| A major question in the
recent fossil record is whether plants and animals have remained essentially
unchanged, but shifted their distribution in response to climate
variability, or remained in place and responded to the varying environment
by some combination of phenotypic
plasticity and evolutionary
change. Changes in the body size of the bushy-tailed woodrat (Neotoma
cinerea) since the last ice
age were estimated from measurements of fecal pellets preserved in fossil
middens (pellet diameter explains 70% of the variance in body size of modern
woodrats). Body size decreased during periods of climatic warming, as predicted
Rule (the inverse relationship between environmental temperature and
body size) and physiological responses to temperature stress in more recent
An animal's size affects such fundamental physiological and ecological factors as metabolic rate, fecundity, longevity, home
range and even extinction rate. Thus, identification of factors that influence body size are critical for understanding the basic ecology and evolution, as well as for predicting responses to changing climatic conditions. The upper left panel shows an
experiment in the 1960s, in which woodrats of different body sizes were subjected to high, lethal temperatures. Note that smaller animals, which can better dissipate heat due to a higher surface area to volume ratio, withstood the highest temperatures.
The upper right panel shows the negative (inverse) relationship between average body size and mean July temperature at the weather station closest to each of the 12 populations measured. The lower left panel indicates the predictive relationship
between fecal pellet diameter and body size in a controlled setting. Fecal pellets abound in middens, providing a reliable, statistical sample of past body size. The lower right panel shows the dwarfing that occurred in the Four Corners area with
global warming at the end of the last ice age (15,000 to 12,000 years ago). The smallest body size was recorded between 8,000 and 4,000 years ago, a period known as the Altithermal in the American Southwest.
U. S. Geological Survey
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