Distinguishing sheep and goats using dental morphology and stable carbon isotopes in C₄ grassland environments

Distinguishing between sheep and goats in C₄ grass environments using new dental morphology criteria and enamel bioapatite stable carbon isotope ratios (δ¹³C) was tested on 35 modern individuals from the Central Rift Valley of Kenya. Two morphological criteria on the second and third lower molars, one of which had been previously partially described by Halstead et al. [Journal of Archaeological Science 29 (2002) 545], were found to be highly reliable in this population. Identification of species using carbon isotope ratios is made possible in some circumstances by differences in the feeding behavior of sheep, which are mainly grazers, and goats, which are mainly browsers. In environments where C₄ grasses predominate, sheep include a higher proportion of C₄ plants in their diet, and thus have higher δ¹³C values than goats. In the present study, the annual range and seasonal variation of carbon isotope ratio of diet of sheep and goats was measured from intra-tooth sequential analysis. Although the ranges of goat and sheep δ¹³C values overlap, those higher than -3.4‰ all belong to sheep; values lower than -5.2‰ all belong to goats. There is no overlap of the mean δ¹³C values by tooth, which range from -11.8‰ to -4.2‰ for goats, and from -3.1‰ to -1.3‰ for sheep. These results suggest that carbon isotope analysis of bone collagen and/or apatite will also distinguish sheep from goats in tropical C₄ grasslands. Application of the δ¹³C criteria to archaeological material must be restricted to C₄-dominated environments, and where potential access to C₃ plants (mobility, foddering) can be assessed. The utility of these morphological and isotopic criteria for differentiating sheep and goat breeds in other regions remains to be evaluated.