Size and Shape in Biology at The University of Chicago

The size of an organism critically influences the relationship between form and function. Organisms change in size over short time intervals through growth and development, and over longer time intervals through evolution. When comparing organisms of different sizes, anatomical structures that are identical in shape are considered geometrically similar, or isometric, whereas those that differ in shape are allometric. In this lab we explore the functional implications of body size for the shapes of various structures in organisms.

Before coming to lab, each student completes the web tutorial and fills in the worksheet posted there to bring to lab.

At the beginning of lab, each group of 2-3 students chooses one of the seven exercises, each of which has a set of bones or other specimens showing a broad range of sizes. Each investigation includes the following steps:

1. Devise hypotheses about whether shape changes with size and how that might affect function of the structures provided.
2. Test the hypotheses by measuring various lengths, areas, or masses on the specimens.
3. Enter the data into a StatView data set, log-transform them, and analyze by linear regression. The slope parameter in this regression is an estimate of the scaling exponent, which indicates whether scaling is isometric or allometric.
4. Interpret the scaling relationship in light of the function of the structure studied.
5. Present the group investigation to the rest of the class orally at the end of the lab period.
6. Complete a written lab report to be handed in one week later.

The seven exercises explore concepts such as the relationship of body size to stress on skeletal elements, the use of lever mechanics to analyze the power of jaws, the relationship of brain size to body size, and the role of heterochrony in evolution. Each exercise is illustrated below with its central questions.

	1: Mussel Shells As mussels grow, do their shells change shape or thickness? Do the relationships between linear measurements and mass follow a power function? How might shell shape and thickness affect the survival of mussels?
	2: Stress and the Human Foot How does foot area scale with body mass in humans? As humans grow in size, does the stress on the feet change, and, if so, how?
	3: Ruminant Jaws Does jaw shape differ among ruminants ranging from a mouse deer to a cow? How can you measure the mechanical advantage of a muscle used in chewing? How does mechanical advantage of chewing muscles change with size?
	4: Cat Jaws How can you measure the mechanical advantage of a muscle used in chewing? How does mechanical advantage of chewing muscles change with size? Do the skulls of giant extinct cats such as sabertooth cats and the American lion follow the same pattern as living cats?
	5: The Cat Cranium How does cranium width vary with skull size among species of cats? Did extinct sabertooth cats and American lions have brain cases substantially larger or smaller for their body size than living cats?
	6: Dog and Wolf Skulls As the hypothetical ancestral dog grows in size from a fetus through mature adult, does the overall shape of the skull change? How do the skull shapes of different breeds of adult dogs compare with the developmental series of ancestral dog skulls? What role, if any, might heterochrony have played in the evolution of modern dogs from a wolf ancestor?
	7: Mammalian Femurs If femurs scale isometrically, then will the strength of the femur be proportional to the load on it as you go from a mouse to a horse in size? Do the femurs of larger mammals differ in shape from those of smaller ones, and, if so, how? What prediction can you make about femur shape and athletic abilities in large dinosaurs?