Fatty acid signature analysis is a technique used to examine the feeding habits of free-ranging polar bears. It is based on the knowledge that chemical components of food are incorporated, largely unaltered, into a polar bear’s tissues. By looking at the fatty acid composition of a polar bear’s fat, and comparing it to its potential prey, we can make some inferences about what that bear has eaten.
Fatty acid structure and nomenclature
Fatty acid analysis uses the components of dietary fat as an indicator of past diet. Animal fats (which are also called lipids) are primarily in the form of triacylglycerols: three fatty acid molecules attached to a glycerol backbone (upper right). Each fatty acid is made up of a long hydrocarbon chain that contains a variable number of double bonds. The physical characteristics of the fatty acid (e.g., length of the carbon chain, number of double bonds) determine its biochemical properties (e.g., solubility, melting point) and each fatty acid is named according to its structure. For instance, the lower right figure illustrates the structure of linoleic acid, which is also referred to as 18:2n-6. This nomenclature reflects the fact that there are 18 carbons and 2 double bonds in the hydrocarbon chain. The first double bond is in the “n-6” position, which is the sixth carbon from the methyl end of the chain. In most fatty acids, double bonds are separated by a single methylene group (– CH2 – ) so the position of all double bonds can be inferred from this shorthand nomenclature. For unusual non-methylene-interrupted fatty acids (see below) the positions of each double bond must be given. These positions are usually expressed relative to the carboxyl end of the molecule (e.g., 22:2∆7,15 has double bonds at the ∆7 and ∆15 positions).
Fatty acid "signatures"
Researchers can often identify more than 70 different fatty acids in the fat stores of marine organisms like polar bears. Most of these compounds will be present in all of the available foods so we may not be able to determine where any individual fatty acid originated in the diet. However, prey species usually differ in their relative proportions of various fatty acids (see right figure). This overall profile across several fatty acids is termed a fatty acid signature.
Because fatty acid stores are strongly influenced by diet, differences in fatty acid signatures among polar bears reflect differences in foraging habits. A variety of multivariate statistical tools can be used to determine if groups of bears (e.g., age or sex classes) have different fatty acid signatures. In addition, Iverson et al. (2004) developed a multivariate statistical model that quantitatively estimates the diet composition of individual predators. The model works by identifying the proportional combination of different prey types that comes closest to matching the observed predator signature.
The importance of calibration
Polar bears, like other mammalian carnivores, also synthesize their own fatty acids in addition to obtaining them through the diet. Further, some ingested fatty acids will be preferentially utilized for energy whereas other fatty acids will be directly stored without modification. The end result of these metabolic processes is that the fatty acid signature of a polar bear will never exactly match its dietary composition. To account for this, calibration coefficients have been developed for polar bears by using captive fed seals (Iverson et al. 2004, 2006) and mink (Mustela vison; Thiemann et al. 2008) as models. Calibration coefficients are determined by comparing the fatty acid profile of the consumer with that of its controlled diet. Differences in these two profiles are the result of specific metabolic processes and correction factors can be applied to the fatty acid data collected from free-ranging polar bears.
Studying diets using fatty acids
The fatty acid profile of an individual polar bear can be readily determined from a small tissue biopsy taken through the skin of an immobilized bear (see pictures to the right). This technique has been used to generate quantitative estimates of polar bear diets across the Canadian Arctic (Iverson et al. 2006, Thiemann et al. 2007, 2008a).
In addition to using fatty acid signatures to study polar bear diets, some unusual non-methylene interrupted fatty acids have been identified in a few species of polar bear prey. These fatty acids are synthesized by marine invertebrates and are most abundant in those marine mammals that feed on benthic invertebrates: bearded seals (Erignathus barbatus) and walruses (Odobenus rosmarus). These unusual fatty acid biomarkers have been used to identify large, adult male polar bears as the primary predators of large pinnipeds (Thiemann et al. 2007).
Fatty acid signature analysis will be a valuable tool for monitoring future changes in polar bear foraging. With the rapid environmental changes that have occurred in the Arctic and are predicted to continue, fatty acid analysis may allow researchers to identify shifts in food web structure and functioning that may otherwise be difficult to detect.
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