Growth and age at maturity of North American tortoises in relation to regional climates

Abstract

North American tortoises (Gopherus spp.) are long-lived species that occur in a variety of habitats. I described growth of tortoises using Richards' growth model based on measures of scute annuli. Gopherus flavomarginatus is the largest species and grows the fastest, and Gopherus berlandieri is the smallest species with the slowest growth rates. Gopherus polyphemus and Gopherus agassizii are intermediate in size, with intermediate growth rates. All species grow relatively fast for 18–22 years, after which time growth rates decrease greatly. Estimates of mean age at maturity are 13.3 years for G. berlandieri, 13.9 years for G. flavomarginatus, 14.4 years for G. polyphemus, 13.8 years for Sinaloan G. agassizii, 14.4 years for western Mojave G. agassizii, 15.4 years for eastern Mojave G. agassizii, and 15.7 years for Sonoran G. agassizii. Several measures of growth did not correlate with precipitation among species of Gopherus, although mean yearly growth was negatively correlated with mean annual precipitation in populations of G. agassizii. Also, a matrix of growth variables did not correlate with a matrix of environmental variables. Although other environmental variables should be measured within the ranges of Gopherus spp., preliminary analyses suggest that the environment is not the main determinant of growth rates of North American tortoises.

Longevity and growth strategies of the desert tortoise (Gopherus agassizii) in two American deserts

“…Previous studies reported that male and female desert tortoises generally attain similar ages (Germano, 1994a;Germano et al, 2002;Turner et al, 1987). Our results bolster that conclusion; our samples show no sexual dimorphism in age (Fig.…”

“…1), matching the results of Germano (1994a). In both sexes, CL and bone lengths are highly correlated.…”

“…Sonoran females reached significantly larger sizes (CL) than Mojave females, but this difference was not observed in male body size comparisons. Whereas the mean CLs of our samples match those of (Germano, 1994a, Table 2) for West Mojave males and Sonoran females, our Mojave females have a strikingly smaller mean and the Sonoran males a larger mean. We interpret these differences as sampling error from smaller samples.…”

“…The estimated longevity in both groups is higher (noticeably so in Sonoran tortoises) than those proposed by (Germano 1992(Germano , 1994a for West Mojave (32 years) and Sonoran Desert (35 years) populations. Maximum age estimates are 47-54 years in Sonoran males and 40-43 years in Sonoran females.…”

Population ecology ofPsammobates oculiferin a semi-arid environment

“…Body mass and SV were used to calculate body condition as a ratio of mass to shell volume (Loehr et al 2004) and as mass scaled to shell volume (Loehr et al 2007a). Because scute ring counts often provide a reliable estimate of age in chelonians (Germano 1994;Germano & Bury 1998;Hellgren et al 2000), we used a hand lens to count scute rings (annuli) on the dorsal and ventral sides of all individuals. We also took digital photographs of dorsal and ventral surfaces, which helped to verify annuli counts made in the field.…”