Underwater hearing (of vertebrates)

Human ear

The inner ear

Fish ears

Odontocete receiving system

CT scan from Darlene Ketten

“Acoustic fat” found ONLY here & melon

How do we test hearing?• Behavioral methods

– Animal trained– Responds

• Go/no-go• 2 alternative choice

• Auditory brainstem response– No training required– Record firing of auditory cortex

• Usually test pure tones• Occasionally test pulses

– Thresholds much lower for pulsed sounds than pure tones

Up-down staircase procedure50% ‘catch trials’ (no signal present)

Envelope following response

Supin et al.

Envelope following response ABR

ABR threshold calculation

ABR

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Behavioral vs. ABR

Yuen et al. 2005

Behavioral vs. ABR

• Behavioral– Requires months to train, months to test– Usually only 1 subject

• ABR– Requires no training, rapid testing

• Can be used to test for transient effects

– Can be done on more species e.g. stranded animals, catch and release animals

• Both require placement of a threshold that varies with conditions

Fish hearing

Carp (goldfish)

Cod

Salmon

Damselfish

Tuna

Popper et al.

3 types of fish ears• General fish

– No hearing specialization– 100-1,000 Hz– Best hearing 100-400 Hz

• Specialized hearing– Goldfish, catfish, etc.– 100-3,000 Hz– Best hearing 300-1,000 Hz

• High frequency adaptations– Clupeids (herring, shad, menhaden, sardine, anchovy)– Swimbladder morphology facilitates broad frequency hearing

range– 1-200,000+ Hz

Cetacean hearing

Human

From: Au, 1993

Pinniped external ears

Elephant seal Harbor seal Sea lion

Kastak et al. 1999

Pinniped in-air hearing

Kastak et al. 1999

Pinniped underwater hearing

Kastak et al. 1999

In air vs. underwater – pressure or intensity?

Phocids (true seals) generally hear equally well in air and underwater – amphibious

Elephant seal – a deep diver hears better underwater (bone conduction in air)

Fur seals hear better in air – primarily terrestrial socialization and mating

Fur seal

Pressure – assumes hearing mechanismIntensity – corrects for acoustic properties of media. Energy flow measureDoes not require knowledge of stimulus mechanism

Elephant seal

Harbor seal

Hearing curves combined

Bottlenose dolphinCod

Sea lion

Catfish

Harbor porpoise

Project “Deep EAR”

• Human hearing attenuates with increasing pressure (chamber experiments)

• Beluga whales (a dolphin species) experience large pressure increases with diving

• Effects on whistling and hearing in free-swimming animals

Ridgway, S. H. et al. J Exp Biol 2001;204:3829-3841

Ridgway, S. H. et al. J Exp Biol 2001;204:3829-3841

Up to 40 tones were presented to the whale during a dive

Depth effects – Beluga whales

“Deep EAR” results

• Increasing pressure (up to 300 m dives)

• Did not affect hearing• Changed whistle

spectra and intensity• One whale only

clicked at 300 m depth

Diving and elephant seal hearing

Kastak et al. 2001

Temporary threshold shifts

• Aural fatigue• Hearing threshold increased• Recovery follows with varying time course

(minutes – weeks)• Experiments in chinchillas and humans

have shown the relationship between TTS and PTS (permanent threshold shifts)

• Good predictor of auditory damage

TTS

Finneran et al 2005

Temporary threshold shifts

• Longer exposures to quieter sounds have the same effect as shorter exposures to louder sounds

• Exposure intensity usually relative to hearing threshold except for impulsive sounds

• The total exposure energy of the sound to which an animal is exposed important

Signal effects on hearing

• Received intensity (source level + range + environmental conditions)

• Frequency• Duration• Timing (spacing between sounds)