Humans as well as many animal species reveal their emotional state in their voice. Vocal features show strikingly similar correlation patterns with emotional states across mammalian species, suggesting that the vocal expression of emotion follows highly conserved signalling rules. To fully understand the principles of emotional signalling in mammals it is, however, necessary to also account for any inconsistencies in the way that they are acoustically encoded. Here we investigate whether the expression of emotions differs between call types produced by the same species. We compare the acoustic structure of two common piglet calls—the scream (a distress call) and the grunt (a contact call)—across three levels of arousal in a negative situation. We find that while the central frequency of calls increases with arousal in both call types, the amplitude and tonal quality (harmonic-to-noise ratio) show contrasting patterns: as arousal increased, the intensity also increased in screams, but not in grunts, while the harmonicity increased in screams but decreased in grunts. Our results suggest that the expression of arousal depends on the function and acoustic specificity of the call type. The fact that more vocal features varied with arousal in scream calls than in grunts is consistent with the idea that distress calls have evolved to convey information about emotional arousal.

It is well established that in human speech perception the left hemisphere (LH) of the brain is specialized for processing intelligible phonemic (segmental) content (e.g., [1][2][3]), whereas the right hemisphere (RH) is more sensitive to prosodic (suprasegmental) cues [4,5]. Despite evidence that a range of mammal species show LH specialization when processing conspecific vocalizations [6], the presence of hemispheric biases in domesticated animals' responses to the communicative components of human speech has never been investigated. Human speech is familiar and relevant to domestic dogs (Canis familiaris), who are known to perceive both segmental phonemic cues [7-10] and suprasegmental speaker-related [11,12] and emotional [13] prosodic cues. Using the head-orienting paradigm, we presented dogs with manipulated speech and tones differing in segmental or suprasegmental content and recorded their orienting responses. We found that dogs showed a significant LH bias when presented with a familiar spoken command in which the salience of meaningful phonemic (segmental) cues was artificially increased but a significant RH bias in response to commands in which the salience of intonational or speaker-related (suprasegmental) vocal cues was increased. Our results provide insights into mechanisms of interspecific vocal perception in a domesticated mammal and suggest that dogs may share ancestral or convergent hemispheric specializations for processing the different functional communicative components of speech with human listeners. Results and DiscussionEach dog took part in one trial in which they were presented with a single sound stimulus from either one of eight conditions in which speech samples were resynthesized to vary the relative salience of segmental (phonemic) versus suprasegmental (speaker cues and intonation) information or from one of two control conditions ( Figure 1). Using the head-orienting paradigm, the sound was played simultaneously from both sides of the subject, and the direction of the subject's initial orienting response (left or right) was recorded. We obtained head-orienting responses from 25 dogs in each condition. Given that auditory information entering each ear is processed mainly in the contralateral hemisphere of the brain via the dominant contralateral auditory pathways [14], it is assumed that if the dog turns with its left ear leading in response to the sound, the acoustic input is processed primarily by the right hemisphere (RH), whereas a right turn would indicate primary left hemisphere (LH) processing [15].A binary logistic regression analysis identified a significant overall effect of auditory condition on head-turn direction [Wald(8) = 37.61, p < 0.001], indicating that the content of the acoustic signals affected the direction of hemispheric lateralization during perception (Figure 2). There were no significant effects of subject sex (p = 0.76), age (p = 0.15), breed type (p = 0.37), current residence (animal shelter or private home; p = 0.16), stimulus exemplar (p = 0.23), sti...

The vocal expression of emotion is likely driven by shared physiological principles among species. However, which acoustic features promote decoding of emotional state and how the decoding is affected by their listener's psychology remain poorly understood. Here we tested how acoustic features of piglet vocalizations interact with psychological profiles of human listeners to affect judgments of emotional content of heterospecific vocalizations. We played back 48 piglet call sequences recorded in four different contexts (castration, isolation, reunion, nursing) to 60 listeners. Listeners judged the emotional intensity and valence of the recordings and were further asked to attribute a context of emission from four proposed contexts. Furthermore, listeners completed a series of questionnaires assessing their personality (NEO-FFI personality inventory), empathy [Interpersonal Reactivity Index (IRI)] and attitudes to animals (Animal Attitudes Scale). None of the listeners' psychological traits affected the judgments. On the contrary, acoustic properties of recordings had a substantial effect on ratings. Recordings were rated as more intense with increasing pitch (mean fundamental frequency) and increasing proportion of vocalized sound within each stimulus recording and more negative with increasing pitch and increasing duration of the calls within the recording. More complex acoustic properties (jitter, harmonic-to-noise ratio, and presence of subharmonics) did not seem to affect the judgments. The probability of correct context recognition correlated positively with the assessed emotion intensity for castration and reunion calls, and negatively for nursing calls. In conclusion, listeners judged emotions from pig calls using simple acoustic properties and the perceived emotional intensity might guide the identification of the context.

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For both humans and other animals, the ability to combine information obtained through different senses is fundamental to the perception of the environment. It is well established that humans form systematic cross-modal correspondences between stimulus features that can facilitate the accurate combination of sensory percepts. However, the evolutionary origins of the perceptual and cognitive mechanisms involved in these cross-modal associations remain surprisingly under-explored. In this review we outline recent comparative studies investigating how non-human mammals naturally combine information encoded in different sensory modalities during communication. The results of these behavioural studies demonstrate that various mammalian species are able to combine signals from different sensory channels when they are perceived to share the same basic features, either because they can be redundantly sensed and/or because they are processed in the same way.Moreover, evidence that a wide range of mammals form complex cognitive representations about signallers, both within and across species, suggests that animals also learn to associate different sensory features which regularly co-occur. Further research is now necessary to determine how multisensory representations are formed in individual animals, including the relative importance of low-level feature-related correspondences. Such investigations will generate important insights into how animals perceive and categorise their environment, as well as provide an essential basis for understanding the evolution of multisensory perception in humans.