Size Matters

by James Morris
Illustrations by Talia Niederman

A newly described dinosaur named Patagotitan mayorum holds the record for the largest animal ever to have lived on land. Does its size matter?

One of the most dramatic but underappreciated aspects of life on Earth is the incredible range of sizes among organisms. The smallest free-living organisms are bacteria, specifically Mycoplasma. The largest are blue whales. In fact, the largest organisms to have ever existed on Earth are blue whales.

Mycoplasma are about 200 nanometers long, or 200 billionth of a meter; blue whales are about 30 meters long. That’s a difference of 13 orders of magnitude, where one order of magnitude represents a 10-fold change in size. In other words, the largest organism that ever lived is 1013 times larger than the smallest organism.

By mass, blue whales differ from mycoplasma by about 21 orders of magnitude. That is, a blue whale is 1021 times heavier than a mycoplasma.

No matter how you measure them, organisms differ tremendously in size.

Where are you and other humans? Regardless of how tall you are, you are somewhere in the middle. And, it turns out, your size influences so many other aspects about yourself – how you move about, what you can see, how you gain and lose heat, how you obtain oxygen from the air and nutrients from your food, even how long you live. In short, your size affects just about everything.

An ant can’t be as a big as you and still look like an ant. Conversely, you can’t be as small as an ant and still look like you. Why not?

Let’s start with a big ant. If an ant increases in size but keeps the same relative proportions (that is, it maintains the same shape and still looks like an ant), it would buckle under its own weight. Weight increases as the cube of the scale factor because weight is proportional to volume. However, strength of the legs increases as the square of the scale factor because strength is proportional to the cross-sectional area of the legs. In other words, the ant’s weight would quickly outstrip the amount of weight its legs could support, and the ant would come crashing down.

Now let’s consider what happens if you were as small as an ant, like Marvel’s Ant-Man. As you get smaller, you have relatively more surface area than volume compared to when you are bigger. Volume decreases as the cube of the scale factor, whereas surface area decreases as the square of the scale factor. So, volume decreases much more quickly than surface area.

You dissipate heat in part through the surface area of your skin. And heat is generated by many chemical reactions that make up your metabolism, which tracks with volume. In other words, with relatively more surface area than volume, your small self loses much more heat than you can generate, unless you speed up your metabolism, eat all of the time, and run around constantly.

The problem in both of these scenarios is that the ant and human kept the same shape, but changed in size. Neither works because an ant looks like an ant in part because of its size. And you like you in part because of your size.

When we imagine large ants and small humans, we think of them as maintaining the same basic shapes. This is known an isometry (“same measure”). Similarly, when certain kinds of salamanders grow, they keep their same basic shape. In other words, an older salamander looks like a scaled-up younger salamander.

A different way to think about scaling is to consider shape changes that occur along with size changes. This is known as allometry (“different measure”). One of the best-known examples of allometry occurs as you grow up. A baby’s head is quite large compared to its body. By contrast, an adult’s head is relatively small compared to its body. Your body, in fact, grows much faster than your head. So your shape changes as your size changes.

The same is true of the growth of many animals. The fiddler crab has one large claw and a second small claw. They begin about the same size, but one claw grows much more quickly than the other claw. So, an adult fiddler crab looks quite different from a young fiddler crab.

We can consider scaling relationships that occur as organisms grow, asking whether they maintain the same shape, like salamanders, or not, like humans. In addition, we can look at different organisms and ask how their sizes and shapes compare to one another.

As early as the 1600s, Galileo Galilei noticed that the bones of larger animals are not scaled-up versions of the bones of smaller animals. Instead, the bones of larger animals are disproportionately wider than those of smaller ones.

J.B.S. Haldane, a British scientist with wide-ranging interests, wrote, “Comparative anatomy is largely the story of the struggle to increase surface area in proportion to volume.” What he meant is that as organisms get bigger, various traits and functions (like weight) track with volume and increase quickly, whereas others (like strength of bones) track with area and increase slowly. As a result, any trait or function that depends on surface area will become modified to keep up with volume so it doesn’t lag too far behind.

Think of the lining of your gut. It’s one large surface area. If your gut were a simple tube, there would not be nearly enough surface area to take in nutrients to supply your body (a volume) with the energy and nutrients it needs. It turns out that your gut is not a simple tube. There are folds that increase the surface area of the lining gut. Along these folds, there are smaller folds, called villi. And, along the villi, there are still smaller villi, called microvilli. This is why the lining of the intestine earns the name “brush border,” resembling a 1970s shag rug.

Seeing where you are in size compared to other organisms not only helps you understand basic aspects of your anatomy, but also has practical applications. In the 1960s, scientists injected LSD into an elephant at the Oklahoma City Zoo to study its behavior. The elephant immediately became agitated, fell over, and died.

The dose of LSD was more than 1,000 times a typical human dose and the largest dose ever given to a living animal. It was calculated by scaling the dose given to cats based on weight. The authors concluded that elephants are very sensitive to LSD. Further examination, however, revealed that the problem was not the sensitivity of the elephant to LSD, but instead the dose of the drug.

Just as adults are not scaled-up babies, and a big ant and small human don’t work, an elephant is not a scaled-up cat. Organisms differ in many ways, but size is particularly important because, in fact, one size doesn’t fit all.

© James Morris and Science Whys, 2017

2 thoughts on “Size Matters

  1. James Morris Post author

    I really like what JBS Haldane has to say about this topic:
    “The most obvious differences between different animals are differences of size, but for some reason the zoologists have paid singularly little attention to them. In a large textbook of zoology before me I find no indication that the eagle is larger than the sparrow, or the hippopotamus bigger than the hare, though some grudging admissions are made in the case of the mouse and the whale. But yet it is easy to show that a hare could not be as large as a hippopotamus, or a whale as small as a herring. For every type of animal there is a most convenient size, and a large change in size inevitably carries with it a change of form.”
    ~ J. B. S. Haldane, On Being the Right Size


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