Hearing without ears: Dolphins

Fun Fact

Dolphins don’t have external ears.

Introduction

My son’s ears stick out. They stick out so much that his swimming teacher jokingly suggested wearing a swim cap might help. This got us thinking: Do dolphins and whales have ears? How do earthworms hear? Why are ears shaped the way they are? Would dolphins and whales create more drag if they had external ears? Are my son’s ears causing significant drag? Doesn’t his long surfer hair make more of a difference when moving through water?

Equipment

coat hangers, string, chopsticks, metal straws or knitting needles

Method

Attach one piece of string to each end of the coat hanger. Create a loop wide enough to fit a finger through at the end of each string. Now loop the strings over your pointer fingers. Ask one of the children to hit the coat hanger with a metal straw or chopstick. What does it sound like? Next, stick your fingers in your ears and slightly lean forward. Ask the child to hit the hanger again. What does it sound like now? Invite the children to try it out themselves. Can they describe the difference?

What’s the STEM?

In humans, the part of the ear that is visible outside the head is called auricle or pinna. Most animals don’t have this type of external ear, including reptiles, birds, amphibians, fish and all invertebrates. They use different ways to detect sound or ‘hear’.

The coat hanger trick works because sound waves carry sound from the source to your ears. Sound waves travel better through solids or liquids than through the air, because molecules, which propagate sound, are further apart in the air. Inside your ear, the sound wave causes your eardrum to vibrate and you hear the noise.

When you put your fingers in your ears, you provide a path that lets more of the vibrations reach your ears. When your fingers aren’t in your ears, you hear a faint, high-pitched, tinny sound from the coat hanger. When you put your fingers in your ears, you hear a deep, resonant tone. The hanger creates the same sound waves in both situations, but in one of them, you provide a path that lets more of them reach your ears.

Dolphins do not have external ears like humans. Scientists believe that the lower jawbone conducts sounds to the middle ear. Dolphins also have two small openings on both sides of their head, which form part of a sophisticated hearing sense that allows them to localise sounds underwater.

Some frogs hear with their mouths. The mouth amplifies the sound and the waves travel through bones and tissue in their head to the inner ear, where hair cells translate them into electric signals sent to the brain. Sound travels through bones and tissue, just like in the coat hanger exploration.

Observations, conclusions and extensions

A similar way to experience sound travelling through bones is to stick a finger in your ear and then ask someone to hold a vibrating tuning fork to your elbow.

You could also look at how sound waves travel through string or wire by making a tin can telephone. Can you replace the tin can with a paper or plastic cup? What does it sound like if you use a wooden or plastic coat hanger? Explore how different materials propagate sound.

Dolphins can’t hear our voices. They hear at a different, higher pitch than we speak. Our voices are too low for them to hear us. A bit like dog whistles. You could investigate!

STEM language

Use your explorations to introduce this STEM vocabulary to the children: vibrations, strike, material, plastic, metal, sound, volume, quiet, loud, propagate.