Vocal communication relies on the ability of listeners to identify, process, and respond to vocal sounds produced by others in complex environments. In order to accurately recognize these signals, animals’ auditory systems must robustly represent acoustic features that distinguish vocal sounds from other environmental sounds. In this dissertation, I describe experiments combining acoustic, behavioral, and neurophysiological approaches to identify behaviorally relevant vocalization features and understand how they are represented in the brain. First, I show that vocal responses to communication sounds in songbirds depend on the presence of specific spectral signatures of vocalizations. Second, I identify an anatomically localized neural population in the auditory cortex that shows selective responses for behaviorally relevant sounds. Third, I show that these neurons’ spectral selectivity is robust to acoustic context, indicating that they could function as spectral signature detectors in a variety of listening conditions. Last, I deconstruct neural selectivity for behaviorally relevant sounds and show that it is driven by a sensitivity to deep fluctuations in power along the sound frequency spectrum. Together, these results show that the processing of behaviorally relevant spectral features engages a specialized neural population in the auditory cortex, and elucidate an acoustic driver of vocalization selectivity.

This study describes and analyzes the vocalizations of the Townsend chipmunk (Eutamias townsendii) in Oregon. Tape recordings of calls were collected primarily from Forest Park in Portland (Oregon) during 1970 and 1971, and sonograms of these recordings were produced. Findings were compared with those of Brand (1970) who investigated E. townsendii vocalizations in California. The prominent calls in the vocal repertoire of townsendii are the quist, the quirt, and the chipper. These calls all appear to be warning calls. The quist note is a a chevron-shaped (A) figure representing a sound which begins at a low frequency (1-2 KC), sharply rises to a peak at about 11 KC, and then drops sharply to the lower frequencies again. Quists are arranged into bursts, and bursts into sequences. The number of quists per burst and the frequency of the top of the note decreased significantly over time. The interval between burst increased significantly, while the interval between quists seems to increase over time. The variation in the quist rate and related character6istics between Oregon and California townsendii populations is probably a result of subspecific differences. There seems to be a trend in the sciurids from the specific vocal system (one definite meaning per call) of the forest dwellers towards the unspecific vocal systems of animal groups which live in the more open habitats. E. townsendii displays a specific vocal system in which each call seems to have a specific function.

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North American red squirrels are a solitary, territorial species that confront various predators. Previous research proposed that squirrels produce predator-specific, referential ‘seet’ and ‘bark’ alarm calls to aerial and ground predators, respectively. To test this hypothesis, I examined alarm call production during natural encounters with predators, conspecific intruders and in a series of predator simulation experiments. Call production patterns were consistent across all types of disturbance and involved protracted bouts where both call types were inter-mixed. Hence, the call types were not predator-specific but rather their patterning reflected the persistence of disturbances of any type. Tests of alternative call functions further indicated that calls were not actually directed at conspecifics, but rather at predators and intruders and might function to deter or repel them directly. These outcomes are consistent with life-history details of red squirrels and contradict the proposal that this species produces predator-specific, referential alarm calls. / xi, 153 leaves : ill. ; 29 cm

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Thesis (Ph. D.)--University of Missouri-Columbia, 2007. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on January 31, 2008) Vita. Includes bibliographical references.

Thesis (Ph.D.)--Murdoch University, 2005. / Thesis submitted to the Division of Science and Engineering. Bibliography: leaves 203-217.

Thesis (M. Sc.)--University of Canterbury, 2004. / Typescript (photocopy). Includes bibliographical references (leaves [127-135]). Also available via the World Wide Web.

Thesis (M.Sc.) - Carleton University, 2008. / Includes bibliographical references (p. 63-67). Also available in electronic format on the Internet.

Thesis (M.Sc.) - Carleton University, 2007. / Includes bibliographical references (p. 78-85). Also available in electronic format on the Internet.