Isotope analysis

Prepared by: Seth Cherry, University of Alberta

Stable isotopes are different forms of the same chemical element that vary in atomic mass due to differing numbers of neutrons. Most elements of biological significance (i.e., carbon, hydrogen, oxygen, nitrogen, and sulfur) have one or more naturally occurring stable isotope. The most commonly used elements in isotopic diet studies are carbon and nitrogen, with the most widespread isotopes of these elements being 12C and 14N, respectfully.  However, the less common and heavier stable isotopes 13C and 15N can be useful biological tracers because even though they have similar chemical characteristics as their lighter counterparts, they react at different biochemical and geochemical rates in ecological systems. This difference in reaction results in fractionation or discrimination in the ratios of 13C/12C or 15N/14N in different organisms throughout a food web. These fractionation events result in various organisms having distinct isotopic signatures. An isotopic signature is simply the ratio of heavy to light isotopes of a given element in biological material relative to the same ratio in an element-specific reference material used for comparison in all ecological stable isotope research.

Diet reconstruction

Stable isotope analysis is a convenient and useful technique that can be employed to reconstruct an animal’s diet. It is based on the concept that isotopic signatures in an animal’s tissues are similar to signatures found in the foods they consume. The diet of an individual can be determined by comparing the isotopic signature found in its tissues to signatures in possible prey species. This can be done for various time frames depending on which tissue is sampled and the metabolic turnover rate of the chosen tissue. Thus, it is possible to determine the diet of a polar bear during various seasons by sampling a variety of tissues at once. When a polar bear is captured, several different types of samples may be taken for stable isotope analysis including: hair, claw, blood, and even breath. Metabolically inert tissues such as hair and claw represent the isotopic signatures of the food a bear was eating at the time the tissue was grown. The isotope signature stored in a metabolically active tissue such as blood represents the foods that a bear consumed approximately 1-2 months before being sampled. Breath samples may be used to indicate the isotopic signature of the energy source being metabolized by the bear at the immediate time of capture.

Perfect technique for remote bears

Stable isotope analysis is particularly useful for research on polar bear diets because polar bears inhabit remote areas that are logistically difficult to access during most times of the year. Because direct observations of polar bear feeding behaviour are usually impossible, we can use stable isotope analysis to provide important insights regarding what polar bears are eating in various regions and during different seasons. The use of stable isotope analysis to examine animal diets is somewhat unique because it allows for the measurement of assimilated nutrients over various time periods. This differs from other methods such as scat or stomach analysis, which only reveal an individual’s last meal. In addition to examining potential seasonal changes in polar bear diet, it may also be possible to implement long-term sampling protocols enabling researchers to determine the effects of a rapidly changing Arctic climate on the feeding ecology of polar bears.

Further reading

Kelly JF. 2000. Stable isotopes of carbon and nitrogen in the study of avian and mammalian trophic ecology. Canadian Journal of Zoology 78 (1): 1-27.

Bentzen TW, Follmann EH, Amstrup SC, York GS, Wooller MJ, and TM O'Hara. 2007. Variation in winter diet of southern Beaufort Sea polar bears inferred from stable isotope analysis.  Canadian Journal of Zoology 85(5): 596-608.

Crawford K, Mcdonald RA, and S Bearhop. 2008. Applications of stable isotope techniques to the ecology of mammals. Mammal Review 38(1): 87-107.

Hobson KA, Stirling I, and DS Andriashek. 2009. Isotopic homogeneity of breath CO2 from fasting and berry-eating polar bears: implications for tracing reliance on terrestrial foods in a changing Arctic. Canadian Journal of Zoology 87(1): 50-55.