Age estimates of the yellow-margined box turtles based on post-mortem examination of confiscated individuals

利用走私查緝個體估計臺灣食蛇龜年齡

Age is an important parameter for understanding growth patterns and population structure of wild animals. In chelonians the age can be estimated either by counting the number of scute growth rings (SGR) or reading lines of arrested growth (LAG) on long bone sections. Unfortunately, the inaccuracy of the first method and the difficulty in obtaining bones for study have resulted in limited in-depth researches on turtle life history and population biology. In order to more accurately determine the age of the protected yellow-margined box turtle (Cuora flavomarginata), we used turtles that died within one year after being confiscated in a rescue facility, and took measurements from humerus and estimated turtle age by counting SGR and LAG on humerus based on the skeletochronological method. Humerus length, width and weight positively correlated with body weight in the yellow-margined box turtle, and the LAG counts were also proportional to body weight, which justified the use of humerus for skeletochronology. Due to absorption of inner LAG in the center of the bone, it was necessary to adjust the LAG counts using juveniles of known age. In juveniles and adult females, SGR counts were always higher than the adjusted LAG, whereas in males both counts were not significantly different. Turtles born in the laboratory had higher SGR counts than adjusted LAG counts, and the former tended to overestimate the age of turtles. LAG was more reliable to estimate turtle age. We used two nonlinear regression models to compare growth patterns, and the logistic model fit the data better. The model suggested high growth rate in early years followed by a decrease, presumably near the age of maturation. It would be possible to take the straight carapace length of turtles into the formula to obtain a more accurate estimate of the ages of the yellow-margined box turtle. According to the observation records of breeding size in the wild, and using this model, the age of sexual maturity for female was 12 years old, and 11 years old for male.

年齡是瞭解野生動物生長模式與族群結構組成的重要數據，但龜鱉類的年齡僅能以背甲輪估計或由附肢骨骼切片得知，前者之不準確和後者之難以取得，限制了對龜鱉類生活史和族群生物學的研究。為能更準確判別保育類之食蛇龜年齡，本研究使用收容後一年內死亡的屍體，測量肱骨形態並同時利用背甲輪計數與肱骨鑑齡估計年齡。食蛇龜肱骨長度、寬度與重量均隨體重增加，且肱骨的橫切片的生長停滯環數目亦和體重成正比，適合拿來做骨骼鑑齡，但須利用已知年齡幼龜肱骨最內圈之停滯環校正骨髓內被吸收之環數。在幼龜與成體雌龜，背甲輪計數顯著大於校正後停滯環數目|成體雄龜則在兩種年齡估計上無顯著差異。於實驗室內出生的幼龜，背甲輪計數結果顯著大於停滯環目，且明顯大於實際年齡，生長停滯環結果較為接近實際年齡。我們使用兩種非線性迴歸模型來比較食蛇龜生長模式的差異，以邏輯生長模式較適用於食蛇龜的生長模式，顯示食蛇龜的生長率會先逐漸增加到一個高峰後下降。未來可以體長準確帶入公式後估計食蛇龜年齡，依照前人野外個體實際繁殖觀察紀錄，並使用本研究模型推估雌龜性成熟年齡為12歲、雄龜為11歲。