The effect of castration and age on the development of the Shetland sheep skeleton and a metric comparison between bones of males, females and castrates

Given its abundance on archaeological sites, the sheep has clearly played a major role in the economy of our ancestors. An understanding of its osteology is therefore important to the zoo-archaeologist. The aims of this study are: (a) to determine when the epiphyses of long-bones of wethers and rams fuse; (b) to understand how castration affects dental and limb-bone development; (c) to examine the relation between age and bone-size; and (d) to find osteometric ways of distinguishing between the three sexes using a collection of 12 male (ram) and 30 castrate (wether) unimproved Shetland sheep skeletons whose ages range from 8 to 52 months (wethers) and 11 to 31 months (rams). They are compared to the 26 adult females (ewes) of the same breed in Davis (1996) Journal of Archaeological Science 23, 593-612. Fusion of the wethers' epiphyses proceeds continuously, and by their fifth year all limb-bone epiphyses are fully fused. Despite much variation in the ages of epiphyses closure, rams appear to fuse their epiphyses considerably earlier than wethers, which corroborates earlier studies. This delay of fusion in wethers allows elongation of long-bones. While castration appears to have little influence on the age at which milk teeth are replaced, rams' teeth may wear slightly faster than those of wethers. Most bones or parts of bones probably cease growing at or before epiphysial fusion. Some important exceptions include the scapula, distal humerus, proximal radius (and astragalus), which appear to continue growing for some time. Measurements of these bones or parts of bones may therefore be less useful for comparing sheep from different sites/periods. Apart from the pubis shaft thickness, long-bone lengths and shaft widths, few dimensions provide a clear distinction between males, females and castrates. Plots of long-bone lengths versus shaft widths do show some shape differences between the three sexes with ewes being short and slender, rams being short and robust and wethers being long and slender. These inter-sex differences however do not appear to provide a clear identification of archaeological specimens of sheep long-bones.