Why Geese Don't Get Obese (And We Do)
CHAPTER ONE
Different Species, Same Problems
Nature is an endless combination and repetition of a very few laws. She hums the old well-known air through innumerable variations.
--RALPH WALDO EMERSON, ESSAYS (1841)
When I was an eager young student at Northwestern University, I had the good fortune to be taught by a physics instructor who took a great interest in his students' academic careers. Like a true physicist, he never could really appreciate my reasons for wanting to study biology. And while his many attempts to get me to switch from a career in the life sciences to one in the physical sciences were ultimately unsuccessful, he nonetheless left me with some advice that I continue to pass on to the young biology students that I now teach. That advice is as simple as it is fundamental: Never forget that the laws of nature are at the root of the life sciences. In other words, to understand howthe human body works, you must first understand something about the physical laws of gravity, electromagnetism, thermodynamics, and matter and energy. For example, it's highly unlikely that Sir Isaac Newton was thinking about the way blood flows to the head of a giraffe when the apple dropped on his head. However, his revelation about gravity led not only to a better understanding of how planets revolve around the sun but also helps us to understand why people sometimes get light-headed when they stand up too suddenly or why a giraffe's blood pressure must be higher than our own.
Don't be alarmed. You won't need a Ph.D. in physics to understand how the forces of nature influence how your body works. In the following chapters we'll see how warm-blooded animals, like ourselves, use heat energy to their advantage, why we have two nostrils, why seals don't get the bends, how sharks use electricity to monitor their surroundings, how the salt content of water determines whether or not a fish will drink (it's the opposite of what you might think!), and why elephants have such big, floppy ears.
The business of studying how the different structures of our bodies--such as the heart, brain, kidneys, and muscles--function, is the science known as physiology. This branch of science may have gotten its name from the Greek physiologoi, which was the name given to an ancient group of well-to-do philosophers. One of their favorite occupations was to debate the principles of nature and how those principles could explain the nature of living things. Many of their conclusions may not have made sense by today's standards, but nonetheless physiology took hold as a science and is stronger than ever as we enter the twenty-first century.1
As is the case with the elephant's ears, it is a common theme in physiology that even the oddest-looking creature appears that way for a reason. In fact, many of our own features that we take for granted are, on the surface, sort of strange looking, too. Why do we have two nostrils, for example? Wouldn't it make more sense to have one largeopening in the nose rather than two smaller ones? And speaking of things that come in pairs, why do we have two eyes and two ears but only one tongue, when all of these structures are used for sensing things in the environment? Why don't we have a forked tongue like snakes? Why is it that some people are skinny, and others cannot seem to keep weight off no matter how hard they try? Likewise, why don't small animals like mice and shrews--who eat their body weight in food each day--get fat? And why might the ability of humans to gain weight actually have been an evolutionary advantage, one that has gone haywire in the modern era of fast foods and sugary sweets? All of these questions and many others like them can be answered if we accept the premise that nearly every change in an animal's form arose because of evolutionary pressures and the need to adapt to the environment.
As animals evolved in splendid ways in response to their environments, the laws of nature often created previously nonexistent problems. When the giraffe's neck got longer, for example, the animal was better able to eat vegetation that other animals couldn't reach. That's an obvious advantage, but the long neck created a new problem--how could blood get all the way from the heart up to the brain, a distance of many feet? Gravity works against the blood, of course, making it hard for the fluid to move upward. It may not seem that gravity would pose that much of a problem, but try connecting several straws together and see how quickly it becomes more difficult to sip from a glass. Somehow the system manages to work, however, because giraffes are extremely successful animals and live long lives. In fact, in order to solve the problem of gravity and get blood all the way up to the head, nature made the blood pressure of the giraffe very high, much higher than our own--a simple enough solution. But we all know that high blood pressure is deadly in people. Are giraffes somehow resistant to the dangers of high blood pressure, and, if so, wouldn't it be nice to know why, so that we may someday apply that knowledge to the human condition?
It's good to keep in mind that all animals, no matter what type of environment they live in, face the same challenges of survival. For example, whether the environment is a desert or an ocean, the body's water stores must be kept at proper levels. Even fish need the right mechanisms to keep from becoming dehydrated. Similarly, a cave-dwelling bat in Malaysia, a llama or a person in the Andes, a fish in the Pacific, and a crab in a tidal pool must all obtain sufficient oxygen from their environment to power the chemical reactions of the cells in their bodies. They all need some sort of pressurized blood circulation to move the oxygen from place to place within their bodies. The ways in which a person and a fish get oxygen and transport it around their bodies, however, are determined by the environments in which they live. Thus, lungs would do a fish no good, and gills wouldn't help us. As another example, a shark living in deep, murky waters needs to know what objects are in its immediate vicinity, just as a rat that is active at night does. But eyes are almost useless in murky water or on a dark night, so sharks and rodents need to rely on other sensory cues to "see" their surroundings. Sharks developed the ability to detect even the tiniest electrical signals given off by prey even when the prey is "playing possum." Rodents solved the same problem by developing an enormously enlarged smell center in their brains, and even the faintest odor tells a rat or mouse all kinds of useful information about what's nearby.
The way in which we maintain relatively constant levels of salt, water, oxygen, and blood pressure is called homeostasis. We will revisit this concept in Chapter 9, but for now it's worth mentioning that homeostasis is the very basis of health. Disease, in fact, can be defined as a state of non-homeostasis. Think of it as a balance between opposing forces. If you eat an entire pepperoni pizza, the salt level in your blood will rise. You will be in danger of falling out of homeostasis. Fortunately, there are hormonal, behavioral, and brain mechanisms that set into motion a chain of events that quickly bring the salt concentration in the bloodback to normal, restoring the homeostatic state. We all need these and many other built-in homeostatic controls, or in a very short time we would succumb to the rigors of the external world.2
Thus, the physical laws of nature and the environment in which an animal lives, combine to produce the incredible (but understandable) variety of shapes, appearances, and behaviors found in the animal kingdom, and even in ourselves. Every species must develop survival strategies to cope with the same basic, fundamental challenges: getting enough to eat, drink, and breathe; circulating blood; adapting to change; keeping warm; and communicating with other members of the species (or with other species). As a good illustration of how these principles come together, try to imagine a warm-blooded mammal so tiny it is barely heavier than a large insect. What problems would this produce and how would those problems be solved? If we had to deal with those same problems, how would they affect us? In fact, such a mammal does exist. It's called a shrew, and, as we'll see in the next chapter, if humans shared the physiological characteristics of shrews and other small mammals we could not possibly exist.
© 1998, 1999 by Eric P. Widmaier
