Physiological resonance between mates through calls as possible evidence of empathic processes in songbirds

Exploring emotional contagion in zebrafish: A virtual-demonstrator study of positive and negative emotions

Emotional contagion, the transmission of emotions within a group, has been extensively studied in mammals but remains largely unexplored in fish. This study aims to investigate whether emotional contagion, specifically in terms of low and high anxiety levels, can be evoked in zebrafish. This freshwater species has been gaining momentum due to its high genetic homology to humans and complex behavioral repertoire, making it well-suited for exploring social behavior. Our hypothesis posits that zebrafish have the ability to transmit positive and negative emotions to one another through visual cues only and that this transmission is robust over time. To test this, we employed a virtual demonstrator fish approach, where videos of zebrafish exhibiting either high or low geotactic behavior were shown to live zebrafish. Geotaxis, the tendency of a fish to spend more time at the bottom of the tank, is a sensitive measure of anxiety, with high geotactic behavior corresponding to high anxiety levels and vice versa. Our findings indicate that the virtual demonstrator successfully transmitted emotional states to the live fish, as evidenced by changes in the time spent at the bottom of the tank, linear acceleration, and fast-turning maneuvers, metrics that quantify anxiety-like behaviors such as geotaxis and erratic movements. Additionally, we conducted a causal analysis using a transfer entropy approach, revealing a significant flow of information from the virtual demonstrator fish to the live fish, indicating the efficacy and potential of this approach in studying emotional contagion. This study provides additional empirical evidence of how visual cues alone from a virtual demonstrator exhibiting high or low anxious behavior can elicit similar behavioral states in bystander fish, highlighting the potential for emotional contagion beyond mammalian and avian models.

Sparse ensemble neural code for a complete vocal repertoire

The categorization of animal vocalizations into distinct behaviorally relevant groups for communication is an essential operation that must be performed by the auditory system. This auditory object recognition is a difficult task that requires selectivity to the group identifying acoustic features and invariance to renditions within each group. We find that small ensembles of auditory neurons in the forebrain of a social songbird can code the bird's entire vocal repertoire (∼10 call types). Ensemble neural discrimination is not, however, correlated with single unit selectivity, but instead with how well the joint single unit tunings to characteristic spectro-temporal modulations span the acoustic subspace optimized for the discrimination of call types. Thus, akin to face recognition in the visual system, call type recognition in the auditory system is based on a sparse code representing a small number of high-level features and not on highly selective grandmother neurons.

Integrative Model of Human-Animal Interactions: A One Health-One Welfare Systemic Approach to Studying HAI

The Integrative Model of Human-Animal Interactions (IMHAI) described herewith provides a conceptual framework for the study of interspecies interactions and aims to model the primary emotional processes involved in human-animal interactions. This model was developed from theoretical inputs from three fundamental disciplines for understanding interspecies interactions: neuroscience, psychology and ethology, with the objective of providing a transdisciplinary approach on which field professionals and researchers can build and collaborate. Seminal works in affective neuroscience offer a common basis between humans and animals and, as such, can be applied to the study of interspecies interactions from a One Health-One Welfare perspective. On the one hand, Jaak Panksepp's research revealed that primary/basic emotions originate in the deep subcortical regions of the brain and are shared by all mammals, including humans. On the other hand, several works in the field of neuroscience show that the basic physiological state is largely determined by the perception of safety. Thus, emotional expression reflects the state of an individual's permanent adaptation to ever-changing environmental demands. Based on this evidence and over 5 years of action research using grounded theory, alternating between research and practice, the IMHAI proposes a systemic approach to the study of primary-process emotional affects during interspecies social interactions, through the processes of emotional transfer, embodied communication and interactive emotional regulation. IMHAI aims to generate new hypotheses and predictions on affective behavior and interspecies communication. Application of such a model should promote risk prevention and the establishment of positive links between humans and animals thereby contributing to their respective wellbeing.

