Roberts used a variety of media examples, such as “Gulliver’s Travels” and “The Little Prince,” to demonstrate how physics dictate the size and shape of land animals.
“Most people are aware that giant spiders are impossible, at least the way Hollywood shows them,” Roberts said. “But it’s important that they know why giant spiders are impossible. Besides, it’s the little ones you need to look out for.”
Roberts explained the effects of gravity on the construction of land animal physiology. Large animals have thick legs and robust skeletons to support their weight, he said.
Hollywood spiders, he said, do not have the prerequisite leg diameter to support the weight of a giant thorax and abdomen.
To illustrate his example, he showed the audience the thickness of the legs of the four land animals with a mass of more than 2,000 pounds.
The elephant, rhinoceros, hippopotamus and the giraffe are the only living species whose members weigh more than a ton, and all have wide legs compared to other land animals, Roberts said.
Gravity is not the only physical constraint on physiology that Roberts discussed. The square-cube law and diffusion of atoms and molecules are two closely-related topics that he mentioned.
The square-cube law is the relationship between surface area and volume. The most simplified explanation of the law is that surface area is squared every time an object doubles in size, and the volume is cubed.
So, the volume grows faster than the surface area. This causes problems in nutrition, Roberts said, because molecules cannot diffuse fast enough to maintain an organism’s metabolism.
Diffusion is the process by which molecules in an area of higher concentration move into an area of lower concentration until equilibrium is reached.
Diffusion and the square-cube law affect organisms in two ways, Roberts said. First, they limit cell size. Cells tend to be about 10 micrometers in diameter on average, as this is optimal for the diffusion of molecules into and out of the cell.
“Growth is not cells becoming larger,” Roberts said of animals, “but more cells being grown.”
Multicelled animals can escape this limitation with distribution systems like respiratory and circular systems, but they face a different problem.
The metabolism of an animal is also based on the square-cube law but does not behave in a linear fashion. A larger breed of dog with 125 times the mass of a smaller breed does not have 125 times the metabolic activity. Metabolic activity is measured in watts.
Roberts recently wrote a dissertation that argued against a recent finding in physiology: a claim in a study that said the metabolism equation should be changed to W = m^(3/4).
Currently, the metabolic activity of an organism is thought to be equal to two-thrids the power of the mass of an organism, or W = m^(2/3). W is watts, representing the metabolism, and m is mass.
The study examined a wider range of land animals than previous efforts, with organisms that fell below one once and above 2,000 lbs.
Roberts said he found a flaw in the reasoning of the group study. In his dissertation, “A New Model for the Body Size-Metabolism Relationship,” he points out that there were two distinct groups in the study.
Smaller animals were examined at their basal metabolic state, he said. This is when the animals are at rest, and no muscles are being used.
Larger animals, such as horses, cannot sit longer enough for the measurements to be taken without dying, and therefore were measured when standing and fighting gravity.
Roberts said this skewed the results of the study, and the two populations must be separated when averaging the metabolic rate of animals.
The crowd sat in rapt attention during the lecture and stayed afterward to make suggestions to get better results from the study. One suggestion was to launch large animals into space, because they could be studied without the effects of gravity.
“I would hope that students understand that size relationships aren’t linear,” Jeremy Weisz, assistant professor of biology, said.
Weisz said he thought the lecture went well, and that students should be aware of the constraints physics place on physiology.
Joshua Ensler/News editor
Joshua Ensler can be reached at firstname.lastname@example.org.