Eric Olson and Professor Beverly Roeder, Integrative Biology

Stable isotope analysis of carbon (13C/12C or 13C) and nitrogen (15N/14N or 15N) has been shown to provide a more accurate estimate of overall diet in various mammalian species (Sponheimer et al. 2003). This technique was used to determine the amount of vegetation and animal protein in the nutritional uptake of free ranging Utah black bears (Ursus americanus). Samples were obtained from harvested bears taken during the fall 2005 and spring 2006 hunt from various regions of Utah via the Utah Division of Wildlife Resources (UDWR). Gender was reported by the hunters at time of harvest. Age was determined by laser analysis of vestigial premolars also collected by hunters at time of harvest (Calvert and Ramsay, 1998). Hair samples used for isotopic analysis were obtained in identical fashion.

Observation shows a traditional sexual dimorphism (males being larger than females) and we hypothesized that this may, in fact, be due to a higher protein intake by male black bears. However, since only fecal analysis and observational studies has been performed in relation diet, there is a marked lack of accurate dietary information for this species (Bacon and Burghardt 1976, Brody and Pelton 1998, Robbins et al. 2004). Stable isotope methods provided an accurate and reliable source of data for a twelve month period in bears where hair samples were obtained (Hilderbrand et al. 1996).

Results that were obtained showed elevated levels of δ15N and δ13C, indicating that tissue samples were formed with access to adequate levels of foodstuffs, or during internal metabolic recycling of protein and fat stores during a catabolic state. This is due to the absence of necessary nutrients (Hatch et al, 2002). Our results suggest that black bear males are more carnivorous overall than females. However, their δ13C values are similar; purport both genders of black bears ate similar C3 and C4 vegetation or ingested prey that had consumed similar C3/C4 foodstuffs. There was nothing in the data to suggest that the difference in trophic level we saw in 15N was due to some other difference in diet (i.e. like a very different food source such as aquatic vs. terrestrial)(Bocherens and Drucker, 2003). All of these bears were clearly eating C3 plants or animals that ate C3 plants. Hair samples from different hunt units indicated that the boars and sows isotopic signatures differed only because of the amount of meat they consumed (O’Connell et al.1999, Robbins et al. 2005). The δ15N isotopic signatures also indicated that Utah black bears increased carnivory with increasing age, and the most carnivorous black bears in the state were using lamb predation to supplement their diet, as illustrated by hunt unit differences and lamb mortality data (see figures). The similar pattern for δ13C to that of the δ15N within hunt units, although smaller in magnitude, may be a reflection of trophic level. Although a regression of hair 15N against 13C showed the two to be correlated; only ~ 24% of the data was explained by this relationship. Increases in 13C of about 1 per mil can be found per trophic level shift. However, the range we found in carbon was about 4 per mil.

When we separately analyzed the individual hunt units that had sufficient data, we found weak to strong evidence of an age effect. Older bears definitely appeared to be more carnivorous than younger bears.

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