Static and dynamic control of emission beamwidth in horseshoe bats

Abstract

Horseshoe bats (family Rhinolophidae) emit their ultrasonic biosonar pulses nasally with their nostrils surrounded by baffle structures known as “noseleaves.” The noseleaves are often characterized by intricate local shape details such as flaps, ridges, or furrows. Furthermore, some of these structures are capable of undergoing non-rigid shape changes over time. One part of the horseshoe bat noseleaf that has both static local shape features as well as a time dynamics is the lancet, a vertical projection on the top of the noseleaf. The most striking static shape features of the lancet are half-open cavities (furrows) and the most obvious in-vivo motion of the lancet is a rotation in the distal-proximal about the base of the lancet. In the present work, the acoustic effects of the furrows and the distal-proximal rotation of the lancet were studied in three individuals of the Greater Horseshoe bat (Rhinolophus ferrumequinum) using numerical methods. The lancet furrows were always found to act as beam-focusing devices irrespective of lancet rotation. The acoustic effect of forward lancet rotation within the range seen in bats (tested angles 0°, 5°, 10°) was always a significant widening of the beam.