Among humans and many other species, parents have a super sense when it comes to a crying baby. Something in that wordless call communicates distress so clearly that it sparks an instinctive response. And the cries of human, chimp and bonobo babies are so compelling that even other species recognize and react to them, including Nile crocodiles. However, to a croc, a human baby’s screams may sound less like a cry for help—and more like a dinner bell.
According to a new study, crocodiles quickly investigate a human baby’s wails because the sounds of distress trigger a predatory response from the hungry reptiles. Interestingly, some female crocs may also respond because the cries somehow appeal to their maternal instinct. From humans to birds to crocodiles themselves, infants of many species use distress vocalizations that let their own kind know about trouble. The report published Tuesday in Proceedings of the Royal Society B: Biological Sciences adds to the intriguing idea that there is something so universal in the nature of such calls that they are understood by other species—even those not at all closely related.
Using audio recordings of human, chimpanzee and bonobo babies’ cries, scientists found that Nile crocodiles not only paid attention but also quickly reacted when they heard infants in trouble. While some of those reactions were likely predatory, others may have been mothers reacting to familiar sounds of a baby in need. “They just react, more because it triggered some probably innate response,” says Élodie F. Briefer, a behavioral ecologist at the University of Copenhagen, who was not involved with the research. “That might be a predatory response, to a prey in distress, or it could be because the sound resembles a bit what their own offspring are doing.”
Julie Thévenet, of Claude Bernard University Lyon, and colleagues collected various infant cries, expressing varying levels of distress, sourced from a bioacoustics research database. Bonobo infants had been recorded in several European zoos, and chimp calls had been collected from a wild population in Uganda’s Kibale National Park. In each vocalization, the infants were soliciting their mothers with varying degrees of urgency, from begging for attention when mom was nearby to crying out during conflict with another individual. Human cries were likewise collected during different situations, from having a bath at home with parents to receiving shots at the doctor’s office.
The team analyzed the calls and identified 18 different acoustic variables, like pitch, number and duration of syllables, and chaotic and harmonic sounds.
They then set up speakers at CrocoParc in Agadir, Morocco, an outdoor facility with numerous ponds where some 300 Nile crocs are free to roam the grounds. After the park had closed for the day, the various primate sounds were played to groups of crocs, who are known to have excellent hearing.
Many of the crocodiles reacted quickly: Some investigated the speakers from the surface, coming to rest just inches from the devices and staring at them. Others approached the speaker underwater, in what appeared to be a predatory fashion—indeed, some actually tried to bite the speakers. But other responses didn’t seem obviously predatory.
“It cannot be entirely ruled out that some individuals (particularly females) responded in a parental care context,” the authors write in the study. Female crocs, and some males, do sometimes respond to the distress cries of their own young, which share some acoustic features with the calls of primate infants. That the crocs are so attuned to the cries of human babies in distress may also mean that the reptiles have been listening for such calls for a very long time, posing a danger to our ancestors throughout evolution.
“The Nile crocodile was indeed an abundant species in the African cradle where the human lineage developed,” the authors write. “Since the cries of babies of all species forming the human lineage probably shared acoustic characteristics with the cries of present-day human babies, they probably always represented attractive stimuli for crocodiles.”
By using software analysis of the recordings’ acoustic elements, paired with video of the crocodiles’ reactions, the team was able to delve into exactly which aspects of the various cries produced reactions in the crocs. They also were able to compare the reptiles’ responses with the responses of a group of human subjects to the same calls.
Surprisingly, in the case of bonobo cries crocodiles actually analyzed an infant’s distress more accurately than their human counterparts despite being far more distantly related. Because humans relied so heavily on pitch, they consistently overestimated the distress levels of bonobo babies, which generally tend to call out at higher pitches in any situation. The higher the pitch of the cry, the more humans falsely judged it to be conveying serious distress. But these differences in pitch didn’t make much difference to croc reactions, which were triggered by other aspects of the cries more characteristic of distress in the primates—such as chaos. “Where we more focus on the pitch, which works better among humans, in these recordings they show that the [audio features] that crocodiles use actually work better across species,” says Briefer.
Charles Darwin himself hypothesized that the ways different species call out in distress might have very ancient evolutionary roots, dating back to the earliest terrestrial vertebrates, because natural selection could promote those vocalizations that were effective even among very different species.
Vertebrates do often react to stress in consistent manners, says Briefer. “We get more tense, and these reactions change the vocal apparatus in the same ways as well,” she says. “That means that we can also understand each other … even across very distantly related species.”
Other research has found intriguing connections to support this idea. Brain imaging studies have shown that dogs can recognize human emotions by listening to our voices. That might be somewhat expected given our species’ long period of co-evolution. But one 2019 study co-authored by Piera Filippi, a cognitive scientist at the University of Zurich who studies vocal and emotional communication across animal species, found that chickadees, birds that learn vocalizations from their parents, recognized distress calls in widely divergent species including humans—and giant pandas, which they’d never before encountered.
But Filippi, who wasn’t involved in the Proceedings B report, also notes that, beyond a few studies, scientists still don’t know much about the behavioral and cognitive responses of many species, including crocs, to different vocalizations. “The more species we test, and the further apart they are from primates phylogenetically, the more complete picture we can get of how vocal communication, and in particular emotional communication evolved,” Filippi says.