The Nile crocodile, Africa’s largest reptile, has an amazing ability to respond to the cries of cubs of different mammal species, including humans. Scientists from France conducted a study in which they found that the crocodiles’ response depends on specific acoustic details of the cries, which can help predators assess the degree of suffering of their prey before attacking.
As part of the study, the scientists used recordings of the cries of bonobos (Pan pan paniscus), chimpanzees (Pan troglodytes) and humans. It turned out that Nile crocodiles are most attracted to cubs in severe distress.
Earlier studies have shown that crocodiles respond to sounds of distress in potential prey and often move towards the source of the sound to make an easy snack. Scientists were interested in whether crocodiles react in the same way in primates.
The sounds of primate cubs as young as 4 years old were transmitted to groups of Nile crocodiles (Crocodylus niloticus) through speakers installed near the ponds where more than 300 of these animals live in a Moroccan zoo. Behavioral ecologist Julie Thévenet of the University of St. Etienne in France and her colleagues then studied the crocodiles’ reactions.
C. niloticus is the largest reptilian freshwater predator in Africa. Adult males typically weigh between 225 and 750 kilograms, although there are records indicating they can reach weights of up to 1,000 kilograms and a terrifying length of 6.1 meters.
Previously recorded sounds of baby primates have been recorded in a variety of situations to cover a wide range of emotions. For example, the cries of human infants in situations of mild distress when they take a bath at home and severe distress when they receive vaccinations.
Their cries were categorized based on 18 acoustic variables and grouped into three “acoustic dimensions” based on pitch, harmonicity, and deterministic chaos.
Tevene and her team evaluated the crocodiles’ response based on the percentage of individuals in the group that responded to the recording. They did not keep a record of the sexes, but when the screams sounded, both males and females turned their heads or moved toward the noise, and some even tried to bite the speakers.
However, the scientists found that the response of the large reptiles depended on the nature of the screams heard. Only about a fifth of the crocodiles responded to recordings of human babies crying in mild distress, while about a third of the crocodiles responded to recordings of babies in severe distress.
The results of the human study confirm that crocodiles and humans use different acoustic criteria when assessing the degree of distress of infant crying.
The pitch of crying is a standard metric that humans use to assess their own children’s emotions. In fact, humans often misunderstand the high pitched cries of bonobos as being highly distressed.
While pitch can often indicate distress, it varies between species and individuals and is less reliable than acoustic characteristics such as incoherence, noise and jitter, which research has shown are more likely to elicit a response in crocodilians.
