Quantitative analysis of rigid and non-rigid motion patterns in rhinolophid and hipposiderid pinnae

Abstract

It has long been noted that horseshoe bats (Rhinolophidae) exhibit large ear motions during biosonar behaviors. Some authors have regarded these motions as rigid rotations of the pinna whereas others have pointed out that the animals are capable of non-rigid motions where the shape of the pinna changes. In the current work, we have analyzed pinna motions in greater horseshoe bats (Rhinolophus ferrumequinum) and the related great roundleaf bat (Hipposideros armiger) to investigate the issue of rigid versus non-rigid pinna motions. The bats were found to produce rigid as well as non-rigid motions. The latter were characterized as “open-close” motions where the size of the pinna aperture was changed by varying the distance between the anterior and posterior rims of the pinna. To quantify this, pinna motions were described by three-dimensional tracking of points on the pinna rims using high-speed stereo vision. The point coordinates were used to compute a matrix that contained the maximum change in distance between any pairs of points on the anterior and posterior pinna rims. Dimensionality reduction methods (e.g., linear discriminant analysis) were applied to the vectors containing all elements of these “maximum distance change matrices” to quantify the relationship between rigid and non-rigid motions.