9723521 Bradbury The adaptive design of many animal sound production systems is strongly influenced by selection for increased call amplitude and efficient projection of those sounds in the direction of intended recipients. In some systems, specific structures have profound effects on both volume and directionality. One such structure which appears as an accessory organ in the sound production systems of many diverse vertebrate groups is an inflatable air sac. While this adaptation is clearly an important feature of these systems, very little is known about how air sacs affect sound volume and directionality. Using newly developed multitrack recording techniques, this study will focus on several species of lekking grouse to clarify the relationship between display call amplitude, the directionality of the call, and the shape and number of vocal sacs. We have chosen grouse because of the degree of interesting variations in air sac position and operation in the different species. Some grouse have paired external air sacs. Preliminary evidence suggests that sage grouse males may use their two-sac system to beam certain components of their mating displays toward females regardless of the males' relative orientation. These data will also be used to re-examine the role of amplitude in mate choice of female sage grouse. Sage grouse leks are one of the best studied systems in which females choose mates solely on the basis of male phenotype; call amplitude is the only parameter which has remained resistant to proper measurement. We now have the means to make these measurements and complete the analysis. Our hope is that this comparative study will generate models that can be applied to vocal sac function in other groups and, more broadly, contribute to our understanding of the importance of amplitude and directionality in female choice biased mating systems.