Reciprocal processes of sensory perception and social bonding: an integrated social-sensory framework of social behavior

Organisms filter the complexity of natural stimuli through their individual sensory and perceptual systems. Such perceptual filtering is particularly important for social stimuli. A shared "social umwelt" allows individuals to respond appropriately to the expected diversity of cues and signals during social interactions. In this way, the behavioral and neurobiological mechanisms of sociality and social bonding cannot be disentangled from perceptual mechanisms and sensory processing. While a degree of embeddedness between social and sensory processes is clear, our dominant theoretical frameworks favor treating the social and sensory processes as distinct. An integrated social-sensory framework has the potential to greatly expand our understanding of the mechanisms underlying individual variation in social bonding and sociality more broadly. Here we leverage what is known about sensory processing and pair bonding in two common study systems with significant species differences in their umwelt (rodent chemosensation and avian acoustic communication). We primarily highlight that (1) communication is essential for pair bond formation and maintenance, (2) the neural circuits underlying perception, communication and social bonding are integrated, and (3) candidate neuromodulatory mechanisms that regulate pair bonding also impact communication and perception. Finally, we propose approaches and frameworks that more fully integrate sensory processing, communication, and social bonding across levels of analysis: behavioral, neurobiological, and genomic. This perspective raises two key questions: (1) how is social bonding shaped by differences in sensory processing?, and (2) to what extent is sensory processing and the saliency of signals shaped by social interactions and emerging relationships?

Rhythm in dyadic interactions

This review paper discusses rhythmic interactions and distinguishes them from non-rhythmic interactions. We report on communicative behaviours in social and sexual contexts, as found in dyads of humans, non-human primates, non-primate mammals, birds, anurans and insects. We discuss observed instances of rhythm in dyadic interactions, identify knowledge gaps and propose suggestions for future research. We find that most studies on rhythmicity in interactive signals mainly focus on one modality (acoustic or visual) and we suggest more work should be performed on multimodal signals. Although the social functions of interactive rhythms have been fairly well described, developmental research on rhythms used to regulate social interactions is still lacking. Future work should also focus on identifying the exact timing mechanisms involved. Rhythmic signalling behaviours are widespread and critical in regulating social interactions across taxa, but many questions remain unexplored. A multidisciplinary, comparative cross-species approach may help provide answers. This article is part of the theme issue 'Synchrony and rhythm interaction: from the brain to behavioural ecology'.

The singing question: re-conceptualizing birdsong

Birdsong has been the subject of broad research from a variety of sub-disciplines and has taught us much about the evolution, function, and mechanisms driving animal communication and cognition. Typically, birdsong refers to the specialized vocalizations produced by oscines. Historically, much of the research on birdsong was conducted in north temperate regions (specifically in Europe and North America) leading to multiple biases. Due to these historic biases these vocalizations are generally considered to be highly sexually dimorphic, heavily shaped by sexual selection and essential for courtship and territoriality. Song is also typically defined as a learned trait shaped by cultural evolution. Together, this framework focuses research specifically on males, particularly during the north temperate breeding season - reflecting and thereby reinforcing this framework. The physiological underpinnings of song often emphasize the role of the hypothalamic-pituitary-gonadal axis (associated with breeding changes) and the song control system (underlying vocal learning). Over the years there has been great debate over which features of song are essential to the definition of birdsong, which features apply broadly to contexts outside males in the north temperate region, and over the importance of having a definition at all. Importantly, the definitions we use can both guide and limit the progress of research. Here, we describe the history of these definitions, and how these definitions have directed and restricted research to focus on male song in sexually selected contexts. Additionally, we highlight the gaps in our scientific knowledge, especially with respect to the function and physiological mechanisms underlying song in females and in winter, as well as in non-seasonally breeding species. Furthermore, we highlight the problems with using complexity and learning as dichotomous variables to categorize songs and calls. Across species, no one characteristic of song - sexual dimorphism, seasonality, complexity, sexual selection, learning - consistently delineates song from other songbird vocal communication. We provide recommendations for next steps to build an inclusive information framework that will allow researchers to explore nuances in animal communication and promote comparative research. Specifically, we recommend that researchers should operationalize the axis of variation most relevant to their study/species by identifying their specific question and the variable(s) of focus (e.g. seasonality). Researchers should also identify the axis (axes) of variation (e.g. degree of control by testosterone) most relevant to their study and use language consistent with the question and axis (axes) of variation (e.g. control by testosterone in the seasonal vocal production of birds).

Emotional contagion in nonhuman animals: A review

Emotional contagion, the emotional state‐matching of an individual with another, seems to be crucial for many social species. In recent years evidence on emotional contagion in different animal species has accumulated. However, despite its adaptative advantages and its presumed simplicity, the study and direct demonstration of this phenomenon present more complexities than previously thought. For these reasons, a review of the literature on emotional contagion in nonhuman species is timely to integrate current findings. In this paper thus, we carry out a comprehensive review of the most relevant studies on emotional contagion in animals and discuss the main problems and challenges of the field. We conclude that more research is needed to broaden our understanding of the mechanisms and functions of emotional contagion and the extent to which this process is present in a wide variety of species. Furthermore, the comparative study of emotional contagion would benefit from the use of systematized paradigms including both behavioral and physiological measures and the simultaneous recording of the responses of the interacting individuals to reliably assess an emotional state‐matching between them and reliable controls.

