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The field of animal vocal communication has benefited greatly from improved understanding of vocal production mechanisms and specifically from the generalization of the source-filter theory of speech production to non-human mammals. The application of the source-filter theory has enabled researchers to decompose the acoustic structure of vocal signals according to their mode of production and thereby to predict the acoustic variation that is caused by anatomical or physiological attributes of the caller. The source-filter theory states that vocal signals result from a two-stage production, with the glottal wave generated in the larynx (the source), being subsequently filtered in the supralaryngeal vocal tract (the filter). This theory predicts that independent indexical information such as body size, weight, age and sex can be contained in both the glottal wave (mostly characterized by its fundamental frequency), and the spectral envelope of the radiated vocalization (mostly characterized by the vocal tract resonances or formant frequencies). Additionally, physiological fluctuations in emotional or motivational state have been found to influence the acoustic characteristics of signals in a reliable and predictable manner that is perceptually available to receivers. While animal vocalizations contain some dynamic attributes, their static attributes are sufficient to provide an effective means of acoustic individual discrimination both within and across call types. In this paper, we draw together a wealth of experimental work conducted within the source-filter framework over the last decade and we review how such experiments have elucidated the communicative value of animal vocalizations.

While vocal tract resonances or formants are key acoustic parameters that define differences between phonemes in human speech, little is known about their function in animal communication. Here, we used playback experiments to present red deer stags with re-synthesized vocalizations in which formant frequencies were systematically altered to simulate callers of different body sizes. In response to stimuli where lower formants indicated callers with longer vocal tracts, stags were more attentive, replied with more roars and extended their vocal tracts further in these replies. Our results indicate that mammals other than humans use formants in vital vocal exchanges and can adjust their own formant frequencies in relation to those that they hear.

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Individual recognition is considered a complex process and, although it is believed to be widespread across animal taxa, the cognitive mechanisms underlying this ability are poorly understood. An essential feature of individual recognition in humans is that it is cross-modal, allowing the matching of current sensory cues to identity with stored information about that specific individual from other modalities. Here, we use a cross-modal expectancy violation paradigm to provide a clear and systematic demonstration of cross-modal individual recognition in a nonhuman animal: the domestic horse. Subjects watched a herd member being led past them before the individual went of view, and a call from that or a different associate was played from a loudspeaker positioned close to the point of disappearance. When horses were shown one associate and then the call of a different associate was played, they responded more quickly and looked significantly longer in the direction of the call than when the call matched the herd member just seen, an indication that the incongruent combination violated their expectations. Thus, horses appear to possess a cross-modal representation of known individuals containing unique auditory and visual/olfactory information. Our paradigm could provide a powerful way to study individual recognition across a wide range of species.animal cognition ͉ vocal communication ͉ social behavior ͉ playback experiment ͉ expectancy violation H ow animals classify conspecifics provides insights into the social structure of a species and how they perceive their social world (1). Discrimination of kin from nonkin, and of individuals within both of these categories, is proposed to be of major significance in the evolution of social behavior (2, 3). Individual recognition can be seen as the most fine-grained categorization of conspecifics, and there is considerable interest in discovering the prevalence and complexity of this ability across species. While individual recognition is generally believed to be widespread (4), there is much debate as to what constitutes sound evidence of this ability (5). To demonstrate individual recognition, a paradigm must show that (i) discrimination operates at the level of the individual rather than at a broader level, and (ii) there is a matching of current sensory cues to identity with information stored in memory about that specific individual. Numerous studies to date have provided evidence for some form of social discrimination of auditory stimuli, but how this is achieved remains unclear. It is of considerable interest to establish whether any animal is capable of cross-modal integration of cues to identity, as this would suggest that in addition to the perception and recognition of stimuli in one domain, the brain could integrate such information into some form of higher-order representation that is independent of modality.A number of species have been shown to make very finegrained discriminations between different individuals (6-8). For example, in the habituation-dishabi...

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Surprisingly little is known about the role of acoustic cues in mammal female mate choice. Here, we examine the response of female red deer (Cervus elaphus) to male roars in which an acoustic cue to body size, the formants, has been re-scaled to simulate different size callers. Our results show that oestrous red deer hinds prefer roars simulating larger callers and constitute the first evidence that female mammals use an acoustic cue to body size in a mate choice context. We go on to suggest that sexual selection through female mating preferences may have provided an additional selection pressure along with male-male competition for broadcasting size-related information in red deer and other mammals.

SummaryMales often face a trade-off between investments in precopulatory and postcopulatory traits [1], particularly when male-male contest competition determines access to mates [2]. To date, studies of precopulatory strategies have largely focused on visual ornaments (e.g., coloration) or weapon morphology (e.g., antlers, horns, and canines). However, vocalizations can also play an important role in both male competition and female choice [3, 4, 5]. We investigated variation in vocal tract dimensions among male howler monkeys (Alouatta spp.), which produce loud roars using a highly specialized and greatly enlarged hyoid bone and larynx [6]. We examined the relative male investment in hyoids and testes among howler monkey species in relation to the level of male-male competition and analyzed the acoustic consequences of variation in hyoid morphology. Species characterized by single-male groups have large hyoids and small testes, suggesting high levels of vocally mediated competition. Larger hyoids lower formant frequencies, probably increasing the acoustic impression of male body size and playing a role analogous to investment in large body size or weaponry. Across species, as the number of males per group increases, testes volume also increases, indicating higher levels of postcopulatory sperm competition, while hyoid volume decreases. These results provide the first evidence of an evolutionary trade-off between investment in precopulatory vocal characteristics and postcopulatory sperm production.