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

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Doppler shift (ii) Reflected sound sometimes in phase

and sometimes out of phase

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out

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Doppler shift (iii) If reflected and emitted sound have similar

intensity, Doppler echo will generate beats

Production of new frequency from old!

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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