Paleo Slide Set: Packrat Middens: Vegetation & Climate Variability in the Southwestern United States
A Temporal Test of Bergmann's Rule

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 from 
Bergmann's Rule (the inverse relationship between environmental temperature and body size) and physi
ological 
responses to temperature stress in more recent experiments.
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. 

Photo Credits:
Julio Betancourt 
U. S. Geological Survey