Echolocation beam shape in emballonurid bats, Saccopteryx bilineata and Cormura brevirostris

The shape of the sonar beam plays a crucial role in how echolocating bats perceive their surroundings. Signal design may thus be adapted to optimize beam shape to a given context. Studies suggest that this is indeed true for vespertilionid bats, but little is known from the remaining 16 families of echolocating bats. We investigated the echolocation beam shape of two species of emballonurid bats, Cormura brevirostris and Saccopteryx bilineata, while they navigated a large outdoor flight cage on Barro Colorado Island, Panama. C. brevirostris emitted more directional signals than did S. bilineata. The difference in directionality was due to a markedly different energy distribution in the calls. C. brevirostris emitted two call types, a multiharmonic shallowly frequency-modulated call and a multiharmonic sweep, both with most energy in the fifth harmonic around 68 kHz. S. bilineata emitted only one call type, multiharmonic shallowly frequency-modulated calls with most energy in the second harmonic (similar to 46 kHz). When comparing same harmonic number, the directionality of the calls of the two bat species was nearly identical. However, the difference in energy distribution in the calls made the signals emitted by C. brevirostris more directional overall than those emitted by S. bilineata. We hypothesize that the upward shift in frequency exhibited by C. brevirostris serves to increase directionality, in order to generate a less cluttered auditory scene. The study indicates that emballonurid bats are forced to adjust their relative harmonic energy instead of adjusting the fundamental frequency, as the vespertilionids do, presumably due to a less flexible sound production.