This article is categorized under:

Cognitive Biology > Evolutionary Roots of Cognition

Psychology > Comparative Psychology

Psychology > Emotion and Motivation

Discrimination of natural acoustic variation in vocal signals

Studies of acoustic communication often focus on the categories and units of vocalizations, but subtle variation also occurs in how these signals are uttered. In human speech, it is not only phonemes and words that carry information but also the timbre, intonation, and stress of how speech sounds are delivered (often referred to as "paralinguistic content"). In non-human animals, variation across utterances of vocal signals also carries behaviorally relevant information across taxa. However, the discriminability of these cues has been rarely tested in a psychophysical paradigm. Here, we focus on acoustic communication in the zebra finch (Taeniopygia guttata), a songbird species in which the male produces a single stereotyped motif repeatedly in song bouts. These motif renditions, like the song repetitions of many birds, sound very similar to the casual human listener. In this study, we show that zebra finches can easily discriminate between the renditions, even at the level of single song syllables, much as humans can discriminate renditions of speech sounds. These results support the notion that sensitivity to fine acoustic details may be a primary channel of information in zebra finch song, as well as a shared, foundational property of vocal communication systems across species.

Challenges in the comparative study of empathy and related phenomena in animals

The aim of this review is to discuss recent arguments and findings in the comparative study of empathy. Based on a multidisciplinary approach including psychology and ethology, we review the non-human animal literature concerning theoretical frameworks, methodology, and research outcomes. One specific objective is to highlight discrepancies between theory and empirical findings, and to discuss ambiguities present in current data and their interpretation. In particular, we focus on emotional contagion and its experimental investigation, and on consolation and targeted helping as measures for sympathy. Additionally, we address the feasibility of comparing across species with behavioural data alone. One main conclusion of our review is that animal research on empathy still faces the challenge of closing the gap between theoretical concepts and empirical evidence. To advance our knowledge, we propose to focus more on the emotional basis of empathy, rather than on possibly ambiguous behavioural indicators, and we provide suggestions to overcome the limitations of previous research .

Auditory Selectivity for Spectral Contrast in Cortical Neurons and Behavior

Vocal communication relies on the ability of listeners to identify, process, and respond to vocal sounds produced by others in complex environments. To accurately recognize these signals, animals' auditory systems must robustly represent acoustic features that distinguish vocal sounds from other environmental sounds. Vocalizations typically have spectral structure; power regularly fluctuates along the frequency axis, creating spectral contrast. Spectral contrast is closely related to harmonicity, which refers to spectral power peaks occurring at integer multiples of a fundamental frequency. Although both spectral contrast and harmonicity typify natural sounds, they may differ in salience for communication behavior and engage distinct neural mechanisms. Therefore, it is important to understand which of these properties of vocal sounds underlie the neural processing and perception of vocalizations.Here, we test the importance of vocalization-typical spectral features in behavioral recognition and neural processing of vocal sounds, using male zebra finches. We show that behavioral responses to natural and synthesized vocalizations rely on the presence of discrete frequency components, but not on harmonic ratios between frequencies. We identify a specific population of neurons in primary auditory cortex that are sensitive to the spectral resolution of vocal sounds. We find that behavioral and neural response selectivity is explained by sensitivity to spectral contrast rather than harmonicity. This selectivity emerges within the cortex; it is absent in the thalamorecipient region and present in the deep output region. Further, deep-region neurons that are contrast-sensitive show distinct temporal responses and selectivity for modulation density compared with unselective neurons.SIGNIFICANCE STATEMENT Auditory coding and perception are critical for vocal communication. Auditory neurons must encode acoustic features that distinguish vocalizations from other sounds in the environment and generate percepts that direct behavior. The acoustic features that drive neural and behavioral selectivity for vocal sounds are unknown, however. Here, we show that vocal response behavior scales with stimulus spectral contrast but not with harmonicity, in songbirds. We identify a distinct population of auditory cortex neurons in which response selectivity parallels behavioral selectivity. This neural response selectivity is explained by sensitivity to spectral contrast rather than to harmonicity. Our findings inform the understanding of how the auditory system encodes socially-relevant signals via detection of an acoustic feature that is ubiquitous in vocalizations.

