Variations in δ15 N and δ13C in Dentin of Risso''s Dolphins (Grampus griseus) and Pantropical Spotted Dolphins (Stenella attenuata)

• Main Authors: Florence Chan Evacitas, 詹斐蓮
• Other Authors: Lien-Siang Chou
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/nwrak6

博士 === 國立臺灣大學 === 生態學與演化生物學研究所 === 105 === 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.