Auditory performance in tigers, the biggest of the big cats, has not been documented in the scientific literature, although claims of extraordinary hearing attributes are legendary in anecdote. The purpose of this project is to examine the hearing and vocal capabilities of the tiger to lay the foundation for the development of acoustical tools for use in conservation. Is the tiger an auditory specialist, an animal that has evolved special hearing attributes that give it an advantage in the wild, or is it a generalist fitting predictably into its auditory niche? A non-invasive set of tools including sound elicited brain potentials will be used to evaluate hearing. Sound measurements in the infrasonic range (frequencies lower than those available to humans) will be used to determine if tigers are able to produce calls containing energy in this range. The working hypothesis is that tigers specialize in low frequency hearing to communicate over large distances. This project will set the stage for the development of 1) studies designed to more thoroughly understand the role of hearing and vocalization in the diverse communities inhabited by tigers, 2) acoustic census protocols designed to more effectively account for free-ranging tiger populations, and 3) acoustic tools designed to deter tigers from encroaching on human habitats, thereby reducing poaching. The tiger is regarded as a flagship species representing the critical need for the conservation of wide-ranging ecosystems including the coniferous, mixed-deciduous forests of southeastern Russia and the thick tropical rainforest of Sumatra. When combined with a comprehensive outreach and educational program designed to impact the public and students at all levels, this project will call attention to the plight of the tiger. Consequently, a major educational goal is to use the high profile nature of the tiger to leverage the conservation of ecosystems inhabited by these seriously threatened big cats.
The main, overarching goal of this project is to understand vocal communication in tigers (Panthera tigris); to understand the relationship between inner ear anatomy and auditory performance on one hand and associated vocal production and vocal sound radiation on another. Given the specialized, predatory nature of the big cat’s lifestyle and the large scale of individual habitats and hunting ranges, one aim was to determine, indirectly, if evolutionary pressures have influenced the biomechanics of inner ear transduction and/or the dynamic of producing and projecting calls. Another goal that was ancillary in nature was to determine if size, and the size of the inner ear in particular, is a major factor influencing the limits of sensory performance among mammals, primarily felids, as well as humans in light of claims that the human inner ear may be specialized in the frequency resolving domain. Goal one was to determine if tigers are auditory generalists or specialists. Animals fitting into the auditory generalist category take advantage of their full, available acoustic landscape and auditory specialists, by definition, attend to select features of their acoustic experience preferentially. Our findings suggest that the tiger is an auditory hybrid in this regard. On one hand, the animal operates in a generalist mode. When considering sensitivity to sound, input-output behavior and the biomechanical properties of the inner ear that shape its ability to discriminate frequency, the big cat appears to operate as an auditory generalist. On the other hand, what appears to be an inner ear adaptation in the response timing (latency) domain was discovered during the course of this investigation. Specifically, response timing delays associated with low frequency sound stimulation are remarkably and unpredictably short. In that light, our work suggests that selection pressures originating in the animals’ acoustic environment, and operating over evolutionary time, altered inner ear biomechanics and enabled rapid responses to low frequency signals. Another goal of proposed work was to consider the relationship between the animal’s large size, and the large size of its middle and inner ears more explicitly, and the biophysical properties of the inner ear that determine its function. Using computed tomography as our primary tool, anatomical studies led us to conclude that inner ear size is a critical determinant of auditory function, including the frequency resolving power of the inner ear, within the cat family, if not the larger class of mammals. Finally, having gained a relatively firm understanding of the basics of tiger auditory biology, our efforts turned to an investigation of vocalization acoustics. Motivated in part by a plan to implement a voice recognition algorithm to assess population abundance in the wild, the acoustic properties of a subset of calls within a relatively complex vocal repertoire were examined. From a biological point of view, we were interested in the relationship that exists between what the tiger hears and the sounds that the big cat produces. Given their status as the largest member of the cat family, it is not surprising that the bulk of acoustic power is carried in the low frequency range of their production bandwidth. Even cursory inspection of the vocal apparatus of the tiger confirms the fact that the size of the structure is positively correlated with overall body size; e.g., tiger vocal folds are 3 times the length of the human structure and other vocal tract adaptations, like the unusually smooth, broad medial surface of the vocal folds, combine with size and hyoid apparatus adaptations to support the production of powerful low frequency vocal radiations with a very wide dynamic range. The broader impact of the work completed here is related to the tiger’s precarious survival status in the wild, where they are in serious jeopardy. Estimates indicate that the tiger population has decreased by 95% and the range of its home territories by 93% since 1900. Four subspecies have undergone extinction in the last 100 years and the species at large is critically endangered. One vexing problem facing tiger conservation biologists today is the high degree of uncertainty regarding in situ population abundance. Without knowing the number and density of animals inhabiting any given territory, the challenge of developing effective conservation protocols is significant, and adding to the challenge is the unfortunate condition that traditional population abundance estimation methodologies have proven unreliable in the case of the tiger. Based on work with captive tigers we have shown that individuals can be identified by analyzing the acoustic features of their calls; that is, individual animals appear to possess an "acoustic fingerprint" that might be used to identify and count individuals in situ. To that end, we have developed a "voice" recognition algorithm that permits the correct identification of individual tigers based on the acoustics of calls with an accuracy of approximately 90%.
