What shape is an egg?

Chances are you're picturing this: It's generally round, a little fatter at one end, and it probably came out of the non-clucking end of a chicken.

The chicken egg is the egg that most of us know, which makes sense considering there's 2.5 chickens for every person on Earth, they're what most of us eat, and we've been indoctrinated with them from a young age.

"This is just a plastic chicken egg."

(Hatching noises)

But is this shape really "egg-shaped"?

In 2017, scientists analyzed the shape of nearly 50,000 eggs from 1,400 different bird species, and plotted them based on how elliptical–or squished–they are, and how pointy–or asymmetrical–one end is.

And chicken eggs are outliers.

Most bird eggs aren't shaped like this.

Bird eggs come in many shapes.

Nearly spherical eggs like owls, pointy cone eggs of shorebirds like the common murre, even the tiny tic-tac eggs of hummingbirds… and everything in between.

Scientists have been wondering for a long time how and why all of these different egg shapes came to be.

But before we inv-egg-stigate those questions, let's take a minute to appreciate just how eggcellent eggs really are.

Dad yolks, gotta love em.

More than 350 million years ago, the ancestors of every land-dwelling, backbone-having animal alive today began to crawl out of the water, and they pretty quickly colonized every terrestrial habitat on Earth.

And one evolutionary innovation made that possible more than any other: Eggs with shells.

Although many other animals lay eggs, the evolution of the shell allowed reproduction away from BLEEP environments.

Wait.

I can't say BLEEP (moist)?

I guess I don't wanna get demonetized, so…

Eggs with shells allow reproduction away from wet environments.

These self-contained life-support systems keep everything inside from drying out, and it meant that reptiles and the early ancestors of mammals could travel, see the world, set up shop in every corner of Pangaea.

The ancestors of mammals eventually moved all the egg and baby-growing business inside our bodies–except for these weirdos.

So many questions.

While reptiles from crocodiles to turtles to snakes to dinosaurs stayed in the egg business.

Reptile egg shells range from leathery to hard, but one special branch of the dinosaur family tree eventually took natural shell-ection to the next level: Birds.

What makes a bird egg so eggstra cool is this hard shell.

It's strong enough that this many eggs can support my full body weight without breaking.

I'm standing on two dozen eggs.

They said it couldn't be done!

I'm so afraid to move.

And that strength comes from the shell's shape.

If you watched my video about the science of igloos, you remember that this shape, called a catenary arch, distributes tension and compression more evenly than a half circle shape.

Along with the microscopic protein and mineral nanostructures in the shell, this shape makes egg stronger than it would be if it was another shape.

This brings me back to our original question: What is an egg's shape?

An oval is a 2-dimensional curve whose name literally means "egg-shaped".

Unlike an ellipse, there's no exact geometric definition for an "oval", but they can be constructed by joining other arcs of different radii.

They generally only have one axis of symmetry, and by rotating an oval along this axis, the surface of revolution we create is called an "ovoid".

That's "egg-shaped"… at least according to math.

But while the shell gives this shape strength, the shell doesn't make the egg egg-shaped.

If you dissolve away an egg shell, which is rich in calcium carbonate, with an acid, like vinegar, it maintains its shape.

The "egg shape" isn't caused by the shell.

It's caused by this squishy inner membrane.

It starts with an unfertilized egg cell, added to a blob of yolk, and squeezed down a stretchy tube called an oviduct.

As it travels it's fertilized by sperm, wrapped in those membranes and inflated with fluid like a balloon.

The shape of the egg is determined when those membranes form.

Over the years scientists have had a lot of hypotheses about why different birds create eggs of different shapes.

One hypothesis is the more spherical the egg, the less shell material it takes to cover.

Another is, depending on how many eggs a mother lays, different egg shapes could snuggle better during incubation.

Or maybe birds that are born ready to walk and feed themselves might grow better in one shape versus another.

And for birds that nest on cliffsides, pointier eggs tend to roll in circles instead of rolling off the edge.

These are just a few of the possible answers people came up with.

But when those scientists from earlier analyzed thousands of egg shapes from across different families of birds, the best correlation they found was between shape and flying ability.

Birds that are better fliers have more streamlined bodies, and their organs are more tightly packed, which puts a limit on how wide an egg you can squeeze through that egg-making tube.

Inside a computer, those scientists were even able to virtually form every egg shape we know of by changing just two things: the stretchiness of the membrane and how it's squeezed.

The most common egg shape among all birds is something closer to this.

A little pointier than our so-called "typical" chicken egg.

Trust me, they're different.

They measured.

Thinking about it in the eyes of eggvolution, it's not that pointy eggs give acrobatic birds an advantage, it's just a natural consequence of having a narrow body.

And we do know the first dinosaurs that laid pointy eggs were the group that gave rise to birds.

But here's one important thing to remember about how evolution works: Body shape might be the best answer when we look at all birds, but that doesn't mean it's the only answer.

Different families of birds might get different evolutionary eggvantages from different shapes.

Even if a bird's body isn't that skinny, maybe a pointy egg really does keep it from rolling off the edge of a cliff, or make it easier for the parents keep it warm.

Natural selection can happen on many levels, and there can be evolution inside of evolution.

So, are all eggs egg-shaped?

Well, you could say "egg-shaped" just means whatever shape an egg is.

But that's not a very satisfying answer.

What it means to me is… there is no egg-shape, because nature's made so many different shapes, for different reasons.

In the end, the question of why different birds lay different shaped eggs isn't settled.

And that's ok.

Because that's how science works.

If it was wrapped up with a neat little bow, we wouldn't have any questions left to incubate, and no new ideas left to hatch.

Stay curious.