746 Lecture 2 Echolocation in Bats
Jan 07, 2016
746 Lecture 2
Echolocation in Bats
Aim Outline properties of sound Hunting behaviour of bats Types of Echolocation sounds Specialisation of
Ear CNS
Auditory behaviour of moths
Properties of sound Sound is wave of rarefaction and
compression has speed 330m/s, c = f * wavelength - determines whether
objects will reflect or diffract sound frequency f intensity -
measured in dB
Harmonics multiples of frequency usually less intense
Hunting behaviour of bats
Taphozous
Pipistrellus
Megaderma
Hipposideros
Echolocation sounds all bats use “ultrasonic” sounds CF -
constant frequency long tone, often with some harmonics velocity
FM - frequency modulated short burst of sound range determination
CF
in free air
FM near ground
or vegetation
CF-FM CF then droop depends on
place
Landing - Rhinopoma
catching - Myotis
What will bat hear? itself ? reflection ?
quieter more variable? Doppler shift in frequency ?
Doppler shift (i) emitted sound
-1.5
-1
-0.5
0
0.5
1
1.5
0 200 400 600 800 1000 1200 1400 1600
Doppler shift (ii) Reflected sound sometimes in phase
and sometimes out of phase
in
out
-1.5
-0.5
0.5
1.5
2.5
3.5
4.5
0 200 400 600 800 1000 1200 1400 1600
-4.5
-3.5
-2.5
-1.5
-0.5
0.5
1.5
Doppler shift (iii) If reflected and emitted sound have similar
intensity, Doppler echo will generate beats
Production of new frequency from old!
-1.5
-0.5
0.5
1.5
2.5
3.5
4.5
5.5
6.5
0 1000 2000 3000 4000 5000
-4.5
-3.5
-2.5
-1.5
-0.5
0.5
1.5
Doppler summary New frequency – depends on ratio of
outgoing sound and incoming sound Incoming sound is reflected off
ground/trees Difference in frequency therefore tells
how fast the bat is flying fnew = fout (v + s)/v
v speed of sound s speed of bat
Echoes From stationary insect
head on- symmetrical sideways on asymmetric
Echoes from fixed Tipula
Moving Tipula
Summary so far Ultrasonic sound
CF FM habitat dependent
Echoes return information moving insects time to return frequency spectrum
shifted broadened
Behaviour to physiology
Specialisation of Ear CNS
Human ear
Bat ear (i) Large pinnae
directional sensitivity extra gain
Tragus elevation
Bat ears (ii) middle ear muscles
reduce sensitivity while emitting?
flying bat
Bat ear (iii) More of cochlea tuned to high
frequencies than in other mammals
Tuning curve auditory nerve
tuned to “best” frequency of emitted CF actually to just above (Why?)
CF lowered in flight Doppler shift as fly towards object
raises return sound frequency
CNS outline
AC ICCN
CNS Auditory
cortex neurons sensitive to pairs of stimuli load/quiet delay time
crucial time map
mechanisms of delay coincidence detection
inhibition of sound delay line
slow axon synapse
control with vocalisation inferior colliculus neurons respond only
30/40ms after vocalisation
Summary - audition Ear and CNS both highly specialised
more sensitive to ultrasonic frequencies achieve increase in sensitivity to echo respond to pairs of stimuli
Moth Auditory system 2 axons in ear
low and high threshold
Behaviour low threshold - fly fast high threshold - stop flying and fall
?like a leaf
Emit clicks - jam bat sonar - phantom echo returns
at wrong time? warning of unpalatability? moths (Euproctis) emit clicks in mimicry
of distasteful moths
Conclusion co-evolution of bats and moths defence reactions
escape auditory camouflage auditory