Abstract: Age assessments could be used to examine the ecological characteristics of amphibians and reptiles, which is of significance for analyzing life-history traits and predicting population dynamics. Skeletochronology could effectively identify individual age and estimate the growth rates of amphibians and reptiles. In this study, 66 individuals (39 females, 27 males) of the Yarkand toad-headed agama Phrynocephalus axillaris were collected from the Lop Nur Wild Camel National Nature Reserve in southeastern Xinjiang. We measured 21 external morphological traits of all individuals. The 3rd section of the 2nd phalange of the right fore limb was removed for skeletochronological age assessment from paraffin thin sections. The results showed that the maximum age of both males and females was five years. There was a significantly positive correlation between snout-vent length and age for both males and females, but the growth rate continuously decreased with age. The lengths of the fore limbs, hind limbs, and tail in males are significantly longer than that in females. The growth rate of the tail base width in males was significantly higher than that in females. At the age of 3 years, the snout-vent length as well as the lengths of the fore limb, hind limb, tail, second fore toe, and the third, fourth, and fifth hind toes were all significantly greater in males than in females. At the age of 4 years, the tail base width and the lengths of the hind limb, tail, fourth fore toe, and the first, second, third, and fourth hind toes were all significantly greater in males than in females. With the increases of age, resource investment to locomotor-related organs in males increased, consistent with the prediction of sexual selection hypothesis.

Key words: Phrynocephalus axillaris, growth arrest line, sexual dimorphism, age determination, sexual selection hypothesis