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Variations in δ15 N and δ13C in Dentin of Risso's Dolphins (Grampus griseus) and Pantropical Spotted Dolphins
|Authors:||Florence Chan Evacitas|
Pantropical spotted dolphin,stable isotopes,dentin,weaning age,isotopic niche,
|Publication Year :||2017|
|Abstract:||齒鯨類的牙齒象牙質每年累積生長層，各層所含同位素比值可以顯示其合成時的食性，因此可以提供個體一生的食性記錄，並可推導出其生活史特徵。 為便於小型齒鯨的牙齒生長層取樣，必須發展比過去micromilling方法更精確且有效的方法。本方法先將牙齒先脫去礦物質後再縱向切片，可減少耗時90%，並多得10%象牙質collagen ，所得碳氮%，及碳氮比皆較穩定，變異明顯縮小。 採用新方法處理1994到2014年間擱淺或誤捕的65隻瑞氏海豚與59隻熱帶斑海豚的牙齒之出生前後生長層取樣，以進行穩定同位素分析，然後就C/N、擱淺地區及年份用Multivariate Adaptive Regression Splines 模式進行校正。兩種海豚都是在出生前生長層的氮同位素最高，較其母親值高約∼2.0 – 4.0‰，此可推導懷孕期時是否有Fetal Enrichment 現象。氮同位素值在早年生長層遞降趨勢可顯示出海豚在哺乳到完全斷乳時其食物蛋白質來源的變化。 斷乳時間的性別差異在瑞氏海豚(雄性 = 1.09歲, 雌性 = 1.81 歲)較熱帶斑海豚(雄性 = 1.14歲, 雌性 = 1.06歲)明顯。不同於熱帶斑海豚，瑞氏海豚斷乳後碳氮同位素有顯著的雌雄差異，此意味可能有性別分群攝食現象。以Bayesian Ｅllipse-based Ｍetrics估算台灣東海岸共域的這兩種海豚的碳氮同位素食性區位發現，瑞氏海豚比熱帶斑海豚的食性區位較寬。此可推測瑞氏海豚可能攝食較寬廣的碳氮同位素食物，也在食性多樣性（trophic diversity）上表現出不同年齡間與雌雄性別間較大的差異。兩種海豚之間，或其潛在食餌生物種之間的同位素食性區位皆有高度重疊，由此可推斷這兩種海豚可能攝食具有相似碳氮同位素的食物，雖然可能來自不同類群的食餌生物。|
Teeth of odontocetes accumulate annual dentinal growth layer groups (GLGs) that record stable isotope ratios reflective of the time of their synthesis. Together, they provide lifetime records of individual feeding patterns from which life history traits can be inferred. To obtain reliable fine-scale stable isotope data from small odontocete teeth, a more precise and efficient subsampling technique than the standard micromilling process was developed. The method, which involved cutting out the GLGs from longitudinal sections of a demineralized tooth, reduced sample processing time by 90% and yielded ~10% more dentin collagen that were more consistent in wt% N, wt% C, and atomic C/N ratios and showed more reliable δ15N and δ13C values than those obtained by micromilling.
This method was then used to subsample the prenatal dentin and postnatal GLGs in Risso’s dolphins (Grampus griseus) (n = 65) and pantropical spotted dolphins (Stenella attenuata) (n = 59) that stranded or were collected as bycatch in Taiwan (1994-2014) for the stable isotope analyses. Age-specific dentin 15N and 13C were corrected for effects of C/N, stranding site, and year using multivariate adaptive regression splines models. In both species, 15N values were highest in the prenatal layers that were about 2.0–4.0‰ greater than maternal values, suggesting fetal enrichment during gestation. Decreasing 15N values in the early GLGs suggested changes in dietary protein sources during transition from nursing to complete weaning. Sex difference in weaning age was more evident in the Risso’s dolphins (i.e., males = 1.09 yr, females = 1.81 yr) than in the pantropical spotted dolphins (i.e., males = 1.14 yr, females = 1.06 yr). Significant differences in 15N and 13C between weaned males and females in Risso’s dolphins, which was not observed in the pantropical spotted dolphins, suggest sexual segregation in feeding habits.
Comparison of the isotopic niches of these sympatric species in eastern Taiwan waters using Bayesian ellipse-based metrics computed from dentin δ15N and δ13C data showed wider isotopic niche in Risso’s dolphins than in pantropical spotted dolphins. This suggests that the former exploit wider range of δ15N and δ13C and had greater measures of trophic diversity among life stages and between sexes than the latter. The highly overlapping isotopic niches between dolphin species as well as among potential prey species suggest that these two delphinid species may consume common prey of similar isotopic niches although they may be of different taxa.
|Appears in Collections:||生態學與演化生物學研究所|
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