Nicolas Mathevon

Although bird songs are classic models for understanding the evolution of vocal communication, their global diversity has long made the development of a unifying framework challenging. By analyzing the acoustic architecture of songs from more than 3000 passerine species worldwide, we show that this acoustic space can be structured around eight elemental motifs. The differential use of these motifs is driven by a combination of species’ biological traits (social organization, morphology, and mating system) and the physics of sound propagation. In tropical rainforests, environmental filtering for transmission efficiency favors structurally simple motifs, such as flat whistles. Conversely, in temperate regions, where high population densities facilitate close-range communication and short breeding seasons intensify sexual selection, the balance shifts toward complex, information-rich motifs, such as ultrafast trills, despite their susceptibility to acoustic degradation. Ultimately, the global geography of birdsong reflects a spatially varying equilibrium between physical environmental constraints and the biological drive for complex communication.

Acoustic communication in anurans is energetically costly and constrained by environmental factors, especially in open savanna ecosystems where extreme temperature fluctuations, desiccation risks, and wind-induced noise pose significant challenges to signal propagation and caller physiology. We analyzed the acoustic ecology of the widespread bubbling Kassina (Kassina senegalensis) in Mozambique, and compared it with that of other locations. We examined the influence of temperature, photoperiod and moonlight on calling phenology. Calling phenology was strongly seasonal driven, while daily patterns showed a bimodal strategy with peaks at dawn and dusk. This crepuscular activity suggests a behavioral adaptation to optimize signal transmission during stable atmospheric conditions and/or reducing desiccation. We identified a thermal window for vocalization (~ 23–27 °C), with activity sharply declining above 30 °C, suggesting a physiological upper limit. Moonlight also had an effect on calling activity. These findings highlight the behavioral and physiological plasticity of K. senegalensis in navigating savanna constraints, offering insights into ectotherm neuroethological adaptations amid climate change.

Passive Acoustic Monitoring (PAM) has advanced ecological research by enabling non-invasive recordings of wildlife vocalizations that provide insight into species presence, behavior, and reproductive activity. This remote-sensing approach is particularly valuable for species breeding in concealed habitats or remote areas where visual surveys are challenging. The Critically Endangered African Penguin, burrow-nesting seabird, exemplifies this challenge. Its highly vocal breeding behavior makes it an ideal case study for evaluating passive acoustic monitoring as a low-disturbance approach to estimating nest density. To evaluate this, we deployed Autonomous Recording Units at multiple sampling points across the Stony Point Penguin Colony, capturing soundscapes spanning different nest densities and environmental conditions. We then developed an automated detector for Ecstatic Display Songs (EDS), the species’ characteristic territorial song, using a Convolutional Neural Network trained on a multi-source dataset covering several breeding seasons, diverse acoustic environments, and both in situ and ex situ recordings. The model achieved high recall and precision and remained robust across diverse environmental conditions, supporting the use of heterogeneous training datasets for reliable bioacoustics detection. Using the automated EDS detections, we then investigated how vocal activity peaks relate to local nest density. A Generalized Additive Model revealed that EDS peaks strongly predicted nest density, with a nonlinear increase that plateaued at high calling rates. Importantly, models trained in one breeding season generalized well to the next. In conclusion, by integrating PAM with deep learning, this study provides a scalable, low-disturbance framework for estimating penguin nest density from soundscape data, supporting colony rangers in monitoring penguin colonies. Highlights: Large-scale acoustic monitoring captured the soundscape of a critically endangered seabird species. Deep-learning-based acoustic detection reliably identified key breeding vocalizations in field recordings. Peak vocal activity strongly and nonlinearly predicted active nest density across sampling points. The vocal-nest relationship increases rapidly and plateaued at high levels of vocal activity. This approach enables scalable, low-disturbance monitoring of seabird nest density.

