Why are eggs egg-shaped?

Here’s a nice bit of news that’s come out of Christchurch in the last few days. The kiwi eggs being incubated at Willowbank wildlife reserve survived the earthquake, and one of them has now hatched into an adorable baby bird, who now carries the name ‘Richter’.  (N.B. Kiwi aren’t adorable – from what I’ve heard they are vicious things – but very very endangered, so anything that warms people to them has to be good)

At first thought it might be surprising that an egg can survive through an earthquake, but eggs are well shaped for that purpose. The round shape means there are no sharp edges.  Sharp edges can act as stress concentrators. It is at these points that structures are most likely to fail. It’s why you can usually rip a piece of paper in half fairly cleanly. Once you have a small tear in it, the stress will be concentrated at the tip of the tear, and the tear will propagate forward. It’s the same reason why a small crack in your car windscreen can easily turn into a larger crack –  the stress is concentrated at the ends of the cracks.

Round things such as eggs obviously don’t have sharp points to them and therefore it is much harder to get a crack formed in the first place. Also, the shape ensures the shell is strong in compression (squashing) but weaker in tension (stretching). External forces are likely to squash it, whereas internal forces (the chick trying to get out) will stretch it, and it breaks more easily.

Egg shells also have a wonderful structure to them, that makes them very strong for their weight. It’s not really surprising eggs have to be like that, when you consider that they have to be built inside the female bird and go through the stress of being laid and then sat on. So I was not completely surprised to hear that the eggs had survived the quake.

Here’s a nice egg strength experiment to do at home, which I got from this Q+A site. It includes a great electron microscope picture of the lattice structure of an egg shell, which will give you some idea of its strength compared to its weight.

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