Dolphins are fatheads - and that's how they hear

Dolphins hear, despite their lack of external ears, because they are "acoustic fatheads". They use the fat inside their heads to focus sound waves - a skill that makes up for their lack of outer ears.

Dolphins hear, despite their lack of external ears, because they are "acoustic fatheads". They use the fat inside their heads to focus sound waves - a skill that makes up for their lack of outer ears.

The discovery was made by Darlene Ketten of the Woods Hole Oceanographic Institution in Massachusetts, who used advanced computer tomography and magnetic resonance imaging systems to build up a picture of how dolphins hear.

Dolphins let out a constant stream of supersonic sounds, such as clicks and high-pitched squeals, which they use in the same way as sonar. The echoes of the sounds are processed by their inner ear - which has twice as many receptors as the human ear - to give them an auditory picture of what objects lie in the ocean around them. But because they often dive at high speed and to great depths, a normal eardrum (such as in bats, which also use echo location methods) would be a liability rather than an asset. Hence dolphins have smooth heads with just pinpricks for ears.

Scientists have known for years that deep inside dolphins' heads are structures called the auditory bulla, containing the dolphin's inner ear. It is encased in a separate bone, connected to the skull by fibrous tissue. The organs in the bulla do the same task as humans' middle and inner ears, where sounds are translated into nerve impulses for the brain.

Scientists also knew that dolphins had an unusual amount of fat in their heads and faces compared with the rest of their bodies, and suspected that it might somehow conduct sound - but were unsure how.

Dr Ketten paid particular attention to the bundles of fat in the dolphins' lower jaws and ear canals - and, by using magnetic resonance imaging scans, discovered that "they have a shape like an ear trumpet".

It is that which helps to channel the huge range of sounds into the bulla, she revealed at a meeting for the Society of Marine Mammalogy last month, reported by New Scientist magazine today.

Start your day with The Independent, sign up for daily news emails
ebooks
ebooksA special investigation by Andy McSmith
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Ashdown Group: Technical Presales Consultant - London - £65,000 OTE.

£65000 per annum: Ashdown Group: Technical Presales Engineer - central London ...

Recruitment Genius: Physiotherapist / Sports Therapist

£20000 - £50000 per annum: Recruitment Genius: A Physiotherapist / Sports Ther...

Recruitment Genius: Sales Executive / Advisor

£8 - £9 per hour: Recruitment Genius: Sales Executives / Advisors are required...

Recruitment Genius: Warehouse Operative

£14000 - £15000 per annum: Recruitment Genius: An opportunity has arisen for a...

Day In a Page

Homeless Veterans campaign: Donations hit record-breaking £1m target after £300,000 gift from Lloyds Bank

Homeless Veterans campaign

Donations hit record-breaking £1m target after huge gift from Lloyds Bank
Flight MH370 a year on: Lost without a trace – but the search goes on

Lost without a trace

But, a year on, the search continues for Flight MH370
Germany's spymasters left red-faced after thieves break into brand new secret service HQ and steal taps

Germany's spy HQ springs a leak

Thieves break into new €1.5bn complex... to steal taps
International Women's Day 2015: Celebrating the whirlwind wit of Simone de Beauvoir

Whirlwind wit of Simone de Beauvoir

Simone de Beauvoir's seminal feminist polemic, 'The Second Sex', has been published in short-form for International Women's Day
Mark Zuckerberg’s hiring policy might suit him – but it wouldn’t work for me

Mark Zuckerberg’s hiring policy might suit him – but it wouldn’t work for me

Why would I want to employ someone I’d be happy to have as my boss, asks Simon Kelner
Confessions of a planespotter: With three Britons under arrest in the UAE, the perils have never been more apparent

Confessions of a planespotter

With three Britons under arrest in the UAE, the perils have never been more apparent. Sam Masters explains the appeal
Russia's gulag museum 'makes no mention' of Stalin's atrocities

Russia's gulag museum

Ministry of Culture-run site 'makes no mention' of Stalin's atrocities
The big fresh food con: Alarming truth behind the chocolate muffin that won't decay

The big fresh food con

Joanna Blythman reveals the alarming truth behind the chocolate muffin that won't decay
Virginia Ironside was my landlady: What is it like to live with an agony aunt on call 24/7?

Virginia Ironside was my landlady

Tim Willis reveals what it's like to live with an agony aunt on call 24/7
Paris Fashion Week 2015: The wit and wisdom of Manish Arora's exercise in high camp

Paris Fashion Week 2015

The wit and wisdom of Manish Arora's exercise in high camp
8 best workout DVDs

8 best workout DVDs

If your 'New Year new you' regime hasn’t lasted beyond February, why not try working out from home?
Paul Scholes column: I don't believe Jonny Evans was spitting at Papiss Cissé. It was a reflex. But what the Newcastle striker did next was horrible

Paul Scholes column

I don't believe Evans was spitting at Cissé. It was a reflex. But what the Newcastle striker did next was horrible
Miguel Layun interview: From the Azteca to Vicarage Road with a million followers

From the Azteca to Vicarage Road with a million followers

Miguel Layun is a star in Mexico where he was criticised for leaving to join Watford. But he says he sees the bigger picture
Frank Warren column: Amir Khan ready to meet winner of Floyd Mayweather v Manny Pacquiao

Khan ready to meet winner of Mayweather v Pacquiao

The Bolton fighter is unlikely to take on Kell Brook with two superstar opponents on the horizon, says Frank Warren
War with Isis: Iraq's government fights to win back Tikrit from militants - but then what?

Baghdad fights to win back Tikrit from Isis – but then what?

Patrick Cockburn reports from Kirkuk on a conflict which sectarianism has made intractable