Refining the Analysis of Hair Samples

Using sections of a bear hair for stable isotope analysis can tell us what food sources are important to a bear as well as when, since bear diet changes with the time of year.

NPS/Kyle Joly

One of the most fundamental questions scientist seek to understand about wild animals is: What do they eat? Food, and the nutrients and energy they contain, is how wild animals "pay their bills" and support their young. One commonly used method of determining the diet of wild animals is called stable isotope analyses, based on the principle that "you are what you eat." Certain isotopes of carbon and nitrogen can be followed through a food web as their ratio varies across food types. Carbon 13 tends to be higher in marine systems (such as salmon, for example) relative to plants or terrestrial herbivores and Nitrogen 15 increases progressively up the food chain (carnivores are enriched relative to herbivores). Scientists have used analyses of bear hair to determine the relative contribution of meat, plants, and fish to bear diets for decades. NPS biologists and their colleagues compared the results from analyzing whole hair to analysing sections of hair. Because hair is a fixed tissue (it is not metabolically active once grown), the tip of the hair reflects a different dietary period than the base, as does each section in between. Thus, a single hair assimilates diet over the entire period of hair growth. The key finding of this paper is that when scientists assess hair in sections rather than analyzing the entire hair, they can gain a greater resolution of the seasonal diet of bears. This will let scientists assess not only what nutritional resources used by bears are most important, but also when. All this from a single hair sample collected at one point in time.

Splitting hairs: Dietary niche breadth modelling using stable isotope analysis of a sequentially grown tissue

Abstract

Stable isotope data from durable, sequentially grown tissues (e.g., hair, claw, and baleen) is commonly used for modelling dietary niche breadth. The use of tissues grown over multiple months to years, however, has the potential to complicate isotopic niche breadth modelling, as time-averaged stable isotope signals from whole tissues may obscure information available from chronologically resolved stable isotope signals in serially sectioned tissues.

We determined if whole samples of brown bear guard hair produced different isotopic niche breadth estimates than those produced from subsampled, serially sectioned samples of the same tissue from the same set of individuals. We sampled guard hair from brown bears (Ursus arctos) in four regions of Alaska with disparate biogeographies and dietary resource availability. Whole hair and serially sectioned hair samples were used to produce paired isotopic dietary niche breadth estimates for each region in the SIBER Bayesian model framework in R.

Isotopic data from serially sectioned hair consistently produced larger estimates of isotopic dietary niche breadth than isotope data from whole hair samples. Serial sampling captures finer-scale changes in diet and when cumulatively used to estimate isotopic niche breadth, the serially sampled isotope data more fully captures dietary variability and true isotopic niche breadth.

Rogers, M. C., G. V. Hilderbrand, D. D. Gustine, K. Joly, W. B. Lealock, B. A. Mangipane, and J. M. Welker. 2020. Splitting hairs: Dietary niche breadth modelling using stable isotope analysis of a sequentially grown tissue. Isotopes in Environmental and Health Studies DOI: https://doi.org/10.1080/10256016.2020.1787404