Bats Flashcards
What is biosonar?
Image-forming system using sound instead of light
Two major sub-orders of bats
Megachiroptera (egyptian fruit bat)
Microchiroptera (horseshoe bat)
First clue to echolocation
Spallanzani and Jurine 1794, blind bats still avoided obstacles
Griffin and Pierce ‘38
Instrument detected high frequency signals
Plugging ears or taping mouth prevents navigation
How is distance and size calculated from an echo?
Time-delay of echo and amplitude
How is elevation assessed?
Move ears independently to compare amplitude
Two types of ultrasonic signals
Frequency modulated (drops from high to low, big brown bat) Constant frequency (horseshoe bat, small FM component
How does echo change as bat approaches target?
Frequency increases
Amplitude decreases
What is a`harmonic?
Integer multiple of the call (call at 20kHz produces 40 and 60kHz echo)
Info from frequency modulated calls
Each echo of varying frequency given info on
Info from constant frequency calls
Target velocity, insect wing beat, one frequency which propagates further (uses doppler shift)
Describe doppler shift use
Speed of object adds to speed of sound to produce higher pitch
If echo returns at higher freq, target is approaching bat (same frequency if stationary)
Wing beat analysis
Wing moving toward bat will produce doppler effect
Frequency and amplitude of echo modulates as angle of wings change (angle is species specific)
Why CF signals useful for doppler shift analysis
Need to compare freq of call to echo for DS analysis
CF have more energy for more range
CF bats sensitive to narrow freq range around CF component of call (acoustic fovea)
Whys is acoustic fovea focused at higher freq than cell?
Returning echo often higher freq
lowers voice so echo returns within AF - DS compensation
Exclusively FM
Predominantly CF
Combined
(uses)
High frequencies reduce background clutter
(search phase) - often from exposed perch. Greater distance. Lower freq to propagate further
Accurate target distance and flutter
Ear characteristics of bats
Enlarged for faint echoes
Specialised flap - pinnae
Peripheral mechanisms
Pinnae help funnel sound into cone rather than sphere (sound width moderated by altering nose leaf
Pinna designed for directional selectivity to frequencies
Ear specialisation
Outer ear contains pinnae (design dictates freq specialisation)
Middle ear - transmitted via ossicles
Inner ear - oval window to cochlea containing basilar membrane hair cells
Neural structure
Ear
Auditory nerve to (hindbrain) cochlear nucleus
(Midbrain) inferior colliculus
(Forebrain) auditory cortex
Process in basilar membrane (in cochlea)
Sound waves come through inner ear into canal
Vibrating of drug stimulates hair cells which excite primary auditory neurones
Specific freq will excite specific part of basilar membrane
Processing in inferior colliculus
Has interneurons sensitive to call-echo delays
Tells about distance
Many neurones around call freq
FM-FM area of auditory cortex used for
Distance coding (neurones respond to delay between call and echo) Each neurone tuned to particular delay and amplitude
CF-CF area used for
Responding to CF call and echo in combination
Encode specific velocities
Character of DF-CF area
30% volume of auditory cortex
Frequency species specific
Analyses type of target
Overcoming cross talk
Energy in call (fundamental harmonic) <1%
Others only hear higher harmonics
How don’t bats deafen themselves?
FM species contract inner ear muscles
CF - call lower than echo. Echo kept in fovea by compensation so call falls outside fovea
