Comparison of Age Structures of Male Blue Sheep (Pseudois nayaur) on Helan Mountain in Two Periods, 1995 and 2004

We analyzed the static life table, age structure and survivorship curve of blue sheep (Pseudois nayaur) using the horns of 102 male blue sheep that died of natural causes in tie field, from April to May, 2004, on Helan Mountain. The age of each horn was determined by the growth ring. We also compared the data from this study with data from 1995. The natural mortality rate between 1.5 and 4.5 years old ranged from 0% to 7.4% in this study. For rams between 5.5 and 10.5 years old the range was 14.9% to 25.0%. The mortality rate for rams with 11.5 to 15.5 years old varied between 40.7% and 100%. The first mortality peak (from 1.5 to 4.5 years old) in this study was similar to the results of 1995, while the second mortality peak (from 5.5 to 10.5 years old) was remarkably delayed. Unlike the results from 1995, 64.7% of individuals during this research survived up to 8 years old, and 7.8% rams survived until after 13 years. The 2004 data indicated that life expectancy of rams was close to that of 1995. However, life expectancy in the older rams (≥5.5 years) from this research increased significantly. We found a significant difference in the mortality rate of rams below 4.5 years old between 1995 and 2004. There was a significant difference in the mortality rate of rams from 5.5 to 9.5 years old between 1995 and this study. The mortality rate of rams over 4.5 years of age differed significantly between 1995 and 2004. Although the blue sheep populations on Helan Mountain have experienced some changes in the last ten years, the survivorship curve of rams was still close to convex type (A type).