Expert: John Locke - 1/29/2009

Question
Hello,

Why is there a limit to how big organisms can be? In what way does this limit hold true for all organisms? How was this limit researched and discovered?

Thank You!

Answer
Thanks for using AllExperts. The size of organisms is limited by a variety of physical factors that govern normal physiological processes. For instance, diffusion of oxygen and nutrients from blood vessels into tissues is highly limited by distance, so larger organisms require more extensive networks of blood vessels to reach all their tissues. This increases the resistance of the blood vessel network, which in turn requires a larger, more powerful heart. Heart muscle is highly energy-intensive, so that increases the overall metabolic requirements, and so forth--in short, larger organisms have more physical demands placed on them by the environment. There is a point beyond which organisms cannot provide for their metabolic demands, given the physical limits imposed on them; this limit determines the effective maximum size of an organism. Let me summarize a few of the major obstacles that are created by large size:

1. Increased metabolism requires a greater consumption of food.
2. Larger sized organisms have a greater surface area from which water can be lost, necessitating more drinking to prevent dehydration.
3. Larger sized organisms must overcome the conflicting demands of mass versus muscle strength. Muscle strength increases with the square of cross-sectional area, while mass increases with the cube of cross-sectional area (see here: http://intuitor.com/moviephysics/ under the section "Scaling"). This means that larger organisms cannot compensate for their increased size simply by increasing their relative muscle mass.
4. The underlying physics of circulatory and respiratory systems do not scale easily, meaning that larger systems are usually less efficient.
5. Somewhat paradoxically, larger organisms can have difficulty losing body heat to cool off, despite their large size.

If I were to pick out one factor that limits organism size, it would be the ratio of surface area to volume. Volume increases at a much greater rate than does surface area, which limits the ability of materials to cross cell membranes at a sufficient rate to supply the organism as required for its survival. Beyond a certain point, the available surface area is not sufficient relative to the volume of the organism for the required respiration, hydration, and digestion. That is, enough materials simply cannot be absorbed to supply the organism with enough nutrients to allow it to survive.

These limits have mostly been discovered empirically; mathematical investigations of organism physiology have discovered empirical relationships between mass, metabolism, and other factors, termed "power law relationships." See examples here: http://en.wikipedia.org/wiki/Allometric_law