Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)



Committee Chair(s)

Edmund D. Brodie Jr.


Edmund D. Brodie Jr.


T. W. Evans


Susannah French


Brett A. Adams


Anthony R. Lowry


The San Luis Valley is a large valley formation in Colorado surrounded on either side by mountain ranges exceeding 4,267 m. Within the Valley, two of the 14 amphibian and reptile species are dwarfed: the short-horned lizard (Phrynosoma hernandesi) and the Great Plains toad (Anaxyrus cognatus). Since its initial reporting in 1968 and confirmation in 1981, no research further investigating this dwarfism has been conducted. I collected morphological measurements to determine the extent and patterns of dwarfism of both species. I then investigated the genetics of both species using mitochondrial DNA to determine whether they are genetically distinct, their colonization histories within the Valley, and whether the Valley functions as a reproductive barrier. Lastly, I report life/natural-history data to determine the effects of dwarfism. Phrynosoma hernandesi and A. cognatus were significantly dwarfed and showed an increase in sexual size dimorphism compared to populations surrounding the Valley. Valley populations of P. hernandesi show high amounts of genetic divergence from populations surrounding the Valley while A. cognatus shows minimal genetic variation throughout its range. Based on the variable distribution of genetic variation in the Valley, historic climate patterns, and fossil records, there are two most likely colonization histories for P. hernandesi: 1.) populations colonized the Valley during a singular event and have since diverged or 2.) populations colonized the Valley during two events that correlate with the two warm, dry periods within the last 0.8 MYA. Dwarfed P. hernandesi consumed diets similar to populations outside the Valley although there is local variation in the diversity of prey items consumed. Phrynosoma hernandesi at Zapata Ranch showed annual variation in body size and morphology while population dynamics correlate with the timing of precipitation. Also, females show a reduced reproductive output, producing fewer neonates but of equal size to non-dwarfed neonates. Collectively, findings from this study suggest that Valley populations represent unique taxa and should be considered for further genetic study to determine their taxonomic and conservation status.




This work made publicly available electronically on December 23, 2010.