Grey long-eared bat.
Credit: iStockPhoto
BRISTOL: 3-D modelling has exposed how bat species have evolved different shaped pinnae - the visible part of the ear - to help them navigate and hunt using echolocation.
The models show how the pinnae have evolved to maximise sensitivity, a feature which could be used to inspire new designs in sonar and radar engineering.
The features could also find applications in biomedical ultrasound, non-destructive testing, wireless communications, and sensory systems for autonomous robots, according to lead author and acoustic engineer Rolf Müller, of Virginia Tech in the U.S.
"My lab has been doing modelling work on the acoustic function of bat pinnae for several years. The novelty here is that we have analysed the pinnae from at least 59 bat species to find the building blocks of evolutionary bat pinnae design across different species."
Shape of bat ear aids navigation
Bats use a sophisticated biosonar system to navigate and hunt. By broadcasting high-frequency calls and waiting for the echo of these calls they are able to generate a sound-map of their landscape. The pinnae play an important role in capturing the incoming ultrasonic waves and focusing them into a signal that is analysed by the inner-ear.
Müller and colleagues from Shandong University in China created 3-D computer models of 100 bat pinnae covering 59 different species, and transformed the models into cylindrical representations. The models were statistically analysed using principle component analysis - a method routinely used to analyse human faces, palms and ears.
"We found that pinna width, bilateral symmetry and width of pinna base versus width of the pinna tip - where the key features to explain a lot of the natural variability," explained Müller.
"It seems that some pinnae favour detection and identification of targets over precise localisation, others seem to have the opposite goal, and some seem to be a compromise, doing different things in different directions, like bifocal glasses."
Big ears help bats form sound-map
Many bats use a combination of active sonar - the analysis of echoes which have been produced by the bats own echo pulse - as well as passive sonar, the analysis of sounds from foreign sources.
According to the study published in Bioinspiration & Biomimetics, the researchers demonstrated how the variability of the visible part of the ear can affect the properties of beam forming - the process by which the incoming signal is diffracted by the shape of the pinna to create a 'beam pattern' through which the bat sees its environment.
Müller believes these variations in pinnae designs have evolved to allow different bat species to navigate and hunt across different terrain, from open plains to dense forests.
