Bats lend an ear to acoustic engineering

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.

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.

 Renderings of shape representations reconstructed from computer-tomographic scans of bat ears (and one nose, shown in the center).

"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."

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.

Prediction of the spatial sensitivity of a bat ear. Sensitivity isosurfaces are rendered in a different color for each frequency (blue colors: low frequencies, red colors: high frequencies). An animated version is available here (4.2 MByte).

The study found that a group of bats that actively hunts for prey in dense forests could be separated from other bats by the width of their pinna openings, which possibly indicates a customised adaption to pressures of that environment.

"In order for this to happen, the ears of bats must be studied further," said Müller.

"An example would be to expand the sample to include more diversity and find more specific relationships between pinna shape and beam forming across different species. Small local shape features that are hard to capture by the present analysis can also have a big impact on the function."

Engineers are interested in what they can learn from bats both in terms of refining signals used in sonar and radar, and in terms of how best to receive signals.

"This research has revealed a wide range of signal types and some remarkable parallels between the signals that are used by bats, and those used in sonar and radar," said Gareth Jones, lecturer of bat ecology at Bristol Universtiy, UK.

"The findings will be of interest especially to scientists working at the interface of engineering and biology, for example to researchers building echolocating robots."

Images from: Müller

Read the original article here:

A method for characterizing the biodiversity in bat pinnae as a basis for engineering analysis

Jianguo Ma and Rolf Müller 2011 Bioinspir. Biomim. 6 026008 doi: 10.1088/1748-3182/6/2/026008