Communication networks and acoustic interactions play a crucial role in shaping animal communities by mediating information exchange between individuals. These interactions contribute to the biophonic component of the soundscape—the acoustic environment of a place—which is increasingly analysed to understand ecosystem structure and functioning. However, soundscape analysis has rarely been applied to investigate the dynamics of information exchange within and between species. In this study, we tested whether two acoustic indices, Acoustic Entropy and Acoustic Complexity, could effectively reflect intra‐ and interspecies responses to playback experiments in a forest bird community. Our results indicate that, in the absence of non‐biotic background noise, playback elicited vocalizations that altered the soundscape sufficiently to be reflected in acoustic index values. These changes in the indices also reflect the number of bird species responding to playback. However, this effect was observed only in experiments conducted under low‐noise conditions, outside the breeding period, and when alarm calls were broadcast—conditions where the acoustic background is likely to be lower. In contrast, under conditions where the acoustic background is likely to be higher, the indices were unable to detect changes in bird acoustic behaviour elicited through playback. Our study highlights that acoustic indices may be a promising tool for investigating the acoustic networks emerging from homo‐ and heterospecific communication within an ecosystem, provided that precautions are taken regarding the experimental environment, ensuring that the acoustic background is not excessively high.

African penguins ( Spheniscus demersus ), a critically endangered seabird, form dense breeding colonies where nesting individuals maintain territories advertised through a loud vocalisation, the Ecstatic Display Song (EDS). This song encodes information about the caller’s identity and body size, but its potential role in distinguishing familiar from unfamiliar individuals – a key aspect of territorial defence – remains unclear. We conducted playback experiments to investigate whether African penguins exhibit the “dear enemy effect”, where territory holders show reduced aggression towards established neighbours compared to strangers. Using a mixed experimental design (independent and repeated measures), on a relatively large sample (N = 49), we observed no differences in behavioural responses, but penguins looked significantly longer towards the speaker when broadcasting a stranger’s EDS. However, in a conservative repeated-measures analysis involving a reduced sample (N = 24), this effect was no longer observed. Together, our findings do not provide clear evidence for a dear enemy effect. Nevertheless, the increased duration of the first looking towards strangers’ EDS in the larger sample suggests a potential for neighbour-stranger vocal discrimination. To further explore the absence of a dear enemy effect in the African penguin, we suggest replicating these experiments with speakers placed closer to nests to simulate greater territorial threats. Our study provides new insights into territorial behaviour, vocal discrimination, and therefore further understand vocal communication in competitive contexts in this critically endangered species. Significance statement: In territorial species, distinguishing familiar from unfamiliar individuals reduces costly conflicts, often achieved through acoustic signals encoding reliable identity cues. Many territorial birds exhibit the “dear enemy effect,” displaying heightened aggression toward unfamiliar conspecifics, but this phenomenon remains unexplored in penguins. African penguins ( Spheniscus demersus ), a critically endangered seabirds breeding in dense colonies, maintain territories and produce complex vocalisations, including the Ecstatic Display Song. Through playback experiments, we found no differences in African penguins’ behavioural responses between neighbours’ and strangers’ calls, suggesting an absence of the dear enemy effect consistent with the colonial ecology of this species. This study is a starting point for elucidate vocal behaviour in competitive contexts in this critically endangered species, informing acoustic-based monitoring and conservation strategies to support their declining populations.

Getting caregivers to respond to their pain cries is vital for the human baby. Previous studies have shown that certain features of baby cries—the nonlinear phenomena (NLP)—enable caregivers to assess the pain felt by the baby. However, the extent to which these NLP mobilize the autonomic nervous system of an adult listener remains unexplored. Here, we show that variations in a listener’s facial temperature, a marker of the autonomic emotional response, reflect the pain expressed by a baby’s cry. Specifically, by conducting listening experiments with cries expressing mild discomfort or acute pain, we demonstrate that NLP modulate the facial thermal response in adult listeners, irrespective of sex and of cry pitch variation. The temporal dynamics of the thermal response is more closely synchronized with the amplitude envelope of the acoustic signal when listening to a cry containing a large level of NLP than when NLP are less prominent. The pain encoded in a baby’s cry thus generates a synchronized emotional response in both adult men and women, emphasizing that our ability to decode the information carried by babies’ cries integrates an immediate activation of the autonomic nervous system before engaging higher-order cognitive processes.

