Why are eggs... egg shaped? Sounds like one of those questions that has a sort of circular logic to it. Like, "Why are oranges orange?" or "Why does salt taste salty?"

But another way of thinking about the question of the egg's shape is, "What is the evolutionary advantage to having egg-shaped eggs?" And that is because... well, we're not actually quite sure. But there are two prevailing theories.

One is that the egg shape, which is really just a sphere tapered to a point at one end, creates a kind of wedge. This shape is called 'ovoid' from the Latin word 'ovum,' for egg.

In some birds, after the egg passes through a series of organs called the oviduct, muscles at the end squeeze down on that wedge to push the egg out, blunt end first. A sphere would be much more difficult to squeeze out. Not that either way sounds particularly fun.

The other main theory about the advantage of the egg's shape is that a spherical egg would be able to roll forever in any given direction if it was accidentally pushed.

But an egg-shaped egg tends to roll in a circle around a central point. This may make it more likely that, when jostled around, eggs will stay in a nest.

This theory is reinforced by the fact that cliff dwelling birds have an even more pronounced egg shape to their eggs. So, strictly speaking, the evolutionary logic behind the shape of eggs isn't circular, it's ovoid.

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