Figure 3.

Directivity index (DI) for the 17 bat species measured to date. The DI compares the emitted source level with that of an omni-directional source producing a signal with the same acoustic power. The species are grouped by family and arranged within families by increasing size (forearm measurements). Vespertilionids, Emballonurids, and Mormoopids emit sound through the mouth. Phyllostomids, Rhinolophids, Hipposiderids, and Megadermatids emit sound through the nose. ^* indicates a recording of a freely flying bat, + indicates that the bat was restrained, □ indicates a field recording, and ○ are recordings of the terminal buzz. Measurements from: ¹Griffin, 1958; ²Möhres and Neuweiler, 1966; ³Simmons, 1969; ⁴Shimozawa et al., 1974; ⁵Schnitzler and Grinnell, 1977; ⁶Mogensen and Møhl, 1979; ⁷Hartley and Suthers, 1987; ⁸1989; ⁹Henze and O'Neill, 1991; ¹⁰Ghose and Moss, 2003; ¹¹Hiryu et al., 2006; ¹²Surlykke et al., 2009; ¹³Jakobsen and Surlykke, 2010; ¹⁴Brinkløv et al., 2011; ¹⁵Jakobsen et al., 2012; ¹⁶Jakobsen et al., 2013; ¹⁷Matsuta et al., 2013; ¹⁸Surlykke et al., submitted. Where measurements were not available from the literature, DI was calculated for a piston source emitting a sound beam with the reported half amplitude angle (the angle where the sound pressure is reduced by 6 dB relative to 0°).