Prematurity is known to increase the risk of organic or functional pathologies from the neonatal period as well as after birth. The impact of prematurity on the acoustic structure of cries, which are vital signals for survival of babies, has received little scrutiny, despite its prognostic potential. Here, we test the hypothesis that the acoustic structure of the cries of preterm babies changes with their growth, ultimately becoming identical to the cries of full-term infants. We recorded and analyzed the cries of 18 premature babies, born between gestational age 26 and 33 weeks, every week while they were hospitalized in a neonatal intensive care unit. Our results show that as premature babies approach their theoretical term, their cries become increasingly tonal, less marked by noisy components, and carrying individual cues. We then compared these cries with those recorded in 25 healthy full-term babies. Despite the evolution of premature baby cries over time, the acoustic structure of these cries remains different from that of full-term infants. These results suggest that the vocal maturity of premature babies is not fully attained at the theoretical age of term.

To make appropriate behavioural decisions, animals continuously process a flow of information provided by different sensory channels. Could temporality, i.e. the order in which independent stimuli are perceived, lead the animal to give greater importance to one stimulus than to another? Here we show that the decision of a crocodile to move preferentially towards the source of water surface waves than towards the source of an airborne sound is irrespective of the relative time of arrival of the sound and water vibrations to the animal, as long as the delay between these two stimuli does not exceed a few seconds. To test whether the late arrival of water waves – which travel more slowly than sound – could explain crocodiles' preference for the source of water waves, we controlled the relative timing of stimulus arrival within a time window of a few seconds. Our results reveal that crocodiles preferentially move towards the source of the water waves, whether they arrive after, at the same time as, or before the sound. This suggests that the temporal integration of information from different sensory channels can occur within a certain time window, where the behavioural decision-making remains independent of the arrival order of stimuli. The maximum delay between simultaneously evaluated stimuli probably depends on animal species and context.

What makes the painful cries of human babies so difficult to ignore? Vocal traits known as 'nonlinear phenomena’ are prime candidates. These acoustic irregularities are common in babies’ cries and are typically associated with high levels of distress or pain. Despite the vital importance of cries for a baby’s survival, how these nonlinear phenomena drive pain perception in adult listeners has not previously been systematically investigated. Here, by combining acoustic analyses of cries recorded in different contexts with playback experiments using natural and synthetic cries, we show that baby cries expressing acute pain are characterized by a pronounced presence of different nonlinear phenomena, and that these nonlinear phenomena drive pain evaluation by adult listeners. While adult listeners rated all cries presenting any of these nonlinear phenomena as expressing more pain, they were particularly sensitive to the presence of chaos. Our results thus show that nonlinear phenomena, especially chaos, encode pain information in baby cries and may be critically helpful for the development of vocal-based tools for monitoring babies' needs in the context of paediatric care. This article is part of the theme issue ‘Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions’.

Nonlinear phenomena (NLP) are often associated with high arousal and function to grab attention and/or signal urgency in vocalizations such as distress calls. Although biomechanical models and in vivo / ex vivo experiments suggest that their occurrence reflects the destabilization of vocal fold vibration under intense subglottal pressure and muscle tension, comprehensive descriptions of the dynamics of NLP occurrence in natural vocal signals are critically lacking. Here, to plug this gap, we report the timing, type, extent and acoustic context of NLP in 12 011 whines produced by Beagle puppies ( Canis familiaris ) during a brief separation from their mothers. Within bouts of whines, we show that both the proportion of time vocalizing and the number of whines containing NLP, especially those with chaos, increase with time since separation, presumably reflecting heightened arousal. Within whines, we show that NLP are typically produced during the first half of the call, following the steepest rises in pitch (fundamental frequency, f _o ) and amplitude. While our study reinforces the notion that NLP arise in calls due to instabilities in vocal production during high arousal, it also provides novel and efficient analytical tools for quantifying nonlinear acoustics in ecologically relevant mammal vocal communication contexts. This article is part of the theme issue ‘Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions’.