An Ecological Study of the Chuckwalla, Sauromalus obesus Baird, in the Western Mojave Desert

Abstract

A marked population of chuckwallas, Sauromalus obesus, was the object of an ecological study in the western Mojave Desert during three activity seasons. The results include data on diel and seasonal activity, reproduction, size, growth, age, sexual dimorphism, molting, population dynamics, and feeding. Members of a second population located during the last year of study were collected to supplement the data on reproduction and to provide specimens for a color and pattern analysis. This herbivorous lizard inhabits rocky areas and exhibits daily activity typical for heliothermic reptiles. Chuckwallas are active only from mid-March to mid-August, and presumably estivate and hibernate during the inactive period. Both sexes live within home ranges, but the consistent occupancy of a home site was not observed. All mature males are reproductively active during the spring. Data suggest that mature females do not produce a clutch of eggs each year, but rather every two years or more during the middle of June. The chuckwalla population exhibits an even sex ratio, low density, low reproductive potential, a high survival rate, and great longevity. Most growth occurs during the spring, the time of maximum vegetational development. Although chuckwallas probably grow throughout their life, the highest growth rate occurs among juveniles. Sauromalus obesus was compared with Dipsosaurus in order to ascertain features of special adaptive significance. The chuckwalla, Sauromalus obesus, is a large iguanid lizard occurring in the desert regions of southeastern California, western Arizona, southern Nevada, southern Utah, and adjacent portions of Mexico (Shaw, 1945). This lizard is restricted to rocky areas and lava flows within its range. It is herbivorous, a feature shared with only one other desert sp-ecies, Dipsosaurus dorsalis, the desert iguana. The present report is based upon a detailed ecological analysis of a population of chuckwallas from the western Mojave Desert of California. The study site was located in Red Rock Canyon at an elevation of 2400 ft near Mojave, Kern County, California, at the northwestern margin of the range of the species. Individuals of one population called the marked population, were marked for identification purposes and investigated over a three-year period. Information obtained from the marked population provided data on the following topics: diel and seasonal activity, movement, reproduction, population size and composition, age, and growth. A second population, called the sample population, was utilized in the last year of field work. Portions of this population were captured and preserved for a study of color, pattern, and reproductive organs. The direct line