Crocodile perception of distress in hominid baby cries.

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.

Sound categorization by crocodilians.

Rapidly sorting the information contained in a stream of stimuli is a major challenge for animals. One cognitive mechanism for achieving this goal is categorization, where the receiving individual considers a continuous variation of a stimulus as belonging to discrete categories. Using playback experiments in a naturalistic setup, here we show that crocodiles confronted with an acoustic continuum ranging from a frog call to a crocodile call classify each acoustic variant into one of these two categories, establishing a meaningful boundary where no acoustic boundary exists. With GO/NO-GO experiments, we then observe that this boundary is defined along the continuum following learning. We further demonstrate that crocodilians rely on the spectral envelope of sounds to categorize stimuli. This study suggests that sound categorization in crocodilians is a pre-wired faculty allowing rapid decision-making and highlights the learning-dependent plasticity involved in defining the boundary between sound categories.

Voice-mediated interactions in a megaherbivore.

Planet Earth is becoming increasingly difficult for large animal species to inhabit. Yet, these species are of major importance for the functioning of the biosphere and their progressive disappearance is accompanied by profound negative alterations of ecosystems1 (Supplemental information). To implement effective conservation measures, it is essential to have a detailed knowledge of the biology of these species. Here, we show that the hippopotamus Hippopotamus amphibius, an iconic African megaherbivore for which little is known about social communication, uses vocal recognition to manage relationships between territorial groups. We conducted playback experiments on groups of hippos and observed their response to vocalizations from an individual of the same group (familiar), a group from the same lake (neighbor) and a distant group (stranger). We found that stranger vocalizations induced a stronger behavioral reaction than the other two stimuli. In addition to showing that hippos are able to identify categories of conspecifics based on vocal signatures, our study demonstrates that hippo groups are territorial entities that behave less aggressively toward their neighbors than toward strangers. These new behavioral data suggest that habituation playbacks prior to conservation translocation operations may help reduce the risk of conflict between individuals that have never seen each other.

Spatial release from masking in crocodilians.

Ambient noise is a major constraint on acoustic communication in both animals and humans. One mechanism to overcome this problem is Spatial Release from Masking (SRM), the ability to distinguish a target sound signal from masking noise when both sources are spatially separated. SRM is well described in humans but has been poorly explored in animals. Although laboratory tests with trained individuals have suggested that SRM may be a widespread ability in vertebrates, it may play a limited role in natural environments. Here we combine field experiments with investigations in captivity to test whether crocodilians experience SRM. We show that 2 species of crocodilians are able to use SRM in their natural habitat and that it quickly becomes effective for small angles between the target signal source and the noise source, becoming maximal when the angle exceeds 15∘. Crocodiles can therefore take advantage of SRM to improve sound scene analysis and the detection of biologically relevant signals.