Rescue-like Behaviour in Mice is Mediated by Their Interest in the Restraint Tool

Acting without the expectation of compensation is called prosocial behaviour. Since prosocial behaviour requires high cognitive and social abilities, it has been thought to be only shown by primates. Although prosocial behaviour has been recently reported in rats, there are still questions regarding this finding. We demonstrated rescue-like behaviour in mice in a previous report. In this study, we investigated the motives underlying rescue-like behaviour for constrained cage-mates among mice. We prepared either a tube containing a ball of yarn or an opaque tube and assessed whether mice displayed the same rescue-like behaviour shown in the case of tube-restrained cage-mates. Mice did not open the lid of the tube containing the ball of yarn but opened the opaque tube lid. Mice showed a high interest in the tube in which the cage-mate had been restrained and prioritized staying in this tube rather than rescuing additional cage-mates. Oxytocin, which increases empathy, had no effect on the lid-opening behaviour. Thus, the rescue-like behaviour of mice is not based on empathy but is related to social interest in the cage-mate and the tube itself. These results suggest that rodent lid-opening behaviour may not conclusively prove the presence of prosocial behaviour.

Negative emotional contagion and cognitive bias in common ravens (Corvus corax)

Emotional contagion is described as an emotional state matching between subjects, and has been suggested to facilitate communication and coordination in complex social groups. Empirical studies typically focus on the measurement of behavioral contagion and emotional arousal, yet, while highly important, such an approach often disregards an additional evaluation of the underlying emotional valence. Here, we studied emotional contagion in ravens by applying a judgment bias paradigm to assess emotional valence. We experimentally manipulated positive and negative affective states in demonstrator ravens, to which they responded with increased attention and interest in the positive condition, as well as increased redirected behavior and a left-eye lateralization in the negative condition. During this emotion manipulation, another raven observed the demonstrator's behavior, and we used a bias paradigm to assess the emotional valence of the observer to determine whether emotional contagion had occurred. Observers showed a pessimism bias toward the presented ambiguous stimuli after perceiving demonstrators in a negative state, indicating emotional state matching based on the demonstrators' behavioral cues and confirming our prediction of negative emotional contagion. We did not find any judgment bias in the positive condition. This result critically expands upon observational studies of contagious play in ravens, providing experimental evidence that emotional contagion is present not only in mammalian but also in avian species. Importantly, this finding also acts as a stepping stone toward understanding the evolution of empathy, as this essential social skill may have emerged across these taxa in response to similar socioecological challenges.

Water restriction influences intra-pair vocal behavior and the acoustic structure of vocalisations in the opportunistically breeding zebra finch (Taeniopygia guttata)

Seasonally-breeding species experience significant and predictable shifts in vocal behaviour; however, it is unclear to what extent this is true for species that breed opportunistically. The Australian zebra finch is an opportunistically breeding species, which means individuals must time breeding bouts based on many environmental factors. Here we tested the effect of experimental water restriction, which suppresses reproductive readiness in zebra finches, on vocal behaviour of males and females. More specifically, we quantified the effect of water restriction on three parameters of vocal behaviour in pair-bonded zebra finches: vocal activity, patterns of vocal exchanges, and the acoustic structure of vocalisations (calls and male song). We found that water restriction caused a decrease in vocal output (both song and call rate). Additionally, water restriction affected the composition of male songs. However, there was no effect of water restriction on the patterns of calling exchanges for monogamous partners (temporal coordination and turn taking). Finally, water restriction had vocalisation- and sex-specific effects on the acoustic structure of song syllables and calls. Because the direction of these effects were vocalisation- and sex- specific, there may be different mechanisms underlying the effects of water restriction on acoustic structure depending on context. These results contribute to the growing body of research highlighting the rich communicative potential of bird calls. Our current results raise the hypothesis that zebra finches may use changes in vocal behaviour and/or the structure of vocalisations of their conspecifics when making breeding decisions.

Vocal contagion of emotions in non-human animals

Communicating emotions to conspecifics (emotion expression) allows the regulation of social interactions (e.g. approach and avoidance). Moreover, when emotions are transmitted from one individual to the next, leading to state matching (emotional contagion), information transfer and coordination between group members are facilitated. Despite the high potential for vocalizations to influence the affective state of surrounding individuals, vocal contagion of emotions has been largely unexplored in non-human animals. In this paper, I review the evidence for discrimination of vocal expression of emotions, which is a necessary step for emotional contagion to occur. I then describe possible proximate mechanisms underlying vocal contagion of emotions, propose criteria to assess this phenomenon and review the existing evidence. The literature so far shows that non-human animals are able to discriminate and be affected by conspecific and also potentially heterospecific (e.g. human) vocal expression of emotions. Since humans heavily rely on vocalizations to communicate (speech), I suggest that studying vocal contagion of emotions in non-human animals can lead to a better understanding of the evolution of emotional contagion and empathy.

Empathy is not in our genes

In academic and public life empathy is seen as a fundamental force of morality - a psychological phenomenon, rooted in biology, with profound effects in law, policy, and international relations. But the roots of empathy are not as firm as we like to think. The matching mechanism that distinguishes empathy from compassion, envy, schadenfreude, and sadism is a product of learning. Here I present a dual system model that distinguishes Empathy¹, an automatic process that catches the feelings of others, from Empathy², controlled processes that interpret those feelings. Research with animals, infants, adults and robots suggests that the mechanism of Empathy¹, emotional contagion, is constructed in the course of development through social interaction. Learned Matching implies that empathy is both agile and fragile. It can be enhanced and redirected by novel experience, and broken by social change.

Acute exogenous corticosterone treatments have few effects on courtship and pair bonding in zebra finches

Stress impacts social relationships. In turn, social relationships buffer the stress response in some species. Studies that have investigated the role of corticosterone (CORT) on courtship, mate choice, mating, and pairing have found mixed results. We therefore tested the role of CORT in these steps of the pairing process in the monogamous zebra finch. Male and female zebra finches received either one of 2 doses of corticosterone (CORT, 10 μg and 20 μg, referred to as low and high dose) or a vehicle control (peanut oil). Subjects were then given the opportunity to pair in mixed sex aviaries. Courtship and pair bonding behaviors were observed over 3 days. Overall, zebra finches of both sexes were equally likely to pair or not pair regardless of treatment, although a high dose of CORT increased the latency to form a pair bond. There were no effects of CORT on courtship behavior in either sex, though the low dose increased undirected (non-courtship) singing in males relative to the high dose. Animals treated with CORT, regardless of dose, engaged in fewer copulations than did control animals. When we examined pairing behaviors, we found a decrease in co-nesting in low dose animals. Our results suggest that acute CORT has few effects on pair bonding, suggesting species-specific effects of CORT on behavior.

Zebra finches identify individuals using vocal signatures unique to each call type

Individual recognition is critical in social animal communication, but it has not been demonstrated for a complete vocal repertoire. Deciphering the nature of individual signatures across call types is necessary to understand how animals solve the problem of combining, in the same signal, information about identity and behavioral state. We show that distinct signatures differentiate zebra finch individuals for each call type. The distinctiveness of these signatures varies: contact calls bear strong individual signatures while calls used during aggressive encounters are less individualized. We propose that the costly solution of using multiple signatures evolved because of the limitations of the passive filtering properties of the birds' vocal organ for generating sufficiently individualized features. Thus, individual recognition requires the memorization of multiple signatures for the entire repertoire of conspecifics of interests. We show that zebra finches excel at these tasks.

Socially-mediated arousal and contagion within domestic chick broods

Emotional contagion - an underpinning valenced feature of empathy - is made up of simpler, potentially dissociable social processes which can include socially-mediated arousal and behavioural/physiological contagion. Previous studies of emotional contagion have often conflated these processes rather than examining their independent contribution to empathic response. We measured socially-mediated arousal and contagion in 9-week old domestic chicks (n = 19 broods), who were unrelated but raised together from hatching. Pairs of observer chicks were exposed to two conditions in a counterbalanced order: air puff to conspecifics (AP) (during which an air puff was applied to three conspecifics at 30 s intervals) and control with noise of air puff (C) (during which the air puff was directed away from the apparatus at 30 s intervals). Behaviour and surface eye temperature of subjects and observers were measured throughout a 10-min pre-treatment and 10-min treatment period. Subjects and observers responded to AP with increased freezing, and reduced preening and ground pecking. Subjects and observers also showed reduced surface eye temperature - indicative of stress-induced hyperthermia. Subject-Observer behaviour was highly correlated within broods during both C and AP conditions, but with higher overall synchrony during AP. We demonstrate the co-occurrence of socially-mediated behavioural and physiological arousal and contagion; component features of emotional contagion.

Acoustic fine structure may encode biologically relevant information for zebra finches

The ability to discriminate changes in the fine structure of complex sounds is well developed in birds. However, the precise limit of this discrimination ability and how it is used in the context of natural communication remains unclear. Here we describe natural variability in acoustic fine structure of male and female zebra finch calls. Results from psychoacoustic experiments demonstrate that zebra finches are able to discriminate extremely small differences in fine structure, which are on the order of the variation in acoustic fine structure that is present in their vocal signals. Results from signal analysis methods also suggest that acoustic fine structure may carry information that distinguishes between biologically relevant categories including sex, call type and individual identity. Combined, our results are consistent with the hypothesis that zebra finches can encode biologically relevant information within the fine structure of their calls. This study provides a foundation for our understanding of how acoustic fine structure may be involved in animal communication.

Stress-induced flexibility and individuality in female and male zebra finch distance calls

Vocal recognition is central to the coordination and organization of behavior in pair-bonding species such as zebra finches. Zebra finches' vocalizations are individualized and support acoustic discrimination processes. Physiological states - such as the ones involved in emotional stress - can modify vocal production and consequently the structure of vocalizations. These modifications might signal the state of the caller but also impair individual recognition processes. This may represent a signaling trade-off, especially in contexts where both pieces of information can be critically important, for example when mates use calls to reunite after social isolation. Here we study the impact of a stress on the individual vocal signature in both female and male zebra finch distance calls. We built a manually curated database of distance calls of several individuals (both females and males) recorded in control and stress conditions. The stress was induced either by social isolation of the bird or using exogenous corticosterone. We developed a machine learning approach to assess the impact of stress on the individual characterization of calls. We show that while calls' spectral structure is significantly modified by stress, it still allows for the correct classification of calls to the caller. Moreover, we also show that the stress-induced modification of calls' structure is not a 'general feature signal' that can be detected as a 'stress' signal regardless of identity. Thus, female and male zebra finch calls' structure show stress-induced flexibility that stays within the range of individual vocal signatures.

Prosody in birdsong: A review and perspective

Birdsong shows striking parallels with human speech. Previous comparisons between birdsong and human vocalizations focused on syntax, phonology and phonetics. In this review, we propose that future comparative research should expand its focus to include prosody, i.e. the temporal and melodic properties that extend over larger units of song. To this end, we consider the similarities between birdsong structure and the prosodic hierarchy in human speech and between context-dependent acoustic variations in birdsong and the biological codes in human speech. Moreover, we discuss songbirds' sensitivity to prosody-like acoustic features and the role of such features in song segmentation and song learning in relation to infants' sensitivity to prosody and the role of prosody in early language acquisition. Finally, we make suggestions for future comparative birdsong research, including a framework of how prosody in birdsong can be studied. In particular, we propose to analyze birdsong as a multidimensional signal composed of specific acoustic features, and to assess whether these acoustic features are organized into prosody-like structures.

Perception of emotional valence in horse whinnies

Background

Non-human animals often produce different types of vocalisations in negative and positive contexts (i.e. different valence), similar to humans, in which crying is associated with negative emotions and laughter is associated with positive ones. However, some types of vocalisations (e.g. contact calls, human speech) can be produced in both negative and positive contexts, and changes in valence are only accompanied by slight structural differences. Although such acoustically graded signals associated with opposite valence have been highlighted in some species, it is not known if conspecifics discriminate them, and if contagion of emotional valence occurs as a result. We tested whether domestic horses perceive, and are affected by, the emotional valence of whinnies produced by both familiar and unfamiliar conspecifics. We measured physiological and behavioural reactions to whinnies recorded during emotionally negative (social separation) and positive (social reunion) situations.

Results

We show that horses perceive acoustic cues to both valence and familiarity present in whinnies. They reacted differently (respiration rate, head movements, height of the head and latency to respond) to separation and reunion whinnies when produced by familiar, but not unfamiliar individuals. They were also more emotionally aroused (shorter inter-pulse intervals and higher locomotion) when hearing unfamiliar compared to familiar whinnies. In addition, the acoustic parameters of separation and reunion whinnies affected the physiology and behaviour of conspecifics in a continuous way. However, we did not find clear evidence for contagion of emotional valence.

Conclusions

Horses are thus able to perceive changes linked to emotional valence within a given vocalisation type, similar to perception of affective prosody in humans. Whinnies produced in either separation or reunion situations seem to constitute acoustically graded variants with distinct functions, enabling horses to increase their apparent vocal repertoire size.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-017-0193-1) contains supplementary material, which is available to authorized users.