The determination of feeding preferences through either direct observation of feeding or the analysis of stomach contents has serious limitations.\u00a0 In recent years there have been a number of studies undertaken using stable isotopes of carbon and nitrogen to determine trophic relationships of a variety of birds and mammals.\u00a0 Since the stable isotope ratio of a consumer reflects its diet, whale tissues analyzed for these ratios have the potential to indicate dietary history.\u00a0\u00a0 This approach would overcome the temporal biases inherent in stomach content analysis or strict observational evidence and has been used for other cetacean species.<\/p>\n
Isotope ratios are determined by the general type of food (original method of carbon fixation, number of trophic levels) that has been incorporated into the animal over the past several weeks or months and it gives a good overall idea of the average diet.\u00a0 With multiple types of food generally available, isotope ratios can indicate, but cannot prove, that a certain type of food was used; they can however, sometimes prove when a food has not been eaten and assimilated.\u00a0 In general, the d13<\/sup>C and d15<\/sup>N values of animals reflect the isotopic values of their diets, even though animals appear to incorporate dietary 15<\/sup>N over 14<\/sup>N.\u00a0 Isotopic analysis of an animal can therefore be used to reconstruct the diet when food sources have different d15<\/sup>N values.\u00a0 Furthermore, the 15<\/sup>N enrichment between trophic levels ranges from 1.3 to 5.3\u2030, averaging 3.4 \u00b1 1.1\u2030 irrespective of biochemical differences or differences in habitat.\u00a0 This characteristic elevation in nitrogen isotope ratio is thought to be due to isotopic fractionation associated with catabolic metabolism.\u00a0 This suggests that the d15<\/sup>N of the animal\u2019s body would be a useful parameter in assessing its trophic level in an ecosystem.\u00a0 On the other hand, the d13<\/sup>C values of animal tissue are very close to those in their diet, and only a small increase in 13<\/sup>C content (about 1\u2030 enrichment) occurs with increasing trophic level.\u00a0 The primary theoretical basis of using d13<\/sup>C as a tracer is that the characteristic ratios of different sources are preserved as the carbon is cycled through organisms and detritus.\u00a0 Consequently, the differences in d13<\/sup>C values have been used for the food habit analysis of animals in an ecosystem.<\/p>\n Since the turnover rate of an isotope in any single tissue is dependent on the tissue\u2019s metabolic rate, measuring the isotopic signature of several tissues from the same individual can provide short-, intermediate-, and long-term dietary information.\u00a0 Analysis of metabolically inactive tissues, such as hair, baleen, teeth, or whiskers, will reflect the diet as it existed during the period of tissue growth, while tissues which are very metabolically active, such as blood, will reflect the signature of the most recent meal.\u00a0 One difficulty in examining isotopic signatures of cetaceans is the lack of knowledge about the turnover rates of any tissues.\u00a0 There is some turnover data available for seals and some basic information on cetaceans, but it is difficult to extrapolate tissue turnover data for other species to cetaceans because of suggestions that cetacean skin has higher turnover rates than other mammals. \u00a0We have assessed diet-tissue fractionation and turnover times for dolphin and manatee skin.<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" The determination of feeding preferences through either direct observation of feeding or the analysis of stomach contents has serious limitations.\u00a0 In recent years there have been a number of studies undertaken using stable isotopes of carbon and nitrogen to determine … Continue reading