Male vocal displays are rarely so dramatically different in closely related subspecies as in Cervus elaphus. Many studies investigated the evolution of the European Red deer low pitched roaring sounds, but little is known about why the Rocky Mountain elk evolved high pitched bugles. We investigated whether sound attenuation characteristics in their respective habitats might have contributed to the evolution of these very different vocal displays. We tested two hypotheses in two representative habitats, the Rocky Mountain National Park, Colorado, and the Vejers Klitplantage Nord, Denmark. Sound attenuation is frequency dependent. We tested the attenuation pattern of playbacks of a harmonic complex consisting of frequencies up to 6 kHz over various distances. Results suggest, first, that in DK a more pronounced low frequency (up to 1 kHz) attenuation occurs, and, second, the variance for a small frequency window around 2 kHz is smaller in Colorado. Attenuation depends also on the ground effect. The ground effect is the acoustic interference between the direct sound wave from sender to receiver and the indirect sound wave reflected off the ground. The effect depends on acoustic properties of the ground and can result in severe attenuation of sounds in particular at lower frequencies. We tested whether Colorado soil is acoustically softer than European soil, negatively affecting low frequencies propagation. We found the opposite true. The smaller ground impedance in DK is the likely explanation for the stronger low frequency attenuation in DK. However, ground impedance differences are so small that they hardly can account for a strong selective pressure against low frequencies in CO. In summary, our data suggest that frequency-dependent attenuation and ground effect are unlikely candidates supporting higher frequency calls in North America. The smaller variance around 2 kHz, although not consistent, seems currently the only observation favoring a high pitched vocalization.
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Rocky Mountain National Park 2010 Research Conference