Bone growth strategies and skeletochronological age estimates of desert tortoise (Gopherus agassizii) populations.

摘要

Turtles are among the longest-lived vertebrates and concepts related to aging are important because they may have a substantial impact on life history evolution. Age at sexual maturity and longevity are essential baseline data which all demographic studies need for the conservation of extant species. The desert tortoise (Gopherus agassizii) is a threatened species and seems to be long-lived, although population longevity is mostly unknown. Skeletal growth layers can be used to estimate age, and skeletochronology has been shown to accurately age many reptiles. My main objective was to determine whether bone, obtained from carcasses, could be used to indicate differences in longevity and growth strategies among wild desert tortoise populations. I used two skeletochronological methods (the Correction Factor and Ranking Protocol) to estimate ages for tortoises from the Mojave and Sonoran Deserts. Using a sample of 9 known-aged tortoises from Rock Valley, Nevada, I determined that the Correction Factor method provided the most accurate age estimates for juveniles, whereas the Ranking Protocol provided the most accurate age estimates for adults. West Mojave tortoises had the smallest females, highest growth rates, youngest age at sexual maturity but shortest longevity of the three populations. In contrast, Sonoran tortoises had the largest females, lowest growth rates, oldest age at sexual maturity and greatest longevity. East Mojave tortoises showed intermediate traits. Annual growth rates in West Mojave adults were similar to those of juveniles, whereas East Mojave, and especially Sonoran adults had significantly reduced growth rates after the attainment of sexual maturity. West Mojave tortoises may have higher overall growth because of the higher desert productivity and more abundant forage after wet years. The cost for Mojave Desert tortoises (especially western populations), without dependable and predictable annual forage and water for health and growth maintenance, may be higher mortality and reduced life expectancy, which females have compensated for by evolving a younger age at sexual maturity. Desert tortoises are potential models for the consequences of increased desertification on desert fauna, especially in species that have not evolved as desert specialists but rather have learned to adapt to these extreme environments.