Skeletochronology Reconciles Differences in Growth Strategies and Longevity in the Common Chuckwalla (Sauromalus ater) with Implications for Squamate Life-History Studies
Accurate, comprehensive data on life expectancy, growth rates, age structure, and sexual maturation are necessary to inform decision-making for conservation action, but they are often difficult to obtain from wild animals. Osteohistology has proven a reliable method for accessing chronological data in extant and extinct vertebrates. Traditional mark–recapture methods require repeated measurements in the field and do not allow comparison with far more extensive data from specimens housed in museum collections, thus reducing statistical power and inviting unjustified extrapolation. We investigate longevity and growth in the large-bodied iguanian lizard Sauromalus ater (Common Chuckwalla). We find highly variable growth to be typical for S. ater occupying a spectrum from rapid growth to sexual maturity within two to three years leading to above-average body size later in life or slow growth early in life with sexual maturity at four to eight years followed by growth to body sizes around the average adult size. We show individual S. ater achieve final body size in about ten years, not decades, and that body size and age in adults are significantly correlated. This study suggests conflicting reports on growth, maturation, longevity, and generation cycle in S. ater are all correct and reflect complex ontogeny in squamates.
Distribution of Sauromalus ater (shaded area) and localities of specimen collection (circles), with the two dominant ecoregions (Mojave and Colorado deserts; dashed outlines) inhabited by the species. The specimens for this study were collected in the southern and western Mojave and the northern Colorado deserts. Numbers indicate number of specimens collected. Distribution data: NatureServe and IUCN (2007).
Exemplary thin sections of Sauromalus ater. (A) CAS 174906, male, 8 years; (B) CAS 231585, male, 9 years, innermost line of arrested growth (LAG) completely resorbed; (C) CAS 174679, male, 5 years; (D) CAS 174542, female, 7 years. Partial resorption of earlier LAGs in A–D. Arrows point at LAGs; eb, endosteal bone; nf, nutrient foramen. Boxes indicate position of Figure 3A and B.
Close-up images showing features from Figure 2 in greater detail. (A) CAS 231585; (B) CAS 174542, black line highlights shift in LAG caused by nutrient foramen. Partial resorption of earlier LAGs in both. Arrows point at LAGs; eb, endosteal bone; nf, nutrient foramen; rl, resorption line.
Comparison of observed first LAG circumferences and 100 simulated yearling sizes based on previous reports. The first preserved LAG was significantly larger than the reported yearling size range in 13 specimens, indicating loss of the LAG corresponding to yearling size in those individuals.
Growth rates to sexual maturity of male (blue) and female (tan/red) Sauromalus ater. Growth rates are in reference to increase in femur circumference (*).
Contributor Notes
Associate Editor: D. S. Siegel.