Human sperm vs. the sperm whale - Aatish Bhatia

View full lesson: http://ed.ted.com/lessons/human-sperm-vs-the-sperm-whale-aatish-bhatia

Traveling is extremely arduous for microscopic sperm -- think of a human trying to swim in a pool made of...other humans. We can compare the journey of a sperm to that of a sperm whale by calculating the Reynolds number, a prediction of how fluid will behave, often fluctuating due to size of the swimmer. Aatish Bhatia explores the great (albeit tiny) sperm's journey.

Lesson by Aatish Bhatia, animation by Brad Purnell.

In 1977, the physicist Edward Purcell

calculated that if you push a bacteria

and then let go,

it will stop in about a millionth of a second.

In that time, it will have traveled less

than the width of a single atom.

The same holds true for a sperm

and many other microbes.

It all has to do with being really small.

Microscopic creatures inhabit a world alien to us,

where making it through an inch of water

is an incredible endeavor.

But why does size matter so much for a swimmer?

What makes the world of a sperm

so fundamentally different

from that of a sperm whale?

To find out, we need to dive in

to the physics of fluids.

Here's a way to think about it.

Imagine you are swimming in a pool.

It's you and a whole bunch of water molecules.

Water molecules outnumber you

a thousand trillion trillion to one.

So, pushing past them

with your gigantic body is easy,

but if you were really small,

say you were about the size of a water molecule,

all of a sudden, it's like you're swimming

in a pool of people.

Rather than simply swishing by

all the teeny, tiny molecules,

now every single water molecule

is like another person you have to push past

to get anywhere.

In 1883, the physicist Osborne Reynolds

figured out that there is one simple number

that can predict how a fluid will behave.

It's called the Reynolds number,

and it depends on simple properties

like the size of the swimmer,

its speed,

the density of the fluid,

and the stickiness, or the viscosity,

of the fluid.

What this means is that creatures

of very different sizes inhabit

vastly different worlds.

For example, because of its huge size,

a sperm whale inhabits

the large Reynolds number world.

If it flaps its tail once,

it can coast ahead for an incredible distance.

Meanwhile, sperm live

in a low Reynolds number world.

If a sperm were to stop flapping its tail,

it wouldn't even coast past a single atom.

To imagine what it would feel like to be a sperm,

you need to bring yourself down

to its Reynolds number.

Picture yourself in a tub of molasses

with your arms moving

about as slow as the minute hand of a clock,

and you'd have a pretty good idea

of what a sperm is up against.

So, how do microbes manage to get anywhere?

Well, many don't bother swimming at all.

They just let the food drift to them.

This is somewhat like a lazy cow

that waits for the grass under its mouth to grow back.

But many microbes do swim,

and this is where those incredible adaptations come in.

One trick they can use

is to deform the shape of their paddle.

By cleverly flexing their paddle

to create more drag on the power stroke

than on the recovery stroke,

single-celled organisms like paramecia

manage to inch their way

through the crowd of water molecules.

But there's an even more ingenious solution

arrived at by bacteria and sperm.

Instead of wagging their paddles back and forth,

they wind them like a cork screw.

Just as a cork screw on a wine bottle

converts winding motion into forward motion,

these tiny creatures spin their helical tails

to push themselves forward

in a world where water feels as thick as cork.

Other strategies are even stranger.

Some bacteria take Batman's approach.

They use grappling hooks to pull themselves along.

They can even use this grappling hook

like a sling shot and fling themselves forward.

Others use chemical engineering.

H. pylori lives only in the slimy, acidic mucus

inside our stomachs.

It releases a chemical

that thins out the surrounding mucus,

allowing it to glide through slime.

Maybe it's no surprise

that these guys are also responsible

for stomach ulcers.

So, when you look really, really closely

at our bodies and the world around us,

you can see all sorts of tiny creatures

finding clever ways to get around

in a sticky situation.

Without these adaptations,

bacteria would never find their hosts,

and sperms would never make it to their eggs,

which means you would never get stomach ulcers,

but you would also never be born in the first place.