Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)



Committee Chair(s)

Edmund D. Brodie Jr.


Edmund D. Brodie Jr.


Michael E. Pfrender


Joseph R. Mendelson III


James A. MacMahon


Barrie K. Gilbert


During the summers of 1999 and 2000 the apparent habitat partitioning of two species of sympatric horned lizards was investigated at the Barry M. Goldwater Bombing Range near Yuma, Arizona. The flat-tailed horned lizard, Phrynosoma mcallii, and the desert horned lizard, P. platyrhinos, overlap in range in portions of the Sonora desert in the southwestern United States. However, upon finer scale examination, it appeared that these horned lizards were partitioning the available habitat according to soil substrate texture. Phrynosoma mcallii appeared to be exclusively found in areas of fine, loose, wind-blown sands, in contrast to P. platyrhinos, which appeared to be associated with areas of hardpan substrates covered by small rocks and pebbles. Subsequently, an experiment was set up to test if soil texture was indeed responsible, at least in part, for the observed partitioning of these lizards among the available habitat. Three experimental treatments were established (Sandy, Rocky, Mixed) which differed only with respect to soil texture. Lizards of both species and sexes were fitted with backpacks of thread and allowed to spool out one pathway in each of the three treatments. Additionally, pathways of P. mcallii found on the Naval Air Facility El Centro CA were also collected. This nearby population of P. mcallii is allopatric with respect to P. platyrhinos. Net displacements, fractal dimensions, and a correlated random walk (CRW) model were utilized to evaluate differences in movement patterns. Similarities in the net displacement and fractal dimension of movement pathways from both species of horned lizards for each of the three treatments suggest substrate composition does not affect species distribution. Additionally, a CRW model severely underpredicted the observed net squared displacement of both P. mcallii and P. platyrhinos among all three treatments and at both study sites, thus indicating that lizards were moving more linearly than would be expected. Other abiotic factors such as temperature or soil moisture could be important variables in habitat preference. Additionally, due to the sister relationship of these species, phylogenetic constraints in morphology and/or behavior may contribute the to lack of differing movement patterns among varying substrates.