The Physics Of Extreme Noseleaf And Ear Shapes In The Horseshoe Bat R. Paradoxolophus

With evolution and natural selection,most bats have developed an ultrasonic echolocation system with high performance,called biosonar,which comprises two primary parts:signal sender(mouth or nostril) and signal receiver(external ear).The signal sender is surrounded by complicated wrinkles(noseleaf) whose position and structure are considered able to change the direction of ultrasonic pulse.Experimental studies have suggested noseleaf really has complicated acoustic function.However, the former studies just explain quite few phenomena based on observation and rough survey.The unique noseleaf structure of Bouret's horseshoe bat(Rhinolophus paradoxolophus) and the acoustic function of its relatively huge external ear,the research object of this thesis,have not got thorough study.This thesis makes acoustic study of the noseleaf and external ear of Bouret's horseshoe bat by numerical implementation for the first time.It mainly analyzes the acoustic function of sella,a unique structure on the noseleaf of Bouret's horseshoe bat,and for the first time it discloses the mystery of the bat's long sella in terms of the evolution of physical function and biological extreme.It also studies the acoustic function of external ear based on the practical process of echolocation,and puts emphasis on the acoustic function of antitragus in receiving ultrasonic pulse.The study reveals that the tip of the external ear could bend when the wrinkles surrounding contract, which will intensify the sidelobe of the beam or bring about new sidelobe in ultrasonic echolocation at low frequency,so as to detect the area beyond the primary detection orientation.The first step is to acquire original biological data.In experiment,high-resolution X-ray micro CT machine is adopted to scan samples of noseleaf and external ear of Bouret's horseshoe bat to get original digital shadow images.Then the cross-section view of the noseleaf and external ear are reconstructed with the method of cone beam reconstruction.These images with 256 gray level are then processed through Gaussian filter and boolean process,and images with clear definition of air and biological tissue could be got.With three-dimensional digital image processing technique,three-dimensional digital image attributes of Bouret's horseshoe bat noseleaf and external ear are acquired.With the three-dimensional image technique,a kind of digital image processing technique,change the length of sella reasonably to shorten or prolong it smoothly in the direction of its intrinsic axis abiding by certain stretching/contraction factor.In contraction,it is shortened by one eighth of the original length in turn until it is one eighth of the original length,which produces a series of 7 new noseleaf structures with shorter sella;in stretching,it is prolonged by one eighth of the original length until it is two times as long as the original,which produces a series of 8 new noseleaf structures with longer sella.In this process,altogether 16 noseleaf structures,including the original noseleaf structure,are produced,with different sella length varying from one eighth to two times of the original.Shortening and prolonging of sella are done respectively on the nose leaves of three different bats to verify that it is a general character.With method similar to the deformation of sella,the antitragus of Bouret's horseshoe bat is revolved by two degrees of freedom.One revolution is done in left-fight direction on the surface vertical to its minimal intrinsic axis;the other is done nearby the pinna.The practical deformation is the composite result of these two revolutions. 5 positions are set for revolution in left-fight direction on the surface vertical to its minimal intrinsic axis:10°left,20°left,30°left,10°right,20°fight,thus producing altogether six cases including the original position.4 positions are set for revolution nearby the pinna:10°,20°,30°,40°respectively,thus producing altogether five cases including the original position.Combination of both revolutions produces altogether 30 external ear structures(including the original external ear structure).Except the three cases which coincide with the pinna,and add the case that is completely cut by antitragus,altogether 28 different external ear structures remain. Based on the above 28 different external ear structures,the bend of the external ear tip caused by contraction of shrinks surrounding is also simulated.The three-dimensional data experiment is done respectively on three samples Bouret's horseshoe bat external ear,two right ears and one left ear.Different from operation on fight ears, the revolution of left ear in left-right direction is from 20°left to 30°right.Near-field sound field distribution of the above deformation is also simulated using finite element analysis.In analysis,it gets the far-field sound pressure distribution according to the near-field data using Kirchhoff integral,analyzes the sensitivity of different structures to space and direction,and finds the numerical function for sella length through survey of the broad width of noseleaf ultrasonic beam in direction of elevation angle.The result suggests that under natural state,sella length is a special value:the sella tends to have worse action of converging ultrasonic beam as it is shortened, while its action of converging ultrasonic beam is very little affected when it is prolonged.Therefore,the unique shape of Bouret's horseshoe bat sella can be predicted by its physical function.The analog computation of external ear reveals that revolving antitragus in leftright direction exerts very little influence on ultrasonic beam forming,while the bend of external ear top resulting from wrinkles contraction is beneficial to the echolocation of Bouret's horseshoe bat.The external ear top of Bouret's horseshoe bat bends when the wrinkles surrounding,which distinctly increases the number of sidelobes and intensifies them.Bats usually makes common environmental detection with low frequency, so sidelobes at low frequency is of particular importance,can give them the ability of out of primary orientation detection with out high frequency emission.