Study: Nile Crocodiles Respond to Distress Calls of Human and Primate Infants

Nicolas Grimault, a bioacoustician at the University of Lyon, and his colleagues have discovered that Nile crocodiles are attracted to the cries of hominid (bonobo, chimpanzee and human) infants. The realization of this study published in the journal Proceedings of the Royal Society B: Biological Sciences took them to a zoo in Agadir, Morocco, which houses more than 300 Nile crocodiles (Crocodylus niloticus), a predator particularly well adapted to hunting primates and other mammals. The researchers placed speakers along four ponds. Each pond contained up to 25 crocodiles. They then played a series of chimpanzee (Pan troglodytes), bonobo (P. paniscus) calls to their mother and cries of human babies, recorded at the pediatrician’s office while they were being vaccinated.

The result is stunning: many crocodiles, both male and female, reacted to the (recorded) cries by seeking the source of the sound, quickly approaching the speaker and sometimes even biting it. On average, about one in five crocodiles reacted to recordings of human babies with low levels of distress, while about a third reacted to the cries of human babies in great distress. "This means that distress is something that is shared by species that are really, really far apart," said Nicolas Grimault, bioacoustics research director at the National Center for Scientific Research. Some reptiles were more inclined to react to recordings with acoustic characteristics known to be correlated with highly disturbed infants, such as dissonance, noise bursts and irregular tones reminiscent of radio interference, it is noted.

According to the researcher’s explanations, the reptiles seemed to identify the most afflicted prey based on their cries. A smart strategy for an animal that is a very opportunistic hunter. "A baby may not cry at the top of its lungs if it is with its mother," adds Stephan Reber, a cognitive zoologist at the University of Lund, who did not participate in the study. According to him, this work raises the possibility that emotions may be communicated from one species to another in more ways than scientists had thought. "If crocodiles can do it, it probably means that many other animals can do it too," he adds.