Environment and Variation in Life History Traits of the Chuckwalla, Sauromalus Obesus

Abstract

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 under a variable but predictable environmental setting.