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

GERMANO, D.J. 1994. Growth and age at maturity of North American tortoises in relation to regional climates. Can. J. Zool. 72: 918-931. 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 1822 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.