happy to see the new findings, the study’s authors didn’t play any noncrying sounds to see how crocodiles reacted.
Another thing missing from the study is the sex of the crocodiles; the authors said that for practical reasons they couldn’t identify which were male or female. J. Sean Doody, a conservation biologist at the University of South Florida who focuses on reptiles, said he wished the authors had found a way around this problem. The information would have helped answer “perhaps the key question,” he said: “Is this response by the crocodiles predation, or is it parental?”
Nile crocodiles, although fearsome predators that sometimes kill and eat humans, are also caring parents. Every type of crocodile and alligator, in fact, tends its young. They may help their babies hatch, carry them to water and defend them from predators (including other crocodiles). Nile crocodile mothers respond to calls from their hatchlings, though fathers are sometimes involved, too.
Knowing whether male or female crocodiles were more likely to move toward a crying speaker might have clarified whether the animals were trying to hurt or help, Dr. Doody said.
It’s possible the answer was both. Some crocodiles tried to bite the speakers. However, Dr. Grimault said, “We saw one crocodile that came and tried to defend the loudspeaker from other crocodiles.” It put its body in front of the speaker and turned to face its fellow predators.
Perhaps, he and Dr. Doody said, crocodiles have evolved to recognize distressed cries because they are caring parents — but they have the flexibility to use the same skill for hunting.
“We underestimate animals in general,” Dr. Doody said. “We underestimate large, sinister-looking crocodilians that scare us especially.”
