Crocodile perception of distress in hominid baby cries

TL;DR

This study demonstrates that Nile crocodiles can detect distress in hominid infant cries and respond based on specific acoustic features, revealing cross-species sensitivity to distress signals and differences in decoding criteria between crocodiles and humans.

Contribution

It is the first to show crocodiles' ability to perceive distress in distant vertebrate species' vocalizations and identifies key acoustic features influencing their responses.

Findings

Crocodiles are attracted to infant hominid cries.

Response intensity depends on specific acoustic features.

Crocodiles and humans use different criteria to interpret distress.

Abstract

It is generally argued that distress vocalizations, a common modality for alerting conspecifics across a wide range of terrestrial vertebrates, share acoustic features that allow heterospecific communication. Yet studies suggest that the acoustic traits used to decode distress may vary between species, leading to decoding errors. Here we found through playback experiments that Nile crocodiles are attracted to infant hominid cries (bonobo, chimpanzee and human), and that the intensity of crocodile response depends critically on a set of specific acoustic features (mainly deterministic chaos, harmonicity and spectral prominences). Our results suggest that crocodiles are sensitive to the degree of distress encoded in the vocalizations of phylogenetically very distant vertebrates. A comparison of these results with those obtained with human subjects confronted with the same stimuli further indicates that crocodiles and humans use different acoustic criteria to assess the distress encoded in infant cries. Interestingly, the acoustic features driving crocodile reaction are likely to be more reliable markers of distress than those used by humans. These results highlight that the acoustic features encoding information in vertebrate sound signals are not necessarily identical across species.