Age Structure, Sexual Dimorphism, and Ontogenetic Melanism in a Captive Population of the Endangered Freshwater Turtle Mauremys reevesii (Gray 1831)

Abstract

The age structure of animal populations plays a pivotal role in understanding their demographic dynamics and reproductive potential. Although carapace growth ring analysis is widely used to estimate turtle ages, environmental variability often causes irregular growth patterns, which can compromise the accuracy of this method. Research based on accurately determined ages remains limited despite its importance, especially when addressing vulnerable populations. This study investigated body size distribution and melanism in a captive population of 131 endangered freshwater turtles, Mauremys reevesii, with precisely known ages. The mean ages were 9.4 +/- 3.9 years (maximum 16) for females and 5.5 +/- 1.8 years (max 12) for males, indicating greater longevity in females compared to males. Females were significantly larger than males, with slower growth but larger asymptotic size, demonstrating female-biased sexual size dimorphism. Sex could be distinguished from three years of age onward. Melanism appeared only in males, first being observed at 4 years of age, and the progression followed the order of eyes, claws, skin, and shell. The stage of melanism in male M. reevesii was significantly influenced by age rather than body size. These findings provide the first detailed age-based dataset on growth and melanism in this critically endangered species. This study offers a scientific basis for more accurate population assessments and the development of targeted conservation strategies, while helping to bridge the gap between wild and captive populations.