a number of scaling relationships between observable properties of a city and the city size. GDP, "supercreative" employment, number of inventors, crime, spread of disease, and even pedestrian walking speeds scale with city population. Examples Some examples of allometric laws: Kleiber's law, metabolic rate is proportional to body mass raised to the power: breathing and heart rate are both inversely proportional to body mass raised to the power: mass transfer contact area and body mass : the proportionality between the optimal cruising speed of flying bodies (insects, birds, airplanes) and body mass raised to the power : Determinants of size in different species Many factors go into the determination of body mass and size for a given animal. These factors often affect body size on an evolutionary scale, but conditions such as availability of food and habitat size can act much more quickly on a species. Other examples include the following: Physiological design Basic physiological design plays a role in the size of a given species. For example, animals with a closed circulatory system are larger than animals with open or no circulatory systems. Mechanical design Mechanical design can also determine the maximum allowable size for a species. Animals with tubular endoskeletons tend to be larger than animals with exoskeletons or hydrostatic skeletons. Habitat An animal's habitat throughout its evolution is one of the largest determining factors in its size. On land, there is a positive correlation between body mass of the top species in the area and available land area. However, there are a much greater number of "small" species in any given area. This is most likely determined by ecological conditions, evolutionary factors, and the availability of food; a small population of large predators depend on a much greater population of small prey to survive. In an aquatic environment, the largest animals can grow to have a much greater body mass than land animals where gravitational weight constraints are a factor. See also (also known as a scaling law) References Further reading External links FDA Guidance for Estimating Human Equivalent Dose (For "first in human" clinical trials of new drugs) Branches of biology Physiology Ecological experiments Scales