A taphonomic study of black bear (Ursus americanus) and grizzly bear (U. arctos) tooth marks on bone
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Tooth mark and gnaw damage modifications on bone from African carnivores have been extensively examined; however, there are little data on North American carnivores, especially on ursids. Tooth mark modifications include pits, punctures, scores, and furrows, while gnaw damage modifications include crenellated margins, edge polish, scalloping, scooping, and crushed margins. Forensically, tooth mark identification and measurements allow for easier identification of species involved in cases containing predator or scavenger alteration. The present study hypothesized that black bear (Ursus americanus) and grizzly bear (U. arctos) tooth marks will be able to be distinguished from one another and the two bear species will have different gnawing patterns. Further, the study hypothesized that ursid tooth marks and gnaw damage will be distinguishable from other documented carnivore alterations. In the present study, black bears and grizzly bears from the Oakland Zoo were fed 56 proximal and distal femoral epiphyses from cattle (Bos taurus). The skeletal remains were cleaned and analyzed at Boston University, School of Medicine for pits, punctures, scores, and furrows. Each tooth mark was photographed and then processed through the open-source software ImageJ (National Institutes of Health) in order to obtain the area, perimeter, length, and width of each tooth mark. The presence of certain gnaw damage characteristics were also recorded for each bone, such as crenellated margins, edge polish, scalloping, scooping, and deep furrows. Statistical analyses were used to distinguish if the epiphysis type (proximal or distal) or bear species were statistically significant factors in the type of tooth mark and gnaw damage. The results indicate a pattern distinctive to ursid scavenging with pits with an average length of 3.53 mm and width of 2.19 mm, an average score width of 1.47 mm, scalloping on the distal epiphysis especially on the patellar surface of the femur, scooping on the proximal epiphysis especially on the greater trochanter of the femur, and deep furrows primarily on the distal epiphysis along the patellar region and condyles of the femur. When comparing the present study to previous ursid studies (Arilla et al. 2014; Domínguez-Rodrigo and Piqueras 2003; Sala and Arsuaga 2013; Saladié et al. 2011) and previous other carnivore studies (Andrés et al. 2012; Delaney-Rivera et al. 2009; Pobiner 2007), there was a statistical significance between ursid pits and fox (Vulpes vulpes), skunk (Mephitis mephitis), and the combination of hyena (Crocuta crocuta) and lion (Panthera leo) pits. Scooping occurred in 35.2% of the entire sample, while scalloping occurred in 29.6% of the entire sample. Scooping has been observed in previous research but not as high of a percentage as the present study, and scalloping has been noted in a black bear study (Carson et al. 2000) and a wolf/dog study (Milner and Smith 1989), but with no numerical data for comparison. The high percentage of scooping and scalloping on long bones could be distinctive characteristics of ursid gnaw damage and could distinguish ursid scavenging from other carnivores.