Life history attributes for the western chuckwalla, Sauromalus obesus, and environmental variables, e.g., rainfall and vegetational growth, were recorded simultaneously during a 7—yr investigation in the Colorado desert of southeastern California. The timing of rainfall was most critical to the growth of annual plants, and the standing crop of winter annuals was greatest when substantial rainfall occurred early (October—January). Chuckwallas preferred to eat winter annuals during the spring. However, broad and opportunistic feeding habits were observed, and the persistence of relatively mild winters and frequent summer rainfall enabled individuals to feed throughout most of the year. Males typically achieved reproductive maturity at 125 mm snout—vent length (SVL) and 2 yr of age. Females achieved reproductive maturity at 125 mm SVL and 2—3 yr only when optimal environmental conditions prevailed. Clutch size was highly correlated with body size, and the mean for the study period was 6.9 eggs. Mean values for other reproductive attributes were: egg mass, 8.4 g; relative clutch mass (RCM), 0.343; and expenditure per progeny (EPP), 0.053. Clutch size, egg mass, RCM, and EPP did not differ significantly among years for a given body size. Mean annual frequency of reproduction was 52%, but ranged from 0 to 95% during the study period. Mean 1st—yr survivorship was 38% but showed considerable year—to—year variability. Egg mortality had the greatest impact on 1st—yr survivorship. Mean survivorship for chuck—wallas older than 1 yr approached 75% and values for males and females were not significantly different. Adults of both sexes appeared to be considerably more susceptible to predation after achieving 165 mm SVL. Consequently, large adult individuals were never common. Annual recruitment was 20% and was largely the result of reproduction. Population densities showed nearly a twofold annual variation, ranging from 15 to 30 individuals/ha. Life table analysis showed that younger females (3—6 yr) contributed to 50% of the replacement rate; mean generation time was 8.2 yr, and the life expectancy was ≈15 yr. Year—to—year consistency of various reproductive attributes (clutch size, egg mass, and RCM), indicative of "boom or bust" iteroparity, is possibly a typical adaption for extremely long—lived lizard species. Relatively mild winters and occurrence of summer rainfall in the Colorado desert greatly promoted early maturity and frequent reproductions. In addition, summer rainfall improved the survivorship of eggs, probably by providing adequate moisture for incubation. Such conditions were responsible for relatively high densities and predation rates. However, under drier conditions maturity was delayed, reproduction less frequent, and egg survivorship much lower; such conditions are typical in the Mojave desert. The relative stability of clutch size, egg mass, and RCM, but apparent variability of age at reproductive maturity and reproductive frequency in this study, may indicate that the life history of chuckwallas evolved un...

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Studies of life histories of lizards have usually failed to address appropriate questions and/or were not of sufficient duration to test current life history theory. Also, studies of insectivorous lizards far outnumber studies of herbivorous species, and the life history of the western chuckwalla (Sauromalus obesus Baird), an important desert herbivore, is scarcely known. I chose to study the life history of ~. obesus over a sufficient period to provide an adequate test of current life history theory. In addition, I attempted to model population parameters for S. obesus from environmental variables.This investigation of ~. obesus was conducted from 1977 -1983 in the Colorado Desert of southeastern California and incorporated 4 segments of data: 1) environmental and general ecology; 2) reproductive cycle and synchrony; 3) life history and demography; and 4) population dynamics.At an 8 ha study site, three hundred and twenty six S. obesus subjected to capture-recapture and observation during 1250 hours of investigation provided data for determining most life history and demographic attributes. About 500 S. abe sus (approximately equal numbers of the sexes) were collected for autopsy from localities within a lS km radius of the study site. A combination of gross morphological and histological measurements enabled the determination of a number of reproductive traits and provided additional life history and demographic information.The main activity period of Colorado Desert ~. obesus is about 8 months, but episodes of feeding occurred throughout the year. Coyotes Size at reproductive maturity for both sexes was about 125 mm SVL.Age at maturity for males and females was 2 and 3 years, respectively.Mean clutch size was 6.9 eggs. Reproduction occurred in 6 of 7 years.Mean annual reproductive frequency was 51%. Mean egg weight was 8.4g.Clutch weights averaged 34.3% of the total body weight. Egg weights averaged 5.3% of the total body weight. For a given body size, there was no annual variation in clutch size, egg size or reproductive effort.The consistency of these traits indicates adaptation to a predictable environment. Relatively high egg weights are an adaptation to counter the harsh environment. Fecundity was highly correlated with body size (age); annual variation was attributable to changes in reproductive frequency. First year and subsequent annual survivorship rates were 38% and 75%, respectively.The adult sex ratio was 1 : 1. Annual recruitment was about 20%, almost exclusively due to reproduction. Annual tail-break frequency The population I studied demonstrated considerable growth (Ro = 1.54).However, because occasional periods of drought cause rapid population decline, over the long run population stability is maintained. Attempts were made to predict population age class structure from winter precipitation regression models. Such a procedure appears reliable for predicting age-specific fecundity and therefore is a useful tool for management practices. ACKNOWLEDGEMENTS

Thermal ecol~gy and movement of the Leopard Lizard, G. wistizeni were studied at Frog Spring, adjacent to the Alvord Desert, and at various locations throughout the Alvord Basin.Seasonal activity is known to oc~ur from early May to mid-October, but abundance. gradually decreases after late July. During favorable late spring, and summer 2 weather lizards exhibit a bimodal pattern of diel activity with some activity occurring at all hours of the day between 0600 and 1800 H. At other times activity was unimodal. Emergence was apparently largely temperature dependent and did not occur until sand temperature approximated 21°C. Retreat appeared to be temperature independent, and was initiated by exogenous or end~genous stimuli at sand temperatures often above body temperatures acceptable for normal activity. For the period of study, juveniles exhibited diel activity patterns similar to adults, Iand were found during all hours in which adults were en-