Structure: Lessons from an Egg

Structure: Lessons from an Egg

Suppose you needed to determine what the top 10 biological designs are. What would they be? I know that one of my candidates would most certainly be the bird's egg.

There are lots of different kinds of laid eggs in our world and I find them all fascinating no matter which class of animal, insect, fish, bird, reptile, amphibian or mammal (yes, the platypus and echidna are mammals and lay eggs) produced them. I would choose the bird's egg, however, as a personal favorite. And for its representative, who could resist that largest of all bird eggs, the ostrich egg?

Why the ostrich egg? There is that elegant shape, so elemental and singular; especially apparent when one tries to describe it without using “egg” or a term derived from the Latin root “ovum”(oval, ovoid, etc.). It is, likewise, hard to list its component parts and features.

There are also those crowd-pleasing statistics associated with the ostrich: largest living bird, fastest bipedal runner, thickest shell by weight (20 percent), world's largest single cell (the embryo), smallest bird's egg relative to the bird's size, and world's largest one-egg omelet, equivalent to about two dozen chicken eggs.

Most important to me, however, is that its structure represents the solving of seemingly irreconcilable contradictions of purpose within one unified form. It never ceases to inspire.

How would you like the following to be in your next design brief? “Smooth gradations of curve of one thin material completely enveloping and sealing a contained volume at a thickness/radius ratio of 1:100, without any additional solid external or internal support. It must be crack-resistant from the outside, but breakable from the inside. It must not exhibit significant drag within a fluid medium, but must resist unintentional rolling. It must seal out bacteria, dust and water, but allow air in and carbon dioxide out. It must mitigate wide swings in temperature.” Oh, and it must resist the compressive force resulting from a 240-pound male ostrich sitting on it.

A bit of a tall order, wouldn't you say? And we haven't even specified the materials or the production methods, yet. Let's take a look at how the egg solves some of those problems, starting with that remarkable thin cladding. The shell is made of water, proteins, fat, potassium and amino acids, and if you look at a section through a scanning electron microscope, you will see that it does indeed have distinct features.

On the outside is a protein layer, called a cuticle, which is stretched over a surface of thousands of funnel-shaped pores that pierce the mineral shell. On the inside of the hard shell is another double membrane that further protects the embryo and allows it to breathe through the shell. Blood vessels in the inner membranous sac bring oxygen to the embryo and take carbon dioxide away. Finally there are anti-bacterial compounds in the albumen of the embryo itself.

The albumen, or white of the egg, also performs other functions. It provides the water necessary for the chick's development as the yolk provides the proteins, fats, vitamins and minerals. As it stores and dispenses this water, however, it is also serving as a shock-absorbing buffer between the hard shell and the embryo. We are all familiar with its gelatin-like properties.

The result of these properties: “…a liquid that cannot be compressed, only displaced, and an elastic substance. When the embryo is pushed against the shell by some forceful impact, the liquid must flow past it and transform the destructive energy into heat. The shock absorption of the egg is further improved by an air cushion located at the thick end of the egg--the same end as the center of gravity. In a falling body the center of gravity moves to the lowest possible point, so in an egg the embryo falls on the air cushion. The air pocket in the egg has another mechanical function. It prevents temperature fluctuations from cracking the shell," writes Helmut Tributsch in "How Life Learned to Live."

Note again how static shape (the ovoid), operating within prevailing forces (gravity) can be employed to respond immediately to a threatening change in conditions (falling). Talk about rapid deployment! Why not let the initiating disruptive force power your safety measures into place? Why not pick structure over energy as your input for this mechanism?

As in the abalone shell, we see flexible proteins and hard minerals combined in alternating layers for durability: composites. We also see baffles and membranes employed to passively trap and filter. Larger water molecules are kept out by these membranes, and the smaller oxygen and carbon dioxide allowed to pass.  

The gross shape of the shell is important for protection. The curved, seamless form distributes the force of a blow through its entire structure, like the domes of our buildings distribute the compressive weight of their materials. Try breaking an egg in one hand without using your thumb. It's not that easy! It is the point load of your thumb or the edge of a mixing bowl that finally cracks the case. Similarly, it is the point load of the chick's beak that is able to concentrate enough force to break the shell.

Also helping the chick is the significant structural implications of being on the inside rather than the outside of an arch. If you make a complete (and anchored) arch of stones the weight of each stone pushes out and down onto its lower neighbor and keeps the structure intact. The more you push down on the arch the more it stays pressed together. In pushing up on one of the stones, however, the force takes a different load path and overcoming the inertia of the single stone is the way for you to break the arch, or the chick to break the shell.

The fact that the egg has no sharp edges allows it to pass easily down the birth canal. Once outside in the nest, however, the egg faces different challenges: staying in one safe place takes priority over moving around. The answer is again reflected in the shape: the ovoid, because of its geometric bias, will tend to roll in a self-homing circle, rather than straight off down the hill as a perfect sphere would. It's interesting to note that many seabirds that dwell on cliffs, where straying from the nest is a singular and final event, produce eggs that are distinctly more biased, even to the point of being cone like. The debate is still on about the main influence on this phenomenon: evolution or physics.

The egg makes the top 10 design list because it is a study in contradictions solved. It needs to move easily, and then stay put. It needs to resist breaking, and then break easily. It needs to seal some things out, but allow others in. It needs to keep some things in, but allow others out. It needs to accommodate a moving embryo, but keep it from being banged around inside. It needs to nourish a living and growing thing within a self-contained space. Remarkably, eggs do just that, all the time, and they do it largely with structure.

In my next essay I will discuss some of the ways people have used the principles exhibited in the egg in our built world. By the way, I really would like to know who your favorite design champion is, so send me a comment.

Tom McKeag teaches bio-inspired design to at the California College of the Arts and University of California, Berkeley. He is the founder and president of BioDreamMachine, a nonprofit educational institute that brings bio-inspired design and science education to K12 schools.

Ostrich egg - CC license by Bo&Ko