Variation in tortoise life history: Demography of Gopherus berlandieri

Abstract

Turtles provide excellent models for studies of life history strategies, but terrestrial species are underrepresented in these analyses. We present a life table and demographic parameters of an inland population of Gopherus berlandieri to contribute to the study of evolution of turtle life histories. Data were gathered during a mark–recapture and radiotelemetry study in the Tamaulipan Biotic Province in southern Texas, USA. A total of 835 individuals were captured, measured, and their ages estimated. Females matured at 131 mm carapace length at an average of 5 yr of age (range: 4–8 yr). Clutch size, as determined by ultrasound, averaged 2.07 ± 0.15 eggs (n = 49). No nests were found, and we estimated clutch frequency with a quadratic model to be 1.34 clutches·female−1·year−1. Survival was estimated from age frequency regression and telemetry. Annual male survival (0.828, 0.834) differed (P < 0.05) from female survival (0.728, 0.774) for both techniques of estimation. Age‐specific female survival ranged from 0.62 to 0.83 for 5‐ to 15‐yr‐old tortoises. Differential mortality of sexes led to a male‐biased sex ratio in older age classes. Age structures of captured individuals did not vary (P > 0.05) among years, and population estimates did not differ among years; therefore, we constructed a life table under the assumption of a stationary population (r = 0.0). Under this assumption, survival from nest to age 4 yr must be at least 0.245, with hatchling survival of 0.528 to maintain a stationary population. Gopherus berlandieri matured at an earlier age, had smaller clutch sizes, and exhibited lower rates of female survival than other Gopherus species. We propose a physiological mechanism for lower female survival that implies trade‐offs among egg size, subsequent hatchling survival, and female health. We maintain that high hatchling survival is necessary for population persistence. In an evolutionary context, we theorize that the selective advantages of small size and the life history strategies of G. berlandieri have been and are critical to its persistence.