  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.
