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al., 1997; Rendall et al., 2005). While vocal anatomy can be specialized in particular species, basic principles of produc- tion are importantly similar across all mammals. The most important point is that, at least for larger-bodied animals, vocal quality reflects characteristics of both source energy and subsequent vocal-tract filtering. Critical perceptual attributes like pitch, tonality, and other aspects of timbre can all be understood based the combined effects of these two components. Origins Reptiles and mammals—Probing the evolutionary his- tory of source-filter production, one might ask if dinosaurs also vocalized using such a system. Films like Jurassic Park (1993) and The Land That Time Forgot (2009) show them doing exactly that, inasmuch as their sounds are remarkably mammal-like. Such portrayals are only weakly grounded in scientific evidence, however, which consists of little more than finding that certain duck-billed, Parasaurolophus dinosaurs had elongated nasal passages forming hollow crests (reviewed by Weishampel, 1997; Isles, 2009). Having ruled out other possible functions, paleontologists have con- cluded that these crests must have acted as acoustic res- onators for vocalization. Unfortunately, there is no evidence as to what the source energy used to excite those cavities might have been. This intriguing example from dinosaurs does, however, underscore the broader point that, as a group, reptiles have a purely valve-like larynx that cannot also produce sound. Some modern crocodilians, geckos, and tortoises and turtles do vocalize, but these species represent the exception rather than the rule for reptiles as a whole. In contrast, a sound-pro- ducing larynx is ubiquitous among the more than 4,500 extant mammal species. Given that all the current major mammalian groups had already emerged by about 93 million years ago (Binenda-Edmonds et al., 2007), laryngeal vocal- ization must have arisen even earlier—but nonetheless after divergence from the reptile line. When mammals underwent rapid proliferation after the disappearance of dinosaurs about 65 million years ago, they carried that vocalizing larynx along. In fact, one could argue that vocalization is as funda- mental to being a mammal as having three middle-ear bones or being homeothermic. Primates as mammals—In spite of fundamental com- monalities, mammals do exhibit significant variability in vocal production as well. Differences can occur in both source and filter components, depending on factors such as overall body-size, hearing range, and niche-specific adapta- tions (Fitch, 2006, Brudzynski, 2010). Vibration frequencies vary widely across species, ranging from infra- to ultra-sonic. Extra-laryngeal vocal sacs can dramatically amplify or atten- uate particular frequency regions, and some species even have a mobile larynx that can dramatically increase effective supralaryngeal tract length during sound production. There is an additional, cross-species similarity to point out, however, which is that all mammalian larynges can evi- dently produce a range of phonated sounds, including both harmonically structured and noisy versions. Broadly speak- ing, this division reflects vocal-fold vibration patterns that are either stable and regular, or unstable and chaotic, respec-   Fig. 1. Schematic views of a human male and a female rhesus monkey vocal tract illustrating the source-filter vocal production process. For both species, panels A and B illus- trate source energy frequency spectrum and supralaryngeal transfer function, respectively. Panel C shows the spectrum resulting from combining source and filter, and panel D shows a narrowband spectrogram of the original sound. F0 refers to the fundamental frequency of the sound, while F1–F5 refer to formants. Rhesus monkeys are signifi- cantly smaller relative to humans than indicated here, have significantly higher F0 and formant frequency values. Note that the rhesus vocalization is shown over a wider frequency range. (Drawings by Michael Graham) Human Voice in Evolutionary Perspective 25 

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