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Hipposideros pinna motion patterns as an inspiration for dynamic biomimetic sensing

Many bat species, e.g., among the horseshoe bats (Rhinolophidae) and Old-World roundleaf-nosed bats (Hipposideridae), have highly mobile pinnae. Experiments with biomimetic reproductions of the pinnae shapes/motion patterns in these species have demonstrated a dynamic enhancement of the performance in acoustic direction-finding paradigms. It could hence be hypothesized that reproducing the full range of pinna mobility patterns in bats will allow realizing the full range of the animals' biosonar capabilities. Pinna motions in rhinolophid and hipposiderid bats have been shown to fall into two distinct categories, being either rigid rotations (i.e., changes in orientation but not in the shape itself) or deformations that change both pinna orientation and shape. To characterize the variability in the rigid rotations, landmarks on the pinnae of hipposiderid bats have been tracked using stereovision. From this kinematics data, an axis-angle representation of the pinna rotations was estimated. The results showed that the axes of pinna rotations can scatter widely in orientation, covering a range of directions that extends 180 degrees in azimuth and 180 degrees in elevation, about 40 times larger than the estimated error of the employed method. Hence, biomimetic reproductions of bat ear mobility should explore how to make use of such a variability.