A comparison of the crania measured during the Bison of the Bighorn Basin Project with a national dataset gathered by Dr. Chris Widga, East Tennessee State University, revealed the bison measured in the project are all morphologically modern. Modern morphology arose in bison approximately 3,000 years ago.

Two categories of plants are recognized based on their carbon-fixation pathways. C3 plants represent 90% of all plants including all trees and herbaceous plants from cold and temperate climates. Their stable carbon isotope values range from -23% to -32%, with an average at about -26%. C3 plants do most of their growing during the spring and early summer.

C4 plants are more competitive than C3 plants during periods of stress, especially during periods of drought. Their stable carbon isotope values range from -9% to -16%. C4 plants thrive in warm summers accompanied by adequate precipitation. Unlike C3 plants, the growth of C4 plants correlates with mean annual precipitation (MAP) and mean annual temperature (MAT).

As generalist feeders of grass biomass, we expect bison stable carbon isotope values to reflect a mixture of the C3 and C4 plants available in their range. Stable carbon signatures from this sample indicate a diet of largely C3 grasses. This is expected considering the specimens from the Project are from mostly high altitude and latitude sites.

Nitrogen enters the soil through the atmosphere, by precipitation, or from bedrock decomposition. Once in the system, nitrogen is taken up by plants and moves up through the food chain, becoming progressively enriched by 2-5% at each trophic level (Ambrose 1991; Bocherens et al. 1994).

The potential of dietary stress can also be assessed by examining d15N values. Ambrose and DeNiro (1986) noted a strong correlation between annual rainfall and herbivore d15N ratios. They suggest enrichment may be caused by physiological adaptations to water stress and low-protein diets in arid habitats. For example, when drought-tolerant mammals are water stressed they will concentrate 15N in their tissues and eliminate 14N in urea, producing greater isotopic variability and more tolerant values than obligate drinkers. In arid or saline environments (<400 mm of precipitation), these herbivores are more depleted than browsers living in the same environment (Ambrose and DeNiro 1986; Heaton et al. 1986).

The Bighorn Basin receives, on average, approximately 177.80 mm of precipitation qualifying it as an arid environment. In comparing the δ13C and d15N values, there is slight correlation between enriched N values and depleted C values. This may be an artifact of these individuals using warmer and more xeric habitats.