Hearing and Echolocation

Background on sound

Important sense for MMs.

Travels faster and farther in water (speed of sound 4.5x faster and 1500 m/s)

Low frequencies travel 100/1000s of kms

Frequency and Intensity

Frequency measured in Hz (cycle/second). Intensity (power): decibels (dB); logarithmic

Human (sonic) range: 20Hz-20kHz

Above: Ultrasonic Below: Infrasonic

Human speech: 0.2-4kHz

Confusion with decibels

Human pain/damage threshold in air: 120 dB (logarihmic)

Air to water comparisons difficult: different reference pressures: 1 uPa in water vs. 20 uPa in air. Subtract 61.5 dB from water value to approx. air valves.

Rough hearing ranges

Odontocetes: Tursiops 0.2-160 kHz, fine directional sensitivity

Mysticetes: no ultrasonics likely: similar maybe lower than humans. Infrasound: likely 10-15 Hz

Pinnipeds: In air up to 32 kHz (otariids higher). In water up to 60 kHz for phocids, 35-40 kHz for otariids. Both prioritize water, but phocids more

Sirenians: 0.1-40 kHz: perhaps infrasound. Less impressive sensitivitiy and directionality

Hearing ability

mix of frequency ranges and sensitivity

How do you hear underwater?

Problems with sensitivity: impedence issue of air/water density change

Problems with directionality: bone conduction and skull vibrations

Typical mammalian ear

Outer ear: pinna, auditory canal

Middle ear: tympanic membrane

Inner ear: cochlea, semicircular canals

Human ear

outer ear: ear canal/tympanic membrane (eardrum), pinna

middle ear: ossicles

Inner ear: cochlea (sensory hairs, interprets sound), semicircular canals (controls balance)

Cetacean ear

outer ear: ear canal (lower jaw, hollow fat/lipid filled pathways)

Cetacean “earbone” (auditory/tympanic bulla) or tympano-periotic complex. House middle and inner ear

Odontocete ear

still have ear canal (not used much underwater, maybe in air). T-P complex isolated from skull. Suspended by ligaments, surrounded by tissue

Mysticete ear

Also blocked ear canal. Waxy plug prevents much hearing on land. Waxy glove finger

Sea lion ear

T-P, cochlea connected to skull, but less than ours. Put blood into tissue and block air space (canal) when diving.

Phocid ear

Must do double duty. Not isolated, but semi-cushioned auditory bulla. Large ossicles – enhance bone conduction

Sirenian ear

cheek bone analogous to pan bone. Dense auditory balla, but not isolated (less directionality and sensitivity)

What is echolocation?

seeing with sounds

high vs. low frequencies

distinctive characteristics: density info/internal observation, active sonar: not always on, invisibility of some objects (similar densities to water, orientation)

Ken Norris Theory

4 parts

1. sound generated in pharyngeal lips/divercula

2. reflects off parabolic skull

3. focused by melon

4. return echo received via pan bone (mandible)

The Goosebeak (larynx)

traditional site of sound generation

sound transmitted and refracted through melon and exit head

Sound generation (odontocetes)

monkey lips/posterior bursa (pharyngeal lips)

sound bounces off of parabolic skull

Evidence for sound from pharyngeal lips in dolphins

site pinpointed by ultrasonic imaging, doppler motion detectors, pressure sensors, x-ray

larynx used for unspecialized sounds

Diverticula

collects/directs air across pharyngeal lips

sound focused by melon and return via pan bone

specialized oils (unique from other lipids in body)

great acoustic conductors

synthesized (not from diet)

Sound focusing by melon

captive studies show sound beam from melon

melon runs right up to pharyngeal lips

put acoustic foam over rostrum (can’t hear)

Sperm whale echolocation

generate sound at front of head

spermaceti: oily (what whalers want)

sound goes through spermaceti, bounce off skull wall and bounce off

River dolphins skull

shape for fish eating

can still echolocate, but not as important

Echolocation clicks

broad band

low travels further

clicks are short to quickly hear short echo

Echolocation sequence of events

1. Random search

2. Approaching object

3. Close investigation

Random search

emphasize distance over detail

wild Tursiops observations: 7 kHz “ping”

broad band but peak freq. low

fair resolution: 21 cm wavelength

long range: 500-600 m (based on 0.6-0.8 sec. intervals)

need to receive echo before next sound goes out

Approaching object

As it gets closer:

1. Increase peak frequency (still broad band). Get more details from higher frequencies. Less distance, so dissipation of high freq. waves are less of an issue.

2. Increase repetition rate of clicks. Echoes return quicker (cover less distance). Wait for echo to return before sending out next click

Close investigation (8-10 cm)

jaw pops and biting. Very high frequency peaks (100-150 kHz)

Other sound skills

Ultrasound: seeing internally? mixed abilities

Sound as a weapon: controversial. emit “bangs”, but sound pressure level snd frequencies (low) unclear. equire 20-250 dB to damage or disorient small fish