Lecture 4: Muessel - Echolocation in Bats

Question 1

Why do bats emit a sonar?

Answer 1

In order to form an image of their environment as the echo returns with distortions based on the environment
- Useful where there is emitted light

Question 2

How accurate is echolocation?

Answer 2

Very accurate- can detect positions of mesh 80um thick

Used for prey pursuit and capture

Question 3

Describe some of the peripheral mechanisms possessed by the bat?

Answer 3

A tragus in the ears to provide elevation information
Enlarged ears
Nose leaves which funnels sounds produced by nostrils
- Morphology varies between species

Question 4

How is the sonar emitted?

Answer 4

Sound funnelled into a cone
Nose leaf controls the width (azimuth)
Precise vocals

Question 5

How is the sonar received

Answer 5

Selective to different frequencies from different directions

Different ear shape –> different selectivity

Question 6

How do bats filter the noise?

Answer 6

Pinnae of ear collects and funnels sound
Sound information is converted into pressure waves by vibration of ear drum at equal frequencies
Information carried to cochlea
Vibrates along basilar membrane

Question 7

What information can be generated from echos?

Answer 7

Distance of object
Angular size + amplitude - loudness (size of producer)
Direction
- elevation, provided by tragus
- azimuth (anywhere in 60 degrees in front), calculated using intensity differences

Question 8

What are the two types of ultrasound calls?

Answer 8

Frequency Modulated (FM) and Constant Frequency (CF)

Question 9

Describe Frequency Modulated (FM) calls?

Answer 9

Calls start at a high frequency and end at a low frequency (calls sweeps through a range)

Short in duration (

Question 10

Describe Constant Frequency (CF) calls?

Answer 10

Long duration calls (5-30ms) which are emitted as a pulse at the same frequency until a FM sweep at the end

Question 11

How do calls change with distance to target

Answer 11

Closer to target = increased repetition

Question 12

What are harmonics?

Answer 12

Integer multiplications of the calls fundamental frequency
Fundamental = 25Hz Harmonics = 50, 75, 100
- This is not seen as different notes just decreasing sound quality
- 2nd and 3rd harmonic have more energy than fundamental (1st harmonic call)

Question 13

What is the benefits of FM calls?

Answer 13

Able to detect the distance of the target?

• Use each echo from the different frequencies
• Good for short range (hence end of CF)

Question 14

What are the benefits of CF calls?

Answer 14

Detecting: velocity of the target, relative velocity, flutter or wing beat
Sensitive analysis as any distortion is due to environment
Increased range of call
Produced around acoustic fovea

Question 15

What is Doppler Shift?

Answer 15

Detecting how the target is moving relative to the reciever

• Coming towards = higher frequencies
• Moving away = lower frequencies

Question 16

What is the benefit of being able to detect flutter or wing beat?

Answer 16

Causes a modulation in amplitude and/or frequency due to change in wing angle
Able to identify species- predator or prey

Question 17

What is an acoustic fovea?

Answer 17

Area in the basilar membrane where it is most sensitive due to increase concentration of neurons.
CF component doesnt match –> avoid deafening
Adaptations of call (Doppler shift) ensures echos return in fovea

Question 18

What differences are there in ecology between the different types of echolocation?

Answer 18

Exclusive FM bats: take prey from surfraces, use high frequencies to avoid interference, can distinguish prey up to 1 cm accuracy.
Predominantely CF calls: forage in open space or ambush from a perch, use low frequency at a distance and change to FM in pursuit

Question 19

Explain the neural pathways involved in echolocation

Answer 19

Ear –> Auditory Nerve –> Cochlear nucleus (Basilar membrane) –> midbrain inferior colliculus –> forebrain auditory cortex

Question 20

What is the purpose of the basilar membrane?

Answer 20

Neuronal processing
Vibration of the ear drum causes a stimulation of hair cells which then cause an excitation of the auditory nerve
Frequency is encoded by position along membrane

Question 21

What link does basilar membrane have to acoustic fovea?

Answer 21

Overexpression of receptors at the acoustic fovea in order to ensure maximal vibration.
Area likely to be thickened and lengthened.

Question 22

What are the three important areas in the auditory cortex?

Answer 22

FM-FM area
CF-CF area
DSCF area

Question 23

What is the purpose of the FM-FM area?

Answer 23

compares the fundamental call (FM1) to higher harmonic (FM2). Compares delays between call and echo
Cells are systematically arranged with parameters mapped

Question 24

What is the purpose of the CF-CF area?

Answer 24

Comparing CF1 to Echo in combination to analyse velocity information

Question 25

What is the purpose of the DSCF area?

Answer 25

Analyses delays due to wing beat and flutter.
Makes up 30% of auditory cortex
Involves maps: frequency radial, amplitude in columns

Question 26

What are 2 problems with echolocation in bats?

Answer 26

Distinguishing cross talk
- calls and echo have no relevance without the other
- fundamental harmonic is heard through skull tissue
Not to deafen each other:
- sensitivity is reduced when call is preduced
- FM species contract inner ear, CF species emit call outside of acoustic fovea.