Bergmann's rule and tetrapod vertebrates, with special emphasis on body size variation of the western rattlesnake

Many ecological and evolutionary rules have been described. Documenting the patterns and evaluating the mechanisms that may produce such patterns are important to understanding ecological and evolutionary processes. My dissertation research focused on testing for broad patterns in ecology and evolution, particularly those that relate to body size.; Bergmann's rule states that there is an increase in body size with decreased environmental temperature within species of endothermic vertebrates. However, the validity of this pattern, and the possibility of a similar trend for ectotherms, has not been established. In addition, the mechanism, based on heat conservation, proposed to account for Bergmann's rule has been questioned. I tested for general patterns of body size variation relative to latitude and environmental temperature for species of tetrapod vertebrates. I found that most tetrapods (mammals, birds, turtles, salamanders, and anurans) follow Bergmann's rule, whereas squamate reptiles (“lizards” and snakes) follow the inverse of Bergmann's rule. I tested a prediction of the heat conservation explanation of Bergmann's rule, but did not find support for the prediction. Thus, heat conservation does not appear to explain Bergmann's rule.; I conducted a thorough analysis of body size evolution in the western rattlesnake (Crotalus viridis) to further understand patterns of size variation for squamate reptiles. For this analysis, I first generated a molecular phylogeny using sequences from two mitochondrial DNA regions to provide an evolutionary framework for addressing questions of size evolution. The phylogenetic analyses revealed two strongly divergent clades. I then examined the relationships between body size and general physical and climatic variables. The eastern clade showed larger body sizes at higher latitudes, and in cooler and more seasonal environments. In contrast, the western clade attained smaller sizes in cooler and more seasonal areas. The differences in size trends between the two clades may be explained by variation in the relationship between mortality rates and climate.; To further understand the processes responsible for body size evolution of C. viridis, I examined patterns of body size variation between island and mainland populations. I found that all three island populations were significantly smaller than nearby mainland populations.