Lec 21 - Echolocation

Echolocation

Navigation by sound. High frequency sounds are emitted to gain information about the surrounding environment from the returning echo, interpreted from sound waves.

High frequency

Greater than 20 kHz

Hertz

1 cycle/second

• kHz = 1000 cycles/second

Cycle

1 wavelength

Bats echolocation frequency range

8 kHz-215 Khz

Human sound frequency range

20 Hz - 20 kHz

Vertebrate groups that echolocate

• Oil bird & cave swiftlets

• Bats

• Cetacea

• Pinnipeds

• Rodents

• Tenrecs & shrews

Tenrec echolocation

By tongue clicks 5-17 kHz.

Environment assessment in fruit bats

Do not echolocate and are dependent on vision and smell. Vision involves interpreting information from energy received as light waves.

Environment assessment in all other bats

Vision, smell, and hearing in the form of auditory echolocation. Hearing involves interpreting information from energy received as sound waves. They can operate in the darkness (frees bird competition) and navigate in caves protecting nurseries. Echolocation is used in mother—young identification as well.

How do bats echolocate?

1. Sound produced in voice box

2. Sound emitted through nose or mouth

3. Sound pulses transmitted through, scattered by, or reflected back.

4. When reflected, returning sound is interpreted.

Mouth adaptations of bats for echolocation

• Funnel shaped mouth to direct sound.

• Mouth aligned to flight path.

Nose adaptation of bats for echolocation

• Nose aligned to flight path.

• Emit sound through nostrils with mouth closed

• Distance between nostrils determines interference patterns

Echolocation calls can be described in terms of:

• Frequency

• Harmonics

• Intensity (signal strength)

• Time (duration, repetition)

Why are bat echolocation frequencies so high?

Short wavelength detect small objects better. High frequencies attenuate rapidly, are unique in the environment, and are easily directed.

Harmonics of echolocation

Base frequency that is lower but can get higher. This can be beneficial in rich environments where different frequencies can detect different textures and sizes of objects. Harmonics help to distinguish between near and far objects, the closer the object the more similar the return time echo from the harmonics.

Intensity of echolocation

Measure of sound pressure, visualized by the amplitude of sound waves. Bats tend to be louder in open environment and quieter in complex environments.

What are adaptations bats have to avoid self-deafening?

• Muscles of inner ear well developed

• Bones housing middle and inner ear insulated from the rest of the skull

• Ears tuned to incoming echoes only

Time of echolocation

When a bat takes insects in flight, the frequency and characteristics of echolocation pulses vary depending on go the bat is in search, approach, or terminal/attack phase.

Interpreting echolocation bats

• Search - low rate of repetition but doing it a lot. High duty cycle.

• Approach - transition to high repetition rate. Time between call and echo decreases. Low duty cycle. Rely on Doppler shift and tight attack.

• Termination

Doppler shift

a change in sound frequency of an echo relative to the original signal caused bu movement of one or both objects.

Low duty cycle approach

• Produce signals small percentage of time

• Do not broadcast and receive signals simultaneously

• Produce short duration, broadband, downward FM calls

• Present in most bats and in cetaceans

• More basal condition

High duty cycle approach

• Produce signals larger percentage of time

• Produce pulses and receive echoes simultaneously

• Produce long duration, narrowband, CF calls; short periods of silence

• Doppler-shifted echoes of prey relative to background

• Sensitive to amplitude and frequency shifts generated by the wings of fluttering insects, against cluttered backgrounds

• Required evolution of sophisticated neurobiological specializations

Wing shape bats

• Aspect ratio: proportion of wing length to width

• Wing loading: ratio of body mass/surface area of the wing.

Open spaced wing shape

Speed and endurance

• High aspect ratio (long, narrow wings)

• High intensity (loud), low frequency FM call, CF calls

• No harmonics

Dense space wing shaped

Maneuverable

• Low aspect ratios (short and wide)

• Low intensity, high frequency FM pulses

Cetacean echolocation

Low frequency (long distance), infrasonic for infraspecific communication.

• The skull-vibration enabled bone conduction mechanism and a pressure mechanism transmitted through soft tissues.

• Bone conduction is the predominant mechanism

How do Odontocetes echolocate?

• Air in lungs shunted through sinuses and nasal systems.

• Air squads out, loud

• Melon (oil & fat filled structure) focuses sounds

• Sound received by dentary

• Asymmetric skylls

• Tympanic bullae not fused, insulated by sinuses filled with oil.