Age structure and growth pattern of a high-altitude lizard population based on age determination by skeletochronology

Age determination provides crucial data regarding the life history of a species. Skeletochronology is considered to be a reliable method for age determination but is seldom applied to the genus Phrynocephalus (Agamidae) as it requires careful bone histological analysis to determine the formation of the lines of arrested growth (LAGs). We analyzed LAGs in different bones of Phrynocephalus vlangalii, a small, high-altitude lizard, and demonstrated that the phalanges are the best materials for skeletochronology in this species. We also found that the proximal half of the phalangeal diaphysis exhibits the most complete LAGs because of the unidirectional longitudinal growth of the phalange. Histologically, as the embryo bone that developed during the embryonic period was preserved even in very old individuals, no LAGs were eroded by endosteal resorption in the phalanges. The maximum age was 4 years for males and 5 years for females. It was determined that the age at maturity was 3 years for males and 2 or 3 years for females. On the basis of age determination, the age structures of both sexes were compared, and age-specific growth differences between the sexes were also analyzed. No differences in the snout-vent length were observed between sexes in any age group. However, the tail length of males aged 3 and above was significantly longer than that of females. Thus, the use of phalanges in the age determination of P. vlangalii is effective and can be widely applied to the ecological studies of this species as well as other small lizards.