Abstract
A mark-recapture study of the fresh water turtle Chinemys reevesii was conducted on a small (190 ha) island, Ishima Island, off the east coast of Shikoku to learn the demographic characteristics of an insular turtle population. Most of the Chinemys reevesii on the island were marked, yielding a reliable estimate of population size (1179 turtles per 15 ha aquatic habitats). The estimated sex ratio of the turtles in the younger age class (3-6yr) was essentially equal, slightly skewed toward males in a higher age class, and significantly male-biased in the older age class (>12yr). A significantly female-biased sex ratio among the turtles found dead was compatible with the male-biased sex ratio in adult turtles. Possible physiological and ecological factors for change in sex ratio with age are discussed with consideration of the insular